Evergreens should be just that – ever green. That means a stable, solid green color all year round. Yet for many evergreens that is simply not true, and many of these plants turn unattractive bronze or brownish colors for months of the winter, only returning to a healthy green color once spring is well underway. This is hardly what we want in our gardens. Most of us plant evergreens expecting to create a dense, permanent green background for our garden, not have a nasty brown one just when other plants are sleeping, leaving the evergreens as the most prominent plants in the garden.
Why Do Evergreens Turn Brown?
There are several reasons why we see these brown colors on many of our evergreens in winter. In its extreme forms this condition is not temporary, but permanent, and results in the death of the brown branches. The cause of this condition is the inability of the plant to supply enough water to the foliage. Since these plants keep green leaves in winter, those leaves continue to lose water, when other plants are protected from this because they dropped their leaves for the winter. Plants have a coating on the leaves, called the cuticle, which varies in thickness from one species of plant to another, depending on how dry an environment they normally live in. In plants that grow naturally in dry places we see that they have a thick cuticle, which allows very little water to evaporate from the leaves. Plants from damper places usually have thinner cuticles. We also see thicker cuticles in evergreens from cold areas, for the following reason.
In winter the air is often very dry, and the winds can be strong. These cold, dry winds speed up the rate of evaporation of water from green leaves. That water must be replaced by water drawn from the soil by the plant’s roots. But here is where the problem starts. If the soil around the roots is frozen, then the water in it will be ice, not liquid. So the roots cannot draw that water up. Rather like sucking on an ice cube, it is hard to get much water from it. We have the advantage that the ice soon melts in our warm mouths, which of course plant roots can’t do, so they cannot get at the water at all.
As a result, a deficit develops in the plant, with it unable to draw enough water to keep the leaves moist. They dry out, turning brown, and then in spring, with the warmer weather, die completely. This is especially a problem with young, newly-planted trees, because the roots only have access to a limited volume of soil, while established plants have roots going deeper down, where there is soil that has not frozen hard.
What Is ‘Winter Burn’ on Evergreens?
When our evergreens turn brownish in winter from this lack of water, gardeners call it ‘winter burn’. When severe it leads to the death of the foliage, or sometimes the whole plant. We can protect our newly planted evergreens from winter burn by watering them deeply shortly before freeze-up, and also by mulching. The heavy watering leaves some water un-frozen in the soil, and that is available for the plants. The mulch reduces heat loss from the soil, keeping it warmer and partially or completely preventing freezing. That is why both of these strategies are recommended for the first couple of winters with newly-planted evergreens, even with such tough plants as Thuja Green Giant.
There is though, another kind of winter burn. This is when the foliage turns a bronzy-brown color, but doesn’t die, and naturally turns green again when the spring returns. This is a reaction of the plant to low temperatures, usually below zero degrees Fahrenheit. The extent to which a plant will do this varies between species, and even between individual plants. Several Thuja plants will do this, such as the Eastern White Cedar, Thuja occidentalis. Wild plants of this tree often turn bronze in winter, even though the plant is hardy to minus 50 degrees. The leaves will turn bronze and brