Overwintering Plants: What Is Overwintering

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It can be quite costly to buy all new plants each spring. There’s also no guarantee that your local garden center will carry your favorite plant next year. Some plants that we grow as annuals in northern regions are perennial in southern areas. By overwintering these plants, we can keep them growing year after year and save a little money.

What is Overwintering?

Overwintering plants simply means protecting plants from the cold in a sheltered place, like your home, basement, garage, etc.

Some plants can be taken in your house where they continue to grow as houseplants. Some plants need to go through a dormancy period and will need to be overwintered in a cool, dark space such as a garage or basement. Others may require storing of their bulbs inside through the winter.

Knowing the plant’s needs is the key to keeping plants over winter successfully.

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How to Overwinter a Plant

Many plants can simply be taken into the house and grown as houseplants when temperatures outside become too cold for them. These include:

  • Rosemary
  • Tarragon
  • Geranium
  • Sweet potato vine
  • Boston fern
  • Coleus
  • Caladiums
  • Hibiscus
  • Begonias
  • Impatiens

Lack of sunlight and/or humidity inside a home can sometimes be a problem, though. Keep plants away from heat ducts that can be too drying for them. You may have to set up artificial light for some plants to simulate sunlight. Additionally, you may have to take steps to provide humidity for the plants.

Plants with bulbs, tubers or corms that need a dormancy period can be overwintered just as dried roots. Examples include:

  • Cannas
  • Dahlias
  • Certain lilies
  • Elephant ears
  • Four o’clocks

Cut back the foliage; dig up the bulb, corm or tubers; remove all dirt from them and allow to dry out. Store these in a cool, dry and dark area throughout the winter, then replant them outside in spring.

Tender perennials can be overwintered in a cool, dark basement or garage where temperatures stay above 40 degrees F. (4 C.) but are not too warm to cause the plant to come out of dormancy. Some tender perennials can be left outdoors through the winter with just an extra heap of thick mulch covering them.

Like everything in gardening, overwintering plants can be a lesson of trial by error. You may have great success with some plants and others may die, but it’s an opportunity to learn as you go.

Be sure when bringing any plants indoors for winter that you treat them for pests beforehand. Growing plants you plan to overwinter indoors in containers all year long can make the transition easier for you and the plant.

Friends that read this blog are always anxious to offer criticism, point out mistakes, and otherwise find things I’ve said that just might not be true in every case. Rather than get all defensive, we’ve learned to engage our fact-checkers, address the questions and, more often than not, learn something in the process.

So when a couple of longtime gardeners began taking exception to some of the things I said in “Winter Protection for Potted Plants,” I paid attention. Much of what they commented on wasn’t about out-and-out mistakes. Most of the corrections and considerations my friends made were of the “not true in every case,” and “you failed to make the distinction,” and, best (or worst, depending on your point of view) “you promised the moon” sort. So let’s start with the moon.

At Planet Natural, we’ve carefully selected only the best indoor gardening supplies — from lighting and hydroponics to growing mediums — to make your indoor growing experiences blossom.

Overwintering plants is an imperfect art. Success is relevant to the conditions and, let’s face it, most seasons present a day or three of special conditions. In general, conditions are changing. It’s not unusual to lose plants that you bring indoors; in fact, it should be expected. Success, measured when those plants are taken back outside and then thrive, is an achievement and should be celebrated. Failure is the way you learn to be successful.

A couple friends lucky enough to have greenhouses said that I over-emphasized their abilities. Greenhouses, especially free-standing greenhouses, are prone to wide temperature swings. A sunny day can raise temps well above outdoor temperatures. Insulating the pots with plants you keep in your greenhouse will moderate the temperature swings in the soil but not in the air. Bottom line: make sure you monitor greenhouse temperatures. This may mean venting on a sunny day in February. It also means choosing to overwinter only plants that can take some temperature extremes.

To help keep night time temperature up, you’ll want to conserve the solar gain you’ve made during the day. One acquaintance of mine uses stones inside his greenhouse, a row lining the sides and a stack he works around in the center. These stones — each requires two hands to lift — warm during the daylight hours and release their heat slowly back into the greenhouse during the evening. Any dense material — concrete, water barrels (with water), even soil in the pots — serves the same purpose. My friend claims that his stones also slow the quick warming of his greenhouse come daylight but I think that’s probably not true enough to be effective.

A more useful criticism came from a friend who pointed out that the conditions in garages and basements aren’t consistent. A basement in Montana may be much cooler than one in the warmer climes of New Mexico. But that basement in Montana may have good southern exposure to sunlight near its small windows that certain plants will love. So consider the micro-climate of the place you choose to overwinter plants. Your garage may have windows and abundant (but short-lived) sunlight during the day. Or it may have no sunlight at all. Sunlight and temperature are the two most important things to consider.

