Hardening Off Tomato Transplants

One of the most important steps in planting comes before seedlings get near the garden. This is the process of hardening off, or gradually acclimating you tomato seedlings to outdoor conditions. These plants have spent their short lives in a warm, sunny, protected place and won’t fare well if you don’t expose them slowly to the elements.
A few days before you’re ready to begin hardening plants off, reduce amount of water you give them, and cease fertilizing until they are planted in the garden.
About 10 days before you intend to plant, put your transplants outdoors in an area where they’ll be protected from the direct sunlight and wind. Leave them out for a few hours and bring them back inside. Repeat this each day, gradually increasing the amount of time they’re outside and the degree of exposure to sun and wind. After a week or so, leave the transplants out overnight. If frost threatens, bring them indoors.
If you harden off your plants properly, they’ll be strong and able to withstand full sun, strong breezes, and all the challenges they’ll meet in the garden.

If you grow your own vegetable seedlings indoors under lights, or if you purchase transplants from a nursery greenhouse, you will need to adapt your seedlings before transplanting them into the garden. This adjustment process is called “Hardening Off.”

Hardening off is the process of adapting plants to the outside, so they can get used to sunlight, wind, rain, cool nights, and less frequent watering and fertilizing. The hardening off period allows your seedling to transition from the comfortable growing conditions under lights or in a warm greenhouse to the normal conditions they will experience in the garden.

I like to allow at least a week to harden off seedlings before transplanting to the garden. Depending on the weather, sometimes two weeks are necessary. Be patient and you will be rewarded with healthy and strong plants.

Why Should You Harden Off Seedlings?

Sunlight is stronger than grow lights and can burn foliage if the seedlings are placed in the direct sun. Light breezes can sap your plants’ moisture and cause weak stems to break. Cooler nighttime temperatures may stunt the plant’s growth or even kill a seedling that is not used to it. Gradual exposure to the outside elements allows the plants to toughen up and become accustomed to being outside.

Steps to Harden Off Your Transplants:

1. Begin Hardening Off Your Plants in a Sheltered Location: About a week or two before your transplant date, place your plants outdoors in a protected spot for a few hours on the first day. I like to situate my seedlings on a patio table under an umbrella to shade them from the sun and shelter from rain. The table is located in an area that is also protected from harsh wind. Allow your plants to remain outside for a few hours, then bring your plants back inside.

Keep an eye on the weather during the hardening off period. Temperatures can drop to unseasonable levels quickly, and high winds can destroy tender foliage and knock over seedling trays. Watch the weather for early-season frost or unsettled conditions and bring transplants indoors until the weather returns to normal.

2. Increase Outdoor Exposure a Little Each Day: Increase the amount of time that the seedlings spend outside gradually to allow the plants to adjust slowly. Continue to harden off seedlings by moving the plants outside while temperatures are warm and then back inside at night when the temperature is cool. I usually add a couple of hours each day as long as the weather cooperates. Alter the shade or move the seedlings to a location that receives morning or evening sun, so they are exposed to a little more sun each day. Allow the seedlings to experience gentle breezes. Even filtered sunlight and light breezes can deplete your plants’ moisture, so check on them frequently and give them enough water, so they do not wilt.

3. Leave Seedlings Outside Overnight: Eventually, allow your plants to stay in full sun and outside as long as night temperatures do not drop below freezing. If it is going to get below freezing move the plants indoors. Resume the hardening off process once temperatures return to normal conditions.

Cool season crops such as broccoli, Brussels sprouts, cabbage, celery, lettuce, onions, parsley, peas, spinach, Swiss chard, and other hardy greens can tolerate low nighttime temperatures of around 45°F once they have had time to adjust. Light frost won’t harm these seedlings after these are hardened off.

Warm-season crops such as tomatoes, peppers, eggplants, melons, cucumbers, and some herbs prefer warm nights, at least 60°F. They can’t stand below-freezing temperatures, even after the seedlings are hardened off. So continue to bring indoors if nights remain cool.

Transplanting Hardened Off Seedlings to the Garden

After your seedlings are hardened off, they are ready to be transplanted into their permanent location in the garden.

Prepare your garden bed ahead of time. If the weather has been dry, water the bed thoroughly the day before you plant.

Choose a cloudy day with no wind and transplant in the late afternoon or evening to give your plants time to adjust without the additional challenge of the sun.

Water the seedlings well after planting.

Hardening off is an important step to reduce stress on your plants. If you harden off seedlings properly, they’ll be strong and able to withstand full sun, light breezes, spring rains, and fluctuating temperatures. Once transplanted to the garden, the plant’s energy will be focused on establishing roots and growing rather than surviving.

You May Also Like:

  • 10 Steps to Starting Seedlings Indoors
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Maloca’s Blog

It is approaching time for everyone who has tomatoes growing in their windows to start “hardening-off” the plants. Basically this is a process of gradually toughening up the plants and introducing them to outside. The tomatoes have been growing in the protected environment of your windows and the outside is going to introduce a whole new set of growing conditions including direct sunlight, wind, lower temperatures, and air pollution. If you were to take the plants directly from your window and just plant them outside, their growth would be significantly slowed or they may even die.

