Find out how to move your indoor-sown young plants to the garden outdoors—without stalling their growth!

How to Successfully Harden Off Indoor-Sown Plants

‘Hardening off’ is the process of gradually acclimatizing indoor-sown plants to outdoor conditions.

For most plants, begin hardening off a week before the final frost date for your area. Our Garden Planner uses data from your closest weather station to recommend when it’s safe to plant out, providing a helpful guide to work back from.

Choose a sheltered spot to harden off your plants. An unheated greenhouse or cold frame is a great tool for this, or you can cluster pots into buckets, crates or boxes to keep the wind off. Don’t place pots directly on the ground where they can easily be knocked over by birds or attacked by slugs.

Begin hardening off on a still, cloudy day when temperatures are fairly steady. Water plants before they go outside. Place them into your sheltered spot for just two hours on the first day. The next day, leave them out for two more hours, with perhaps an hour’s direct sunshine in the morning. Gradually increase the length of outdoor time and direct sunshine over one to two weeks. You can then leave them out overnight if there’s no danger of frost.

In cold winter regions, plants – particularly tender plants such as tomatoes and peppers – will need to be prepared for the cooler nights early in the growing season. Towards the end of the hardening off period, cover your crops with fleece or row covers to protect them overnight. Once crops have been planted into their final positions, be alert for unexpected cold snaps and cover tender crops if necessary.

It’s a good idea to grow a few more plants than you need so you can hold some back just in case. Bought-in plants may also need hardening off, particularly if they have been kept in sheltered conditions.

Read our full page about transplanting seedlings.

Also: Be sure to check out the Almanac Garden Planner! We’re offering a free 7-day trial to create your best garden.

Secret of Seeds Series Episode 7 – How to Harden Off Seedlings in 7 Days or Less

Hardening off your seedlings can be simple if you know the schedule you should begin hardening off your plants. What does “hardening off your seedlings” mean? The hardening off process accustoms your seedlings to the outdoor environment. Just planting your seedlings outdoors after growing them inside without accustoming them to the sunlight would be the same as you going out to sunbathe on the first hot spring day without sunscreen. If we take a little time and make sure to harden off our seedlings so they are ready for the outdoor environment we will have a lot better luck having them live and thrive. It will take 4-7 days to ready your plants to live outdoors permanently. Watch our High Performance Garden Training Video to follow the idea hardening schedule and learn the shortcuts to save time hardening off your seedlings!

The Ideal Schedule to Harden Off Seedlings

Day 1: Set the plants out in an area that is free of wind and is above 60 degrees Fahrenheit. You want your seedlings to only get two hours of sun. The plants can stay outdoors as long as it is warm, wind free and has little sunlight. Be sure to check the water frequently! The seedlings have a tendency to dry out fast when they are set outside.

Day 2: Place the plants outdoors for three hours in the sun. A slight breeze will also be good for your plants. Make sure that the temperature is warm and keep watch of any dramatic weather changes.

Day 3: Set the plants in the sun for four hours in a breeze and during the warmth of the day.

Day 4: Set your seedlings outdoors earlier in the day so they experience some cooler temperatures. Let them get five hours of direct sunlight and keep out of heavy wind conditions.

Day 5: Leave the plants outdoors all day in the breeze and in the sun. Be sure to check their water as they will need to be watered more than once on a warm day. Placing your plants in a shallow dish of water allows for them to wick up the moisture that they want and saves you time!

Day 6: Let your plants stay outdoors all day and all night if the temperatures will be above freezing. The hardening off process is almost complete!

Day 7: Plant your seedlings into the garden. If a frost is predicted during the next seven days of your plants life make sure to cover them.

Are their shortcuts so you can do this faster? We have 3 shortcuts that are riskier but save so much on time and energy.

Shortcuts For Hardening Off Your Seedlings

Shortcut 1) Buy or build a mini greenhouse to put over your plant and plant them out the first day. Watch the training video above to find out how you can create a mini-greenhouse with woven wire, plastic and burlap. This is a great frame to use when you need to cover your plants when it is cold. If the temperatures are low leave the plastic on. If it is warm outdoors taking the plastic off and putting a burlap over the mini-greenhouse frame for a week is a great way to accustom your plants to the outdoors. I slowly adjust the burlap until by day seven it is completely off of the frame and the plants receive full sunlight. You can build a mini greenhouse with no tools that can also serve as a trellis with your plants. I use this system in my gardens to help with vertical growing and plant protection. Watch one of our earlier training videos How to Build Your Abundantly Easy Trellis so you can build this one of a kind system in your garden!

