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Cold Hardy Plants

When we talk about cold hardy plants, it is important to understand that both 'cold' and 'hardy' are relative terms. And we need to look at both of these two terms in order to determine which plants we might be able to grow successfully in our gardens year-round - especially during the coldest part of the year.

What are Cold Hardy Plants?

When we refer to cold hardy plants, we are talking about plants that can cope with the temperatures in an area, sometimes in summer but more commonly, during the coldest part of the year.

We look at cold hardiness when we are trying to work out whether we can grow plants outdoors where we live, and also when we are looking at whether perennial or biennial plants will make it through a winter where we live unscathed.

Choosing cold hardy plants for your garden means looking at the conditions in winter where you live, and the characteristics of the specific plants you are considering.

Understanding Your Winter Climate

Remember, what seems 'cold' in one area can seem rather mild when compared to what we might think of as 'cold' in another area. Those in milder temperate climates might experience few frosts during the coldest months, while in other areas, it can remain far below zero for months at a time.

So before we can seek out cold hardy plants for our gardens, we need to understand the condition that we are likely to experience in our gardens over the winter months.

Hardiness Zones

One crucial thing to determine in order to understand your winter climate is the hardiness zone where you live. In North America, for example, there are the USDA plant hardiness zones. These are also often applied in Europe, and in other parts of the world. Though there are other regional systems, this is the most common system used.

These zones range from 1-13, and are defined by the average annual minimum winter temperatures.

1a (-60 to -55 °F/-51.1 to -48.3 °C)

1b (-55 to -50 °F/-48.3 to -45.6 °C)

2a (-50 to -45 °F/-45.6 to -42.8 °C)

2b (-45 to -40 °F/-42.8 to -40 °C)

3a (-40 to -35 °F/-40 to -37.2 °C)

3b (-35 to -30 °F/-37.2 to -34.4 °C)

4a (-30 to -25 °F/-34.4 to -31.7 °C)

4b (-25 to -20 °F/-31.7 to -28.9 °C)

5a (-20 to -15 °F/-28.9 to -26.1 °C)

5b (-15 to -10 °F/-26.1 to -23.3 °C)

6a (-10 to -5 °F/-23.3 to -20.6 °C)

6b (-5 to 0 °F/-20.6 to -17.8 °C)

7a (0 to 5 °F/-17.8 to -15 °C)

7b (5 to 10 °F/-15 to -12.2 °C)

8a (10 to 15 °F/-12.2 to -9.4 °C)

8b (15 to 20 °F/-9.4 to -6.7 °C)

9a (20 to 25 °F/-6.7 to -3.9 °C)

9b (25 to 30 °F/-3.9 to -1.1 °C)

10a (30 to 35 °F/-1.1 to 1.7 °C)

10b (35 to 40 °F/1.7 to 4.4 °C)

11a (40 to 45 °F/4.4 to 7.2 °C)

11b (45 to 50 °F/7.2 to 10 °C)

12a (50 to 55 °F/10 to 12.8 °C)

12b (55 to 60 °F/12.8 to 15.6 °C)

13a (60 to 65 °F/15.6 to 18.3 °C)

13b (65 to 70 °F/18.3 to 21.1 °C)

The USDA plant hardiness zone map for the US can be found online here.

Plant hardiness maps for other regions using this broad system to define hardiness zones according to winter temperatures can also be found online.

It is important to understand, however, that knowing your plant hardiness zone is only one piece of information that you need to gather in order to understand which plants you will be able to grow.

Factors that Influence Winter Temperatures in Your Garden

It is important to understand that your climate zone gives only a broad idea of the likely minimum winter temperatures experienced in your area. There are a number of different factors that can determine the actual temperatures experienced on your particular property.

Winter temperatures are also influenced by a range of factors, including:

  • Topography and position within a landscape.
  • How sheltered or exposed a location you are dealing with. (Wind chill factor etc....)
  • The built environment. (For example, temperatures will typically be higher in built-up city sites.)

Factors Other Than Temperature To Consider

When deciding what you can and should grow in your garden over the winter months, temperature is only one factor to think about. Even if plants can cope with the temperatures experienced in your area over the coldest part of the year, they might not be able to come with other things.

