This might seem like a very easy question to answer – just check the hardiness zone listed for it, put your zip code into the USDA map site, and there is the answer – right? Most sources list zone 5 as the coldest place to grow Thuja Green Giant, which means it is considered hardy to minus 20 degrees Fahrenheit (approximately minus 30 on the Centigrade scale, for all you scientifically-inclined gardeners – odd isn’t it, that we use metric money, but then use an ancient European temperature scale everyone else abandoned many, many years ago?). So that should settle the matter, and there is nothing else to talk about. . .
What About Wind Chill?
At this point someone is going to say that we have forgotten wind chill, that factor that separates two days at the same temperature into ‘tolerable’ and ‘brutal’ because of a howling northerly wind. Wind Chill certainly affects us, but not plants. Why? Because wind chill is a measure of how rapidly heat is drawn away from our bodies, something which happens because we are warm-blooded. Plants, on the other hand, are not, and their internal temperatures are basically the same as the surrounding atmosphere or soil. Apart from a few plants that grow so fast in spring – mostly bulbs – that they actually generate enough heat to melt their way through the snow, all plants are at the same temperature as the air, so Wind Chill is irrelevant to them.
Winter Burn
Wind does affect plants though, and it adds complications to just how hardy a plant can be. The usual advice given to gardeners about growing plants at the limits of their hardiness is to find a ‘sheltered spot’ in the garden for them. This usually means south-facing and protected by hardier plants from cold northern winds. This advice is of limited value, because what evergreens need protection from in winter is dry winds – whatever their temperature and direction. You may have seen the leaves plants in summer shrivel and burn in hot, drying winds, but although direct sun may play a part, it is the dryness of wind that matters. We may feel a hot summer wind as dry or damp, but in winter all winds are relatively dry.
This is because the amount of water the air can hold increases with temperature. Yes, many people don’t realize that a cold winter wind is much, much drier than a balmy summer breeze. For example, at 100 degrees Fahrenheit the air can carry over 50 grams of water in a cubic meter. At 50 degrees that falls to less than 10, and at 32 degrees less than 5. By the time we reach minus 20, the hardiness limit for Thuja Green Giant, it holds just 1 gram of water. Larger amounts turn into dew, mist, fog or rain. Winter air is very, very dry.
How does this effect plants? Water is constantly being lost from the leaves and foliage of plants, by a process called transpiration. That water must be replaced from the roots, and when the ground freezes that cannot happen. So over time the foliage loses more water than it can replace, leading to drying, and the effect we call ‘winter burn’. In evergreens, this is the primary factor controlling hardiness. The other is the ability of roots to resist cold. Since soil is much warmer than the air, roots have much less cold-resistance than stems and branches. This is why plants left in pots outdoors die in winter, but the same plant in the ground will survive perfectly well.
What Does this Mean for Thuja Green Giant?
What all this means in the end is that if you can either increase the ability of the roots to take up more water, and/or reduce the amount of water lost from the leaves, then Thuja Green Giant can resist lower temperatures than usually suggested. Also, if you neglect these factors, it can easily die, even in zone 5. This is true for all plants, but especially for evergreens, which are vulnerable, because they carry their leaves into winter. From a practical point of view, this weakness is going to be worse in younger plants, so if you can keep your plants growing until they reach a good size, they are much more likely to survive in the long-term. So how about some tips for doing that?
Protecting Thuja Green Giant from Winter Cold
There are three ways to protect Thuja Green Giant, and other evergreens, from cold winter temperatures, especially when they are young.
Encourage Deep Root Growth
Deeper soil is warmer and doesn’t freeze. (How deep your soil freezes depends on where you live, of course.) The deeper the roots of your trees go, the more water they can continue to reach in winter, and the less their risk of winter burn. When you prepare the soil for planting, try to dig it as deeply as possible, so that the roots have plenty of soil to grow down into.
Water Your Trees in Late Fall
This is a very effective way to protect against winter damage. Let a hose run gently for several hours over the root zone of your trees, so that the soil is thoroughly wet. Do this as close to freeze-up as you can. Increasing the water content of the soil around the roots does two things. Obviously, it puts more water near the roots, so they can take up plenty. More subtly, it increases the specific heat of the soil, so it takes more winter cold to make it freeze. Specific heat is the amount of energy needed to change the temperature of something. It takes five times as much heat to warm wet soil as against dry soil. so that when the cold penetrates, we can think of it as being ‘used up’ in the top few inches, instead of freezing down a foot or two. The deeper roots can still get at liquid water and keep the foliage moist.
Mulch your trees
Besides its other properties, mulch is a good insulator. Soil covered with mulch, especially plant-based mulches, not stones, will freeze much less, and often not at all. Once you have completed that late-season watering, mulch around your trees too.
Use Anti-desiccant Spray
Widely used by professionals, but neglected by home gardeners, these products, which are entirely natural and made from an extract of pine trees, cover the foliage with a protective coating that reduces dramatically the rate of water loss. They have an equally dramatic effect on survival, especially helpful with new planting, and reduce or often eliminate winter burn, even on vulnerable plants at the limits of their hardiness.
So How Hardy is Thuja Green Giant?
You can see the picture is complex. The bottom line is that if you help your young trees in the ways suggested here, their survival is going to be greatly improved. In zone 5, and even into zone 4, your trees will leave winter as healthy as they entered it, and you can push the hardiness significantly – this is indeed a tough plant.