The first part of “hardening-off” is to toughen your plants up by reducing the amount of water they are getting (but don’t cut off water completely), and if possible by reducing the temperature by a few degrees, for their last week in the window. The result will be a shorter, more fibrous plant that will suffer less when it transitions to outdoors.

The second part of “hardening-off” is to acclimatize the plants to outdoors. This is a gradual process where you take them outside for longer and longer periods. This can take up to a couple of weeks and it is best to be patient since introducing them to outside too quickly may do more harm than good to the plants. Here are a few things you will want to consider during this process:

It is best to start with only a few hours of sunlight, or if possible partial shade, and then increase the amount of sunlight gradually so that in 7 to 10 days the plants can do a full day of sunlight.

Remember that the outside will speed up the rate that the soil will dry, so be sure to keep the soil moist enough to prevent wilting.

Wind can damage the plants by breaking the stems of un-staked seedlings, or by whipping the seedling around to the extent that the roots are loosened and then damaged. Therefore it is important to choose as sheltered place as possible for the first week of taking the plants outside. If you don’t have a naturally sheltered place to put them you may consider using a “cold-frame” system to keep them sheltered: I am taking my tomatoes out in larger Rubbermaid bins without their lids for the first week. This way they get the sun and outdoor temperatures with less risk from the wind.

Cold temperatures can pose a serious threat to tomato seedlings. Be sure to bring the plants indoors if the temperature is going to drop (especially if it is below 10C) since this will put the plants into shock and seriously slow down their growth if they are fortunate enough to survive.

The air quality in southern Ontario is far from perfect and unfortunately smog can do a lot of damage to sensitive plants. Therefore, it is important to consider the air quality when transitioning your plants to the outside. Ontario has a great smog warning system and you can sign up to get smog warnings for where you live to be sent to you by e-mail (go to http://www.airqualityontario.com/alerts/signup.cfm to sign up). If it is a smog day, like today, it may be best to keep the plants inside or reduce the time they are outside.

You may also need to stake the seedlings, especially the ones that are tall and thin. Last year I used twine and old chopsticks (but you could use almost anything a similar size). I found it worked best if I put the chopstick into the pot as far away from the plant as possible (to prevent damaging the roots), tied one end of the twine to the chopstick, loosely looped the twine around the plant to provide some support, and then tied the other end to the chopstick. It is important not to tie tightly around the plant because it may cause damage; instead you want it so that the plant has some room to move and grow while also having some support.

If you do not have a method to take your plants outside, you can either contact me directly or send a message over the list to see if there is someone with a backyard or balcony interested in caring for the plants for the last few weeks before they are planted in the garden.

If you have any further questions or comments, please feel free to contact me

Using Cold Frames In Spring: How To Harden Off Seedlings In A Cold Frame

Whether growing your own transplants or purchasing seedlings from a local nursery, each season, gardeners eagerly begin to transplant starts into their gardens. With dreams of lush, thriving vegetable plots, imagine the disappointment as the tiny plants begin to wilt and wither away. This early season frustration, most often caused by injury at or after transplant, can be easily avoided. “Hardening off” plants before being moved to their final location not only improves the likelihood of survival but ensures a strong start to the growing season. Let’s learn more about using a cold frame for seedlings to harden off.

Cold Frame Hardening Off

Seedlings which have been started indoors or in greenhouses have been exposed to conditions much different than those which occur outdoors. Grow lights emit enough light to nurture and encourage growth in seedlings, but the strength of the light is not comparable to that of direct sunlight.

Additional factors, like wind, may damage delicate transplants. These outdoor variables can make adjusting to new growing

conditions quite difficult for young plants. While these seedlings can sometimes overcome environmental stressors at transplant time; in many cases, the issue is so severe that the transplants are unable to recover.

The process of “hardening off” refers to the gradual introduction of the plants to the new environment. By exposing transplants to new conditions over time, usually about a week, plants are able to increase defenses against these harsher conditions. Using cold frames in spring is another way to help harden off your seedlings.

Hardening Off Plants in a Cold Frame

Many gardeners choose to use cold frames as a means to begin hardening off plants. As the name suggests, cold frames are most often used to offer protection from low temperatures early in the growing season. In addition to temperature regulation, cold frames can also aid in protection from strong winds, moisture, and even direct sunlight. Seedlings in a cold frame can be well protected from these elements, making this an easy way to harden the plants off.

The use of a cold frame allows gardeners to easily and efficiently harden off seedlings without the hassle of repeatedly moving seed trays to and from a sheltered growing area. To begin hardening off plants, place them into a shaded cold frame on a cloudy day for a few hours. Then, close the frame.

Gradually, increase the amount of sunlight the transplants receive and how long the frame remains open each day. After several days, gardeners should be able to leave the frame open for the majority of the day. Cold frames may still need to be closed at night, as a means to control temperature and protect new plant starts from strong winds as they acclimate.

When the cold frame is able to remain open both day and night, the seedlings are ready to be transplanted into the garden.

4 Ways to Use a Cold Frame

A few years ago, while driving with my family through the countryside of Bulgaria, I noticed how strikingly similar the landscape was to that of central Virginia. I also noticed that nearly every home had a garden plot, a grape arbor, and a cold frame of some sort. It made me realize how ancient and universal the practice of using cold frames is.

A cold frame is a simple structure that utilizes solar energy and insulation to create a microclimate within your garden. For those of you who have harvested and eaten a salad of fresh greens in February or have flowers blooming well past frost, you know the attraction of using cold frames. You also know how easy they are to make and use. Although we have a greenhouse on our farm, space is always limited, so we rely heavily, especially in spring and fall, on our cold frames to overwinter plants, extend the growing season, start seeds, and harden off plants.

For whatever purpose you want to use a cold frame, you need to keep in mind a few basic factors. First, some plants fare better in cold frames than others, with low-growing, cool-season plants being the best suited. Second, the type of cold frame you use dictates how much protection you can offer your plants. In all cases, the main conditions you need to monitor and control are temperature, sunlight, moisture, and wind exposure.

1. Overwintering dormant plants

In a sunken cold frame, the ground insulates and the sun gives warmth, but you still need to eliminate pockets of cold air that can harm overwintering plants.

If you love to use tender or tropical plants in your garden but don’t know what to do with them come fall, a cold frame provides a simple solution. It is not a greenhouse: You won’t be able to keep your plants growing lushly through the winter months. But you can provide plants with the right conditions for a gentle dormancy, and they will be eager to resume growth come spring. I keep tender plants in a sunken cold frame, as it provides the best and most consistent insulation. Although my garden is in USDA Hardiness Zone 7, I can reliably overwinter plants that are hardy to Zones 8 or 9—and occasionally Zone 10 if we have a mild winter.

To keep a tender plant in a cold frame, cut it back as much as possible before the first fall frost. If the plant is not already in a container, lift and sink the root mass into a large plastic pot, with plenty of room for soil to insulate it. Pack the cold frame tightly with pots, and add leaves or mulch over the soil surface and into any significant air gaps. Make sure all the pots are well watered.

Once you have your tender plants stored in a cold frame, you will need to control the weather conditions to maintain a healthy dormancy. Keep the soil moderately moist but not wet; the plants won’t need much water over the winter and will rot if overwatered. They should not receive much sunlight, either, because this will encourage active growth. I like to use a white plastic cover on my cold frame as it limits the amount of light penetration and moderates spikes in daytime temperatures.

Temperature fluctuations can be harmful to dormant plants and should be minimized by venting the cold frame. As a general rule, if it’s 35°F to 40°F and sunny, you should open the frame partway, while at 45°F to 50°F, you may need to open it up entirely. Using a sunken cold frame or siting your aboveground frame near a windbreak offers much-needed protection from exposure. This is especially important for those plants that may retain some foliage in winter.

Pay attention to temperature

While heat and humidity are important for germinating seeds, excessive heat (above 90°F) can damage fragile seedlings. A min/ max thermometer hung on an inside wall of the cold frame is a great way to monitor temperature fluctuations.

2. Giving seedlings an early start

Seedlings grown indoors can move outdoors weeks earlier if you use a portable cold frame.Prevent disease on seedlings started directly in a cold frame through frequent venting.

Whether you are starting seeds in flats or sowing them directly into the soil, a portable cold frame provides the opportunity to get your plants going a few weeks early, and it eliminates the transplanting shock that many plants face because they will be better acclimated from the outset. If you are seeding in the early spring or fall, focus on cool-season plants, as they tend to have lower temperature thresholds for germination. Keep in mind that seedlings are more susceptible to extreme weather conditions than established plants.

If you are sowing directly into a portable cold frame, have it in place two weeks prior to seeding to warm the soil for germination. Whatever method you are using to start your seeds, make sure to keep the seedbed evenly moist. Once seedlings have germinated, the cold frame should be vented more frequently to discourage damping off by increasing air circulation.

If you start your seeds in a greenhouse or indoors under lights, you can start them a good six weeks earlier than usual and transplant them to a cold frame you’ve placed in your garden. It helps to have the cold frame in place at least two weeks prior to transplanting to warm the soil. Again, you will need to pay attention to the degree of sunlight, moisture, temperature, and wind. The frame also provides a windbreak while the plants are still small.

Because you’re encouraging active growth, you will want to use a transparent cover of plastic or glass. The soil will dry out more quickly inside the cold frame than outside, so be sure to keep the soil moist, especially while the plants are acclimating to their new site. Keep in mind that more plants die of excessive heat and drought in cold frames than from cold damage. Proper ventilation is particularly important for cool-season plants. If you have established transplants, vent the frame when the outside temperature is 40°F or higher.

If your plants are closer to the seedling stage, you may want to wait until the outside temperatures are 45°F to 50°F before venting.

Three types of cold frames

We use three different types of cold frames on our farm, depending on the plants we are working with and what we are trying to achieve.A sunken cold frame with cinder-block walls and a rigid plastic cover is best for overwintering tender plants and hardening-off seedlings.A plastic hoop tunnel warms the soil for spring seeding and protects frost-sensitive plants in spring and fall.A portable wooden frame with a rigid plastic cover extends the harvest season of cool-season vegetables and allows us to direct-sow seeds earlier in the spring.

3. Hardening off young seedlings

Instead of carrying plants in and out of the house every day, simply open and close the lid of the cold frame to acclimate plants to the outdoors.Like a car on a sunny day, cold frames can heat up significantly. Proper venting will limit possible damage to your plants.

When plants are moved from a warm, sheltered location—such as a greenhouse or indoors—into the garden, they must be gradually acclimated to fluctuations in temperature, sunlight, moisture, and exposure. Generally this is done by carrying the plants outside and back in again for gradually longer periods of time over the course of a week or two. The same effect can be achieved by opening and closing a cold frame over a five- to seven-day period.

The key to a trouble-free hardening-off period is to keep track of the extended weather forecast and plan accordingly. If I am moving out cool-season or young perennial plants from my greenhouse, I will wait for a stretch of weather where the lows don’t fall below 35°F. Even if the temperature drops after this period, plants hardened off and growing in a cold frame will be fine. For warm-season plants, I wait until the temperatures have stabilized and we are within two to three weeks of our last frost date.

In general, wait until seedlings have formed multiple sets of true leaves and are well rooted before moving them into cold frames. Once the plants are packed closely into a frame, start venting the frame during the warmest part of the day, gradually increasing the length of time the frame is left open. If you are not able to tend to the frame during the day, try to time the onset of your hardening-off period with cloudy weather, and start by venting the frame just a crack, gradually increasing the open gap each day.

As plants acclimate to cooler temperatures, more direct sunlight, and wind exposure, their foliage will often thicken and darken in color. New growth is also a good sign that the transition is going well and your plants are ready for their final move into your garden.

4. Extending the season past frost

If you can’t bring plants to a cold frame, bring a cold frame to the plants. Some PVC and rebar make the frame for a hoop tunnel. Plastic over the tunnel keeps plants warm at night, but it should be lifted during the day.

If you dread the coming of the first fall frost, you can extend your growing season with a plastic hoop tunnel. While not a typical cold frame, a hoop tunnel provides the same benefits by insulating the plants inside and keeping frost at bay.

An easy and nondisruptive method is to sink 12- to 18-inch metal stakes into the ground and slide 6-foot sections of electrical conduit or PVC pipe over them, creating an arch. You can then stretch plastic over these hoops and protect the plants underneath from nighttime temperatures and frost. These plants have been hardened off naturally to colder temperatures by the onset of fall and generally don’t need to stay covered during the day, provided the outside temperatures are above freezing.

Want to build a cold frame?

For design ideas and building instructions, check out these links:

  • Build a Simple Cold Frame (video)
  • Build a Cold Frame (article)
  • Build a Cold Frame with a Lightweight Lid
  • Give Your Cold Frame a Warm Bed

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Hardening Off Your Seedlings

Moving is one of life’s most stressful events. Imagine how trying it would to move from a perfect climate where it’s always 70 degrees, calm and sunny, to a harsh and windy climate where it gets really cold at night and the sun is burning hot during the day.
Put yourself in your seedlings shoes. If you had to move from San Diego to Montana, wouldn’t you want some time to adjust?
You’ve started your seeds. Kept them hydrated just right. Transplanted them. Maybe fed them a diluted dish of fertilizer or two. They are tall now.
Your seedlings may look like they are ready to go it on their own in your garden, but be kind, prepare them for the extremes of your garden with a process called ‘hardening off.’
The author of ‘Grocery Gardening’, Jean Ann Van Krevelen, said you shouldn’t skip the step of hardening-off your seedlings. Young plants may not make it if planted directly into your garden with out a transition.
“When seedlings are grown inside in a controlled climate, they don’t have the opportunity to develop the strength and structure to live out in the elements. They need to get acclimated to their new home, “ said Van Krevelen.
To harden off your seedlings, gradually introduce them to the outdoors. It helps to store your seedlings in trays, at this point, to make transporting the plants easier.
“Take your seedlings to a protected location outside for one hour for the first day,” she said, “Do this each day for a week. Add one hour for each day of the process. By the end of the week, you’ll be at 7 hours and the plants will be ready to be transplanted,”
While inside, seedling stems haven’t been exposed to winds. Plants, like us, need to start our workouts and gradually increase the intensity to become strong. So early on in the hardening off process, provide seedlings shelter.
“Don’t put them in direct sun. Don’t put them in a windy location. Keep in mind, they are just babies,“ said Van Krevelen.
If you want to help your plants beef-up early, you can add a fan to the area where you are storing your seedlings. Use the fan to gently move the air. Too much direct breeze from a fan could dry out the seedlings and do the same damage wind would in the garden.
Gardeners have different approaches to the watering aspect of the hardening off process. Van Krevelen keeps her seedlings evenly moist from grow light to garden.
“Provide consistent moisture. Seedlings are susceptible to any extreme until they are established,” she said.
Horticulturalist Erica Shaffer agrees. “Don’t send your babies into the big, bad world of your garden thirsty and hungry,” she said.
Good gardeners aren’t perfect. And the process of hardening off doesn’t have to be executed perfectly or uniformly to be highly successful. If you forget to take your plants out one morning before work, just start back up the next day. If the spot you chose for them becomes too sunny as the day went on, all is not lost. Plants are a forgiving lot and will hang in with you as long as you give them a little attention.
There is a bit of hassle involved in schlepping the plants outdoors and back in again each day over a week. But after gently caring for your baby plants for weeks, the added effort is good insurance that your plants will leave your nest safely and do well in your garden. After all, don’t you want to shield every thing you love from unnecessary stress?

What is “Hardening Off” Your Plants and What Does it Mean?

If this were any other year, we’d still be enjoying Spring’s annual show of Daffodils, Hyacinths, Tulips and Bluebells. Instead, Mother Nature has fooled us into thinking it’s late April and time to go plant shopping. Nurseries are scrambling to fill our overwhelming desire and insatiable need for annuals, perennials, herbs and vegetables.

The weather is so conducive and our urge to plant is so overwhelmingly strong, we’re willing to shop until we drop and plant until our knees and backs cry “UNCLE!” Release that shovel, grab a cold drink and relax.

Let me teach you about “hardening off” your springtime annuals and perennials and why it’s one of the most important steps to giving your plants the right start this spring.

So, what exactly does “hardening off” your plants mean? If you have ever spent all morning picking out your favorite plants at the local nursery, planted them lovingly, only to have them wither and look pitiful for a few weeks, your plants weren’t hardened off.

Did you wonder why those huge, robust and tropical Elephant ears that were so stunning in the garden center became shredded on that windy day? Have you ever have some Hosta that looked like they were sunburned, even though you only left them in the sun for a day or two?

Hardening off is all about acclimating your plants to their new home. They need to be slowly introduced to their new digs.

Generally, the plant material you buy from your favorite greenhouse has spent all winter and early spring under very controlled conditions. The greenhouse temperatures are normally consistent. Water and fertilizer are given at regular intervals and the plants have been “babied” their entire lives. Gentle greenhouse fans barely rustle the plants’ leaves.

In your garden, spring breezes that make for superb kite flying can shred plants recently planted in the ground. Because they haven’t been exposed to sun, wind and rain, the cuticle of the plant is soft and tender, allowing any change at all to throw the plant into a tizzy. They need to be hardened off- you can do it successfully a few ways.

The first way to harden off is by withholding water. You can stress the plant enough that it actually responds by “toughening up.” By waiting to water until the plant exhibits a wilted appearance, you build a stronger plant. Doing this for a week to ten days will toughen up the plant enough to place it in the ground. I think of my plants as my “little babies” and I just don’t have the heart to do this. I prefer hardening off my plants another way.

I place my plants under a covered patio if I know that they have been grown in a greenhouse. I leave them under the protective cover of the patio roof for 3 or 4 days and water normally. Then, I’ll move the plants outside, on a sunny day, for a few hours each morning. I bring them back in about 11 o’clock. I’ll do this for 3 or 4 days, lengthening the time I leave them outside each day. Then, around the 9th or 10th day, I leave them outdoors permanently.

I find this method less stressful than watching my plants wither.

Of course, if temperatures get below 45 or 50 degrees, I’ll cover them with a frost blanket or sheet for the night. Make sure to pull the covering off in the morning. Additionally, when you bring your houseplants outdoors this spring, treat them the same was as you do your new greenhouse plants. I’ve seen many cases of eager gardeners placing their houseplants outside on a beautiful day, only to see them sunburned and withered at the end of just one day. The heartbreak IS preventable.

A third way to harden off your plants is to place them in a cold frame. I got one for Christmas this year and have really enjoyed starting plants from seed in it. Unfortunately, my cold frame isn’t large enough to accommodate the plants I am hardening off. I find the method of “babying” them easy and stress free.

Plants that have been exposed to the elements for a week or more in the nursery don’t need to be hardened off. Just be sure that, once they’re planted, you remember to cover them if we happen to get low temperatures at night. This has been a wacky spring, Mother Nature just might have a frosty night up her sleeve!

By treating your plants right in the beginning, they will return the favor and reward you with vigor and stupendous growth all season long. So, get out there and shop- just make sure that you harden them off becomes a regular part of your planting process!

Hardening Off Vegetable Seedlings

Back to Seedling Care

Hardening is the process of exposing transplants (seedlings) gradually to outdoor conditions. It enables your transplants to withstand the changes in environmental conditions they will face when planted outside in the garden. It encourages a change from soft, succulent growth to a firmer, harder growth.

  • Begin hardening transplants 1-2 weeks prior to setting out plants in your garden.
  • The easiest way to harden transplants is to place them outside in a shaded, protected spot on warm days, bringing them in at night. Each day, increase the amount of sunlight the transplants receive.
  • Don’t put tender seedlings outdoors on windy days or when temperatures are below 45° F. Even cold-hardy plants will be hurt if exposed to freezing temperatures before they are hardened.
  • Reduce the frequency of watering to slow plant growth, but don’t allow plants to wilt.
  • A cold frame provides an excellent environment for hardening off transplants.
  • After proper hardening, tomato plants can usually tolerate light and unexpected frosts with minimum damage.
  • The hardening process is intended to slow plant growth. If carried to the extreme of actually stopping plant growth, significant damage can be done to certain crops. For example, cauliflower will produce thumb-sized heads and fail to develop further. Cucumbers and melons will stop growing if hardened too severely. They may be left outside overnight if the temperature will not fall below 50° F.

Please e-mail us when you have questions or problems – http://extension.umd.edu/learn/ask-gardening

So just exactly what does it mean to harden off vegetable plants and flower seedlings before planting outdoors?

And, why is it so important?

Those two questions are often asked by gardeners every spring. And with good reason.

A young tomato plant grows indoors, awaiting life in the garden.

Whether raising plants from seed at home, or purchasing transplants from a nursery or greenhouse, the process of hardening off is a big key to a plants short and long term health and success.

Why You Need To Harden Off Vegetable Plants & Flowers

In a nutshell, hardening off is the process of toughening up young, tender plants for life outdoors vs. Mother Nature.

Whether raised in nurseries or at home, tender seedlings are simply unprepared for life outdoors.

Inside in a controlled environment, there is no such thing as a strong wind or heavy rain to whip and damage tender foliage.

Both flowers and vegetables need time to adjust from their cozy life indoors before being planted outside.

Nor are there any worries of intense sunlight or blazing daytime temperatures to quickly dry out plants. Or even a cold night that might bring a damaging or deadly frost.

But hardening off plants allows tender transplants time to slowly adjust and prepare for all of those harsh outdoor conditions.

And in the process, keep them safe from injury, transplant shock, or even complete failure.

The Process – How To Harden Off Vegetable Plants And Flower Seedlings

For those who grow their own seedlings indoors at home, the hardening-off process should begin about three weeks before planting day.

Begin by setting plants outside on warm days (around 55 degrees or above) in a protected area.

We use 1 x 12″ boards screwed together and placed on the ground around our flats. It keeps the wind from knocking them over, but still allows them plenty of sunlight and air.

While hardening off, we use 1 x 12″ wooden boards placed around our plants to protect them from harsh winds.

Porches and patios are also ideal for this task.

It gives plants their first taste of outdoor living, while still having a bit of protection from full sun or heavy winds.

For the first week or so, bring plants inside at night to keep them safe from cool or freezing temperatures.

As spring continues to warm the air, allow transplants more and more time outdoors. In fact, as long as night time temps stay above 45 degrees, keep them out around the clock.

Seedlings indoors have it easy. But outdoors, a whole new world awaits!

As planting day approaches, plants should be spending nearly all of their time outdoors. Only bring indoors if a frost, high winds, or a heavy storm is in the forecast.

By following this process, your plants will be more than ready to handle the shock of transplanting.

What About Nursery & Greenhouse Plants?

Store purchased plants are usually a bit larger and more robust than those grown at home.

But even so, most have still spent all of their life protected indoors.

After hardening off, plants are ready for Mother Nature.

And a bit of hardening off for a few days or a week can go a long way towards helping them to adjust to outdoor life.

Start by sitting out newly purchased plants outside in a semi-protected area

Keep them from strong winds and heavy rains, but allow them to stay outside around the clock unless a frost or storm is in the forecast.

These plants are usually much larger, so a week or so is usually more than enough time to harden them off.

Now it’s all about getting those plants off and running! (See : The Ultimate Planting Day Guide – How To Start Your Plants Off Right In The Garden!)

A Few Exceptions To The Rule

There are a few exceptions where you do not need to harden off vegetable plants and flowers.

If you are purchasing late in late spring, or from nurseries who have already placed their plants outdoors – simply buy and plant!

Happy Spring and Happy Gardening! Jim and Mary

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How & Why To Harden Off Vegetable Plants & Flowers Before Planting Tagged on: hardening off flowers hardening off plants hardening off vegetable plants how to harden off plants transplanting vegetable seedlings

Transplanting veg makes a lot of sense. It maximises your outdoor growing space as crops are in the ground for less time. It negates the need for fiddly (and wasteful) thinning out. It enables you to position your crops at exactly the spacings you want. It allows your veg to be raised in the nurturing environment of a greenhouse. This last benefit, though, has a bit of a caveat to it, because that nurturing environment is nothing like the outside world. Cue ‘hardening off’.

I remember being abruptly introduced to this concept about 20 years ago. I’d sown some broad beans under glass in early March and, even though I say so myself, they were looking mighty grand: plentiful, lush leaves in rude health. “Brilliant!” I thought, “these are going to romp away in the garden”. I’d also clocked that broad beans were super-hardy as they’re one of the few vegetables that can be sown on the plot during February. “Surely they can just be placed outside, right?” Twenty minutes after I’d made that assumption I was proved horribly wrong. It was like someone had taken a flame gun to them. Frantically digging the alarmingly wilted beans back up and placing them under glass was a pointless exercise; the damage had been done.