If you’re bringing a potted plant indoors for the first time and aren’t sure what conditions they’ll survive, give them the best. For me, this has meant bringing them into the kitchen or dining room close to a south, east or west facing window. Water moderately — let the soil dry out completely before you do — and give the plants a quarter turn each week to evenly distribute light. Trimming plants back when first brought in can also be beneficial. With less mass to support the plant stands a better chance. Don’t feed them while indoors — orchids are the exception.

Know your plants. Some plants, like calla lilies and dahlias do best in a cool dark place. Some, like agave plants, like cool places but plenty of light. Some, like geraniums, need moist conditions. And some, like begonias and decorative sages, need warmth and light. Because winter days are short and don’t always offer enough sunlight to keep plants healthy, we (and my critical friends) can’t recommend grow lights enough for the plants that require brightness. These can be simple, T5 fluorescent fixtures, or bright spots specifically designed for sun-loving plants. Either way, lights can add warmth and interest to your indoor setting, as they highlight the plants they’re sustaining.

Here’s specific advice on overwinter exotics and other landscape plants. What plants do you overwinter indoors, and how do you do it? We want to know.

Simply put, overwintering is the process of plants reacting to “winter” conditions such as freezing temperatures, ice, and snow. Some plants will need no intervention to survive. Others will require special attention or care to prevent them from subsiding to winter conditions. I discuss the process and options below.

Why Overwinter?

Growers, gardeners, and farmers have little time to spare. Aside from the usual fall preparations going on inside your greenhouse, there are other considerations for plants that might be exposed to the elements. While overwintering might sound like it requires extra work, it can save you both time and money in the long run. There will be a bit of time front-loaded into these preparations, but they are worth the effort.

Here are just a few advantages of overwintering to consider (depending on the tactic you choose):

    • Earlier spring harvests
    • No need to replant
    • Avoid repurchasing same plants each year
    • Chill sweetening root crops (more on this later)

Your Decision Depends on Hardiness

Each plant will have different requirements for overwintering techniques. If you’re not sure what your plants can withstand, consult the USDA Plant Hardiness Zone Map.

Just type in your zip code and the map will pop out your hardiness zone. If you have plants outdoors that won’t survive the winter in your specific zone, you need to overwinter those plants or you risk losing them to the cold. When purchasing plants, you can often find this information on the display card or someone working at the garden center or nursery can help you figure this out.

For instance, we are in zone 5 and 6 here in southern New Hampshire. This means that the annual minimum temperature is between -15 to -5 degrees Fahrenheit. We can then identify the plants that will be able to withstand these temperatures with or without intervention and then plan accordingly.

Plants that cannot survive will have to be brought inside, placed under a cold frame, or… well let’s take a look at the several options.

Overwintering Options

One tip is to plant cool-weather crops such as kale, beets, broccoli, and spinach in the early to mid-fall. They will establish root systems in October and November, fall dormant during the cold dark months, and then emerge early in the spring. So while your neighbors are struggling to dig up ground and get in seeds, you’ll have sprouting plants.

Other plants are less resilient. Less cold hardy perennials will need to be brought indoors. Planting in pots expedites this process, as the plant can be moved much easier. If you have room on a windowsill or in a room with moderate amounts of sunlight, you can bring herbs and succulents such as chives, mint, rosemary, parsley, and oregano indoors. The humidity levels should be monitored and a fan will keep the stems stronger. The plants will grow slower in these conditions but they will most often survive with adequate attention.

If you don’t have the space for this option, then here is an alternative. Once the plant goes dormant, remove them from the garden and store them in a dark, cool place such as the basement. The plant will not grow in these conditions, but it will survive. Light and warmer weather will jumpstart growth. But beware, bringing these plants indoors might shock the plant and potentially kill the root system. For such plants, take a root cutting and keep them in the house in soil over the winter is another safeguard.

Most perennials, as the name suggests, will survive through the winter with minor bed preparations. These plants will persist each year but there are a few steps to help them thrive year after year.

Some, like Hardy hibiscus, will remain dormant throughout the winter. They naturally enter dormancy in hardiness zones 4 through 9. You will see the leaves fall off and the plants will appear dead after the first frost. But not to worry. Come spring they will return.

With this plant, and others that can survive the elements, we suggest trimming dead stems back to the ground. This prevents new growth during brief periods of warmth during the winter. You can also place a layer of mulch over the roots, 8 inches deep, to insulate the plant against the cold. This also prevents early sprouts that will likely not survive and delay the plants ability to flourish in the spring.

More On Chill-Sweetening

Root crops such as beets, carrots, and turnips have defense mechanisms to ensure survival during the freezing months. The best part of this phenomenon is that it actually makes them taste better come harvest time. The survival tactic works by converting starches into sugars. The stockpile of sugar keeps the plants from freezing, and, in turn, they are more crispy and snappy as a result.

And, of course, one suggestion to avoid this entire process is to place the plants in a greenhouse. Specifically, a heated greenhouse in colder regions. We have a plethora of options to consider for growers of all needs and requirements.

Did we miss any useful overwintering tips? We’re all ears! Let us know below and let others in on useful tactics for preserving those beloved plant until next season.

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