Shortcut 2) A wall of water or an Aqua Dome act as a mini greenhouse for your plants. They cut down on UV rays and keep them warm during cold nights . The Wall-of-Waters are about $9-$15 for a three pack. The only issue with these tools is that when you try to reuse them after your first seasons they tend to leak. They also put your plants at risk. If a heavy wind picks up these plastic tubes of water can easily be knocked over and squash your plants. When you take the wall of water off your plants will go through a shock although they are tougher than plants that were brought directly from being inside. An Aqua Dome is a rigid structure that should last many seasons and works the same way as a Wall-Of-Water. I love mine and recommend this gardening tool heartily. The best place to purchase them is at Sustainable Living Today. When I was shopping around recently one Aqua Dome had almost doubled in price since the last time I bought one.

Shortcut 3) Plant the plants outdoors if you know the weather will be cloudy for a week. If your area gets cloudy, cool, windless days for a week you can plant your plants immediately. This is an especially risky shortcut because of how quickly the weather can change. Because of the high elevation I cannot use this shortcut because the sunlight is so intense that it burns plants and people even on cloudy days. If you live in a lower elevation and are confident in your weather patterns then I would say give this a try… And then tell me all about how it goes! I would love to see pictures.

Which methods will you choose? Do you have time to follow the schedule or will you use one of the shortcuts? I would love to see pictures and hear all about your process!

Share this article with the struggling and the wanna-be gardeners in your life. The High Performance Garden Community is the place for them to find the support and training they need to begin the journey of transforming their gardens. I look forward to seeing you next week where we will discover how to transplant your seedlings that you have hardened off.

Until next time may your garden be easy, fun, productive and always organic!

Lynn Gillespie

Email me any questions you have about hardening off your seedlings or any type of gardening question. I would love to help you achieve the high performance garden of your dreams!

Back to the HPG Community Posts

Hardening Off Plants and Seedlings

Planting the seedlings you’ve raised carefully indoors is a proud moment. But be sure to acclimatize them to their new outdoor home first, or you’ll risk losing your plants and wasting all that hard work. This is a process known to gardeners as hardening off plants.

Start hardening off your seedlings about a week before the final frost date for your area. Our Garden Planner uses data from your nearest weather station to give an indication of when it’s safe to plant outside.

Choose a sheltered spot to harden off plants, and start hardening off on a still, cloudy day when temperatures are fairly steady. Water the seedlings before they go outside so there’s less risk of them drying out. Avoid placing plants on the ground where they can easily be knocked over by birds or nibbled on by slugs.

An unheated greenhouse or cold frame is a great tool for hardening off transplants. Place seedlings and plants into the structure for a couple of hours on the first day, then gradually increase the length of time they’re in place by two or more hours per day. After a week they can then be left there overnight, as long as there’s no danger of frost.

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You can use shade cloth or row covers to protect seedlings from strong sunlight – just drape it over the top and tuck it in at the sides so it won’t blow off.

In regions with cold winters, plants will need to be prepared for the cooler nights experienced earlier in the growing season. This is especially important for tender plants such as peppers, which are easily damaged by low temperatures. Near the end of the hardening off period, use row covers to protect foliage from cool temperatures. Once crops have been planted into their final positions in the garden, be ready with crop protection if late cold snaps are forecast.

Grow more plants than you need so you can hold a few back, just in case. Purchased plants may need hardening off, too. Hardening off takes time but will give you stronger, more resilient plants that will ultimately be more productive.

Learn more about hardening off seedlings and plants in this video.

More Gardening Resources

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Our popular Vegetable Garden Planner can help you map out your garden design, space crops, know when to plant which crops in your exact location, and much more.

Need crop-specific growing information? Browse our Crops at a Glance Guide for advice on planting and caring for dozens of garden crops.

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Watch more videos on gardening techniques and other self-reliance, DIY topics on our Wiser Living Videos page.

Get a Jump on the Planting Season: Build Your Own Cold Frame

The trees are bare and the ground is cold, hard, and forbidding in New England. If you’re an amateur gardener who wants to grow your own food, it can be a little intimidating. There are plenty of hardy plant breeds that will grow in winter, or you can do something a little more fun and science-y. For centuries, farmers and gardeners have used a simple device called a cold frame to extend their growing season by using the natural heat and light of the sun to keep plants healthy and warm with relatively little effort or expense.

The following is an excerpt from Four-Season Harvest by Eliot Coleman. It has been adapted for the web.

Gardeners should dedicate a monument to the cold frame. It is the simplest, most flexible, and most successful low-tech tool for modifying the garden climate. It’s simple because it is basically a box with a glass top and no bottom that sits on the soil. It’s flexible because it can be made as long, as wide, or as tall as the gardener wishes. And it’s successful because it is a tried-and-true garden aid that has been used in one form or another since ancient times (sheets of mica predated glass). The cold frame was the foundation for the early development of intensive commercial horticulture.

How the Cold Frame Works

The cold frame lessens climatic stress in a number of ways:

Temperature. A single layer of glass creates a microclimate in which the nighttime temperature inside the frame can be as much as 20° warmer than the temperature outside, although the average difference is 7° to 10°F The daytime temperature inside the frame, even on a cloudy, early spring day, will be 10° to 15°F warmer than outdoors. On a sunny spring day, the temperature can rise high enough to cook the soil and the plants if you don’t vent off the extra heat. Both daytime and nighttime temperature differences depend on the time of year, the angle and intensity of the sun, the rate of outdoor temperature change, and the initial temperature in the frame.

Moisture. Much of the havoc that freezing can wreak on winter vegetables is a function of how wet the plants are. High humidity helps protect plants from cold but plants sitting in a puddled soil just soaked by a rain before freezing will be more stressed than one that is drier. The glass roof of the cold frame protects the crops inside from pounding winter rains.

Wind. The wind can make a cool day feel very cold. Weather forecasters always mention the windchill factor. The same conditions affect plants. Wind cools by removing ambient heat and evaporating moisture. The stress of winter wind alone can mean the difference between life and death for hardy vegetables. Even the slightest windbreak will help. That was proven by two beds of spinach planted a few Septembers ago to winter over outdoors. One was covered lightly with a mulch of pine boughs and the other left uncovered. Even though you could look through the thin layer of pine boughs and clearly see the spinach, that minimal amount of wind protection was significant. Ninety percent of the protected spinach survived the winter, compared to ten percent of the unprotected crop.

Building the Cold Frame

As previously mentioned, a cold frame is a bottomless box that sits on the soil and has a glass cover. Thus, there are two parts-the sides (the box) and the top (the glass). The sides can be made of almost any material-boards, concrete blocks, bales of hay, logs, and so on, all of which have their virtues. From our experience, we suggest making the sides out of boards. This will give you a frame that is long-lasting, easy to construct, easy to use, reasonably light, and movable.

The top covering is called a light. In the old days, lights were 4 to 6 feet square and made of overlapping panes of glass. They were heavy and required two people to carry them. Today’s home gardeners often use old storm windows as lights. Storm windows are easy to find and the size is right for covering cold frames. Modern lights can be glazed with translucent materials other than glass, such as plastic, polycarbonate, or fiberglass. Depending on its size, a cold frame is covered with one or more lights.

A cold frame can be any width that the lights will cover and any length or height. Traditional home garden cold frames measure 4 to 6 feet front to back and are 8 to 12 feet long. They are laid out with the long dimension running east to west. The frame should be just tall enough to clear the crops you plan to grow. In the standard design, the back wall is 12 inches high and the front wall 8 inches high, so that there is a slight slope to the south.

Some experimenters have built frames with the lights at a 45° angle facing south to maximize midwinter sun input. Such frames don’t work as well as the traditional low-angle models for two reasons. First, you don’t need maximum heat in midwinter for hardy crops. All they require is the protection of the frame. Second, there seems to be some benefit to having the glass roof near the plants as if it were a covering of snow. The environment inside the traditional low-angle frames better meets the needs of hardy crops.

Recommended Reads

Get a Jump on the Planting Season: Build Your Own Cold Frame, Part 2

Get a Jump on the Planting Season: Build Your Own Cold Frame, Part 3

The Differences Between a Greenhouse and a Cold Frame

Posted December 30th, 2007 by Garden & Greenhouse in March 2007

Recently a friend sent me a cartoon which perfectly described what having a greenhouse is about. The picture was of two small flowers growing in the ground outside peering wistfully into a greenhouse. The greenhouse is filled with beautiful blooming plants bathed in perfect light and watered by a bikini clad woman. One of the small flowers outside said to the other, “Oh man, that’s the life”.

One of the most common misconceptions of people looking to purchase or start using a greenhouse is that if it’s called a greenhouse, it will grow plants. Often a structure is called a greenhouse if it can hold plants with no regard for the environment it will create. You could easily purchase a “greenhouse in a bag” for a few hundred dollars, but you would not be happy for very long zipped up in a plastic bag. Chances are your plants won’t like it either. Several greenhouse makers recognize the need for some basic industry standards to help consumers make more informed choices. Until those standards are developed, however, gardeners must spend a little time to educate themselves or run the risk of making a costly mistake.

There is an important place for entry level products as long as you know what to expect before making an investment of money or time. Most products called greenhouses on the market today are actually large cold frames. The good news is that there are many good options. Some of the least expensive products on the market can provide good results if the uses and limitations are understood. Even a “greenhouse” in a bag can give plants a head start if used properly.

Understanding the difference between a greenhouse and a cold frame is the key to achieving the best results. Shane Smith, author of the definitive resource on greenhouse gardening; The Greenhouse Gardener’s Companion, defines a greenhouse as a structure that is heated. A properly designed greenhouse will allow the gardener to control the temperature and environment within a specific range.