OK, so it was a sunny day with a firm breeze, making this an extreme case, but let’s all learn from my mistake. ALL veg, if grown under cover, need to be acclimatised to the outdoors before being planted out permanently. Why? Because the waxy cuticle that protects the upper and lower leaf surfaces from desiccation via excessive sunshine and wind isn’t encouraged to fully form under glass, and it takes time for the plant to build up this horticultural armoured plating. An increasingly harsher environment prompts this formation.

To ‘harden off’ your plants, the standard recipe is to place them outdoors in a relatively shady, sheltered spot for a fortnight, bringing them indoors again at night. I’ll add to this that one week in the shade and then one in increasing sun is ideal if you can be faffed (if you’ve no shade then horticultural fleece is very handy shading material). Also, if your plants were grown in strong light under glass and didn’t become stretched and soft, then you can get away with a week (or even a few days if you employ said fleece as a cloche once they’re planted out).

Just another brief word on cloches – they’re another tool that can be employed to make this process quicker (perfect if your courgettes are bulging out of their pots). We used to grow marrows as a commercial crop on my parents’ smallholding, and one of the jobs I remember being very satisfying was taking a sharp knife to slash holes in the clear polythene tunnel cloches that covered the transplants. Planting out veg under such cloches while they’re tiny offers another transition towards life outdoors, freeing up valuable indoor propagation space. If cutting holes in your cloches sounds alarming, simply open up the sides and ends a little every day for a week, before removing these covers completely.
Lucy Halsall is Editor of Grow Your Own Magazine which contains a wealth of information about growing your own vegetables plus even more resources on the website including the new growing guides section.

How to Harden Off Tomato Plants to Prepare Them For the Home Garden

Harden off your tomato plants before you set them out in the home garden. Outdoors can be a shock for them if they’re not ready. It’s up to you to prepare them for the real world.

Until now, your tomato seedlings have had an easy life. You’ve pampered them with plenty of light, water, food, and warmth.

That’s about to change.


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In the outdoor garden, your tomato plants have to stand on their own roots and deal with sun, wind, and critters.

About 7-10 days before planting them outdoors, help acclimate them to temperatures, sunlight, and wind.

This process is called “hardening off.” Without hardening off, your tomatoes are more susceptible to sunburn, windburn, and breakage.

Gradual exposure helps your plants toughen up and reduces the possibility of injury. With proper hardening off, they’ll have an easier transition to the garden and begin producing fruit sooner.

Steps for Hardening Off Tomatoes

Take these steps to harden off your tomato plants.

  1. Start slowly. For the first day, set plants outside in the shade, next to the house, or in a protected area for an hour or two. Eastern or northern exposure out of direct sunlight also works. Bring seedlings in at night.
  2. Raise exposure. Gradually increase the amount of time your plants are outside each day for several days, to include some direct sunlight. Light breezes help strengthen stems as long as movement isn’t too severe.
  3. Monitor seedlings. Check them regularly for wilting and water appropriately if they droop. Move them to shade or a sheltered location if seedlings they appear stressed.
  4. Finally, leave tomatoes out overnight. Make sure the forecast is for temperatures to be above 50°F during the night time.

Once your seedlings handle their outdoor sleepovers without problem, it’s time to move them into their new home: your garden.

More about Planting Tomatoes

Planting tomatoes: top garden tips to help you succeed …
How to buy tomato plants …
Strengthen a growing tomato plant to prepare it for the home garden
Pick the best home garden spot for growing tomato plants
Preparing your soil for planting tomatoes in the home garden
Tomato transplanting mistakes to avoid when setting out plants …
Tomato worms – cutworms: keep them away with …
Watering tomatoes when planting and just afterwards …
Protecting young tomatoes from frost and freezing …
How and when to mulch tomatoes
Planting tomato plants: Frequently Asked Questions (FAQs) …