For example, plants may be perfectly suited to cold, but might not thrive in the humidity levels in your area. An area with dry cold winters will be suited to different plants than an area with high humidity over the winter months.

High rainfall in winter might also threaten certain plants, even where they can cope with the winter temperatures, especially where waterlogging is likely to arise. Where I live, winter wet is often more troublesome to plants than winter cold.

Where snow falls heavily in winter, this can also potentially pose a threat to some plants. Again, damage can occur to plants – even those that are hardy enough to cope with the winter temperatures where you live. So this is something else to consider.

Understanding Plant Hardiness

When we talk about plant hardiness, we need to think about this not in terms of absolutes, but in terms of degree. Not all 'cold hardy' plants will be able to cope with the same minimum temperatures.

Here in the British Isles, the Royal Horticultural Society has developed a guidance system known as plant hardiness rating. Plants are given a rating designation ranging from H1 to H7 based on the temperatures with which they can cope in Britain. These roughly correspond to USDA plant hardiness zones as follows:

H1a – plants for a heated greenhouse that require temperatures above 15 degrees C. (USDA 13)

H1b – plants typically grown in a greenhouse year round, but which can cope outside in summer in hot, sunny, sheltered locations. Plants with this rating require temperatures between 10 and 15 degrees. (USDA 12).

H1c – plants that can be grown outside in summer while daytime temperatures are high enough to promote their growth. These require temperatures between 5 and 10 degrees C. (USDA 11)

H2 – plants tolerant of low but not freezing temperatures. 1-5 degree C. (USDA 10b)

H3 – plants that are half hardy, hardy in coastal/mild areas in Britain. 1 to minus 5 degrees C. (USDA 9b/10a)

H4 – plants hardy through most of Britain in an average winter. Minus 10 -minus 5 degrees C. (USDA zone 8b/ 9a)

H5 – plants hardy in most of the British Isles even during a severe winter. Minus 15 to minus 10 degrees C. (USDA zone 7b/ 8a)

H6 – plants hardy in all of the British Isles and northern Europe. Minus 20 to minus 15 degrees C. (USDA zone 6b/ 7a)

H7 – extremely cold hardy plants that are hardy even in the most severe European climates including exposed upland locations. Can cope with temperatures below minus 20 degrees C. (USDA zones 6a down to 1)

Plants with a rating of H4 to H7 can all be considered 'hardy' or cold hardy plants. Though of course, the degree of hardiness changes depending on the specific minimum temperatures that plants can cope with.

One important thing to remember is that just because a plant can cope with winter temperatures where you live, that does not necessarily mean that it will cope with other winter conditions. Nor does it mean that it can cope with summer conditions where you live.

So cold hardiness is only one of many factors that you need to take into account when deciding what to grow in your garden.

Winter Growing in Milder Climates

In milder temperate climates, it is often possible to grow edible crops outdoors right through the winter months. As well as having a main sowing season in spring, you may also have a second sowing season in late summer and fall.

Annual Growing: Cool-Season Crops

Cool-season crops grown in summer in colder areas can also be hardy enough to be grown through the winter where temperatures in winter do not fall as low.

These might include a range of alliums (garlic, winter onions), root crops like beets/ beetroot, fava beans/ broad beans, winter peas, and other hardy green vegetables, for example.

The milder the winters are where you live, the more annual crops will be able to make it through the winter months successfully, still in active growth. And the more potential there will be for winter gardening, and year-round cultivation.

Another interesting thing to think about is that plants typically grown as annuals in cooler climes can be perennial in warmer climate gardens. Though, whether these can be grown as perennials will depend on the winter temperatures you experience where you live.

Perennial Growing: Chill Hours

In warmer temperate zones, cold hardiness might not be a key concern in perennial growing areas. Instead, you may need to think carefully about the requirements of plants in terms of how much cold they require.

Many fruit trees and other fruiting plants require a certain number of hours below a certain temperature in order to fruit the following year successfully.

Remember, cold temperatures in winter can be important, and beneficial when growing certain temperate climate plants. So in warmer regions, it may be necessary to look for low-chill hour options when choosing fruit trees etc. for your garden.