A greenhouse is either operated as a hot house or a cool house, depending on the zone it’s in and how much heat is added. A hot house maintains a minimum nighttime temperature of 55 degrees or greater. A cool house would be one that will maintain a minimum nighttime temperature of 45 degrees. In most cases, a maximum greenhouse temperature would be around 90 degrees.

In contrast to a greenhouse, a cold frame is any structure that is generally used to extend either end of the growing season by a few weeks. Cold frames are not designed for heat, so the temperature cannot be closely controlled. No external heat is added, so they depend solely on the heat generated from the sun’s radiation. Many cold frames are covered with a single walled glazing. Without insulation, only a minimal amount of heat from the day will remain overnight. They are not a good choice for winter growing or wintering over in cooler climates. Cold frames are most suitable for the few weeks before and after the last frost. With a cold frame, you can get a jump on spring by a few weeks before the soil was fully warmed and before the threat of frost has passed. Warming the soil temperature by even a few degrees can drastically increase the success of early plantings. In fall a cold frame can guard against an early frost giving some winter crops an opportunity to develop strong healthy roots before facing a long cold winter. In an area with a short growing season a cold frame may be all that is needed to give tomatoes that extra few days of precious sun to ripen.

Many people think that if you can walk in a structure it’s a greenhouse. They are often surprised to learn that cold frames come in all shapes and sizes. A cold frame can be small structures on the ground with a hinged lid or it can be a long tunnel structure used by commercial growers. Greenhouses are easily used as cold frames if they are not heated, however, just adding heat to a structure will not turn a cold frame into a greenhouse.

There are some types of structures that cannot be safely or effectively heated. Factors to carefully consider include: the structure’s size, the structure’s ventilation capabilities, the type of covering used, and the frame. The size of the structure is the first consideration. Small structures are more difficult to heat than larger structures because they overheat easily. Any structure with low overhead clearance or less than 64 square feet of floor space will heat very quickly with the addition of external heat. The overheating may cause damage to the plants if the air temperature rises above 90 degrees. Ventilation is the second consideration. Structures that do not have ventilation will not be able to expel excess heat. If the heat source is a gas heater, the air will be moisture laden causing more potential issues from the excess humidity. Controlling a greenhouse environment requires the exchange of fresh air and the addition of heat.

Another important factor to consider when deciding to add heat is the type of covering used. Single wall covering such as glass, fiberglass, film and single wall polycarbonate do not provide any insulation. Heating a single wall structure is not much different than heating the outside air. Most of these coverings are not compatible with ventilation so they temperature can fluctuate between hot and cold rapidly. The framing is the final consideration. Some plastic or resin framing should not be near external heat sources since some materials have a low melting point.

If you are interested in growing tropicals, growing outside your zone, starting seeds early, wintering over sensitive plants in cooler climate, or actively growing in the winter you may need a greenhouse to accomplish your goals. Some seeds will need minimum soil temperatures of 50 degrees in order to germinate and establish themselves, but many seeds will germinate at 65 degrees or higher. Many of our summer vegetables, tomatoes for example, are tropical plants which thrive in the warmer temperatures. Warmer soil and air temperature alone is not enough for some plants to flourish. Some plants are day-light sensitive which means that even if they have the heat they require, they will not flourish in months where there is little daylight without supplemental light. Many greenhouses are designed to support grow lights and can be used to grow even sensitive crops all year. A greenhouse can be used to grow in most places all year long, provided you are willing to add the heat, ventilation, and in some cases, the light necessary.

Once you decide what you want to grow and when you want to grow it, you can determine if you need to use a greenhouse or a cold frame. Since most products are called greenhouses, make sure to look at the covering, the ventilation, and for a sturdy frame. If you are unsure if a product is a greenhouse or cold frame, ask how and where you add a ventilation fan. If a fan cannot be added, then heating the structure will be difficult. Price may be another clue. There are few greenhouses available for less than $1,000, although there are also quite a few high quality cold frames that sell for thousands of dollars. Regardless of whether you use a greenhouse or a cold frame, the plants inside will be the envy of the garden.

Read More Articles

Cold Frames and Cloches

Greenhouse Maintenance – Getting 100,000+ Miles out of Your Greenhouse!

Maintenance on Indoor Gardening and Greenhouse Equipment

The Benefits of Using a Cold Frame

Michelle Moore is the general manager for The Greenhouse Catalog. She is an Oregon State University Master Gardener and has nearly 20 years experience working with greenhouses. She lives in Oregon with her husband where they are gardening outside of a greenhouse for the first time.

Garden centre – Potting Shed Restaurant Woodbank Nursery Harden

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