More Tomato Planting Tips on our Pinterest board …

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Hardening off tomato plants, or gradually exposing seedlings to the outside environment, strengthens stalks and prepares tomato plants for the transition to the garden. Without hardening off, young seedlings may snap in the wind and foliage may scald and sunburn, damaging the plants. Exposing tender stems and foliage gradually to the effects of the wind builds strong stems that stand up to the weather when planted in the garden.
Choose a Sheltered Location to Harden Off Plants
Choose a sheltered location for hardening off your tomatoes. Look for an area that receives filtered or morning light and provides a windbreak. The object is to expose young plants to the outside without causing stress or damaging foliage. Areas near the foundation of the house (out of direct sunlight) or along a fence make the ideal place for introducing tomato seedlings to the outside.
Bring Tomato Plants Inside at Night
Bringing your tomato plants back inside at night during the hardening off process protects them from chilly nights. Leaving them outside poses the risk of losing the plants. Although it may seem like a bit of a chore, moving plants inside is a necessary part of the hardening off process and will reward you with lush green plants with abundant fruit in late summer.
First Exposure
Place tomato seedlings in a sheltered location when weather is warm and mild. Two to three hours of exposure for the first day is sufficient. Place the seedlings in the same location for 2 to 3 days. Watch your tomato plants carefully and provide additional water as outside conditions can dry the soil quickly. Tomato seedlings typically require daily watering.
Gradually Increase Exposure to Sun and Wind
Move the seedlings a foot or two away from the building or fence, ideally into an area that receives some direct sun. This allows seedlings to experience natural winds, but still provides some protection. Gradually move the plants further from the fence or building over the next few days. Check plants often for any signs of damage or stress. Move stressed or wilted plants inside for the rest of the day and set them out again the next morning.
Move to Direct Sunlight
Once plants have acclimated to the outside, begin moving them to direct sunlight. An hour or two of direct morning sun for the first day is adequate. Gradually increase the amount of sunlight until plants tolerate several hours of direct afternoon sun without signs of wilting.
Transplant Seedlings to the Garden
Your tomato seedlings are ready to transplant to the garden once they can tolerate full sun without wilting or showing other signs of stress.
Many gardeners neglect to properly harden off tomato plants or in are in a hurry and do not allow plants enough time to sufficiently acclimate to outside conditions. Although plants may survive in the garden after a day or two of hardening off, the stress associated with drastic changes in the growing environment may set a tomato plant back for a month or more. Taking the time to harden off your tomato plants gets them off to a good start and produces healthy, robust plants.

I’m Too Big for My Britches!

In most cases, your plants will need to be transplanted into larger pots weeks before they can be planted into the garden. A pot-bound plant will stunt and take longer to recover once outside. A series of 4″ pots (which can still fit into the same trays as the cell trays) are usually large enough. Pick up the seedlings to be transplanted by the root ball or the leaves (if they are very small) – not the stem, which can crush. Place the seedling in the bottom of the empty 4″ pot, pinch off any leaves that are below the top rim of the new pot and cover the entire stem with soil-less mix (you can use the seed starting mix if you have some remaining). Water the soil and add more as needed to fill the pot. Those little hairs on the stem will turn into new roots and give you much stronger plants. Follow the same guidelines for hours of light, fertilizer and water with the 4″ pots as with the seed trays until the time is right to plant them in the garden.

I’m Tough! Well Maybe Not So Much . . .

Even with all the care and “training” your plants have received, Mother Nature can be very hard on them. Preparing them to go into the garden is called Hardening-Off and takes about a week. The process is a schedule and each step can be one day or a few days for each step. First day(s) – bring your young tomato plants outside and place them in full shade during the day, but bring them back inside at night. Next step is a few hours in the sun – early morning or late afternoon, move them into the shade during the hottest part of the day and then inside at night. The next step is all day (approx. 8 hours) in the sun and covered with an old bed sheet or a floating row cover such as ReeMay at night. Next step – all day in the sun and stay out over night uncovered. Because of the exposure to direct sun, wind or lower humidity, the plants will need more water than they required during their final week inside.

If there is any frost warning, make sure to bring your plants in early.

Into The Warm Dark Earth.

It’s time to plant into the garden and your plants are ready! When to plant is an educated guess every year. Wait until the soil temperature (not the air) is above 60° and the normal danger of frost is past. The day before you plan to plant into the garden, do not water the plants in their pots. Try to plan planting on an overcast day, the early evening or in the rain. As when you transplanted into 4″ pots, remove the lower leaves and plant deep into the soil. This is a great way to compensate for those plants that might have become leggy waiting to go into the garden. You can “trench plant” rather than digging deep. See diagram below:

Dig the hole, fill it with water and allow the soil around the hole to absorb the water. Add soil amendments or fertilizer as needed and fill the hole again with water, remove your young tomato plant from the pot and plant in the mud covering the stem with soil. Don’t worry if the plant appears to be lying on its side, and don’t try to bend it upright. Within a few days it will face the sun on its own. Most tomato plants can get BIG and they appreciate room to grow. Pruning (discussed in Part 5) can help control the size to some extent, but if you are growing a variety like Brandywine, they will not be contained to 2 feet high and still produce fruit. The rule of thumb is to give each plant 4 square feet. If you plant them too close, you will get less fruit produced on each plant.

Lean On Me.

Tomatoes are vines and they need some support if you want them to grow vertically. Choose what is best for the mature size of your plants; cages, trellis, stakes, tee-pee, ladder, bridge, spiral, fence, basket weave or any combination. Put those supports in now, when you first plant the seedlings out into the garden. You do not want to break off stems trying to fit a cage over the plant 2 weeks from now.

I’m Shy – Cover My Knees Please

Tomatoes do benefit from mulch and it can make the garden look a little neater. But what type is best? Red plastic mulch warms the soil, suppresses weeds and encourages ripening but . . . looks . . . odd in the garden? Organic mulches look nice, maintain the moisture and can provide some nutrients but also can hide some pests like slugs. I love to plant dwarf white clover between my tomatoes as a green manure. It becomes a living mulch of soft green, fixes nitrogen to the soil, attracts bees (which can be an issue for some people) and suppresses weeds. Here is an example of Cabbages mulched with dwarf white clover. In the past my entire garden was permanently mulched with dwarf white cover.

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