Cold Climate Permaculture Garden Design Tips

Those growing in colder climates will typically find it far more important to seek out cold hardy plants. We should think about creating growing areas in our gardens with plenty of cold hardy perennial plants that will come back year after year.

But when creating a permaculture, food-producing garden in a cold climate, it will also often be necessary to take certain steps in order to grow as much food as possible.

In colder climates, even cold hardy plants will typically die back or enter a period of dormancy over the winter months. So at this time of year, outdoors, there may be far less food around.

That means that we need to use design strategies to make sure we can keep plants in active growth over as much of the year as possible – to feed ourselves and our families even when the garden is 'sleeping' outside and to maximize our yields.

Creating a Sheltered, Milder Micro-Climate

When we are contending with lower winter temperatures and more extreme winter conditions, it is especially important not only to think about which plants we choose but also where we choose to grow.

Remember, there are a number of factors that determine the temperatures that we actually experience where we live above and beyond our broad climate.

We can create a more positive microclimate for winter growing (keeping temperatures higher and creating a calmer and more protected environment) by:

  • Considering topography, and using its characteristics to our advantage. (Benefiting from south-facing slopes in the northern hemisphere, for example, and avoiding frost pockets.)
  • Using shelter belts and windbreak hedgerows, and positioning hardier plants to protect and shelter more tender ones.
  • Making use of south-facing walls or other structures in the northern hemisphere, since these provide warmer areas for cultivation.
  • Introducing water into our designs. (A pond or other body of standing water can create a milder microclimate in the surrounding area.)
  • Using mulches to protect plant roots and keep the soil a little warmer in growing areas.

I have a wonderful microclimate in my forest garden, which I have created in a small stone-walled orchard. The stone walls retain heat, and along with trees and shrubs will shelter the area, meaning that I can harvest a few hardier leafy plants from the area even in the depths of winter.

How to Extend your Growing Season

Beyond these things, we can also extend the growing season where we live by introducing man-made structures such as greenhouses or polytunnels into our designs.

Unheated greenhouses or polytunnels can often provide sufficient protection from the cold for year-round growing of annual crops – especially if these structures are designed carefully so that they remain frost free.

I live in an area in Scotland that experiences winter temperatures that can drop to around minus 5 and rarely, in extreme winters, to minus 10 degrees C.

I am able to keep a range of crops in active growth in my unheated polytunnel year-round, (sometimes will a little extra protection) though only the hardiest of crops like brassicas (like kale) can successfully be grown outside here all winter. (More due to wet and sometimes stormy conditions rather than temperature lows).

Though I also preserve plenty from the rest of the year to use through winter, it is also nice to have a variety of fresh greens to harvest over the coldest part of the year.

In particularly cold regions, even things like dwarf fruit trees can potentially be grown under cover, even when they will not survive outdoors.

Where space for walk-in structures is not available, we can still utilize mini polytunnels, row covers, cloches or other forms of protection to create an even more sheltered and milder microclimate for our crops.

By creating the right undercover growing areas, we can grow a range of crops year-round, just as growers do outside in warmer areas.

How much warmer it is inside a greenhouse or a polytunnel than it is outdoors will depend on the specific location and design.

Adding thermal mass will help to keep your undercover growing area a little warmer over the winter months without the need for heating.

Materials like earth, stone, brick, clay, and water have good thermal mass. They absorb the heat energy from the sun during the day and then release it slowly when temperatures fall.

That is why, in colder climate areas, it can be beneficial to consider an earth-sheltered greenhouse or walipini. Or to incorporate these types of materials in the structure, or inside in things like paths or bed edging.

Heating Undercover Growing Areas in Winter

If you cannot keep off the cold sufficiently for the crops you wish to grow where you live in an unheated structure, there are also sustainable ways to heat an undercover growing area through the coldest part of the year.

For example, you might heat using pipes passing through a composting heap, or a solar hot water heating system. You might also create a hotbed to gently provide heat from decomposing materials from below.

Of course, there is also potential to use a wood-fired system using your own or locally grown wood, or a renewable electricity system to heat a growing area in winter.

But remember, seeking out cold hardy plants suited to cultivation in your area should always be the first step. And it is only once these have been considered that you might also consider taking further steps to increase the length of your growing season and grow your own food year-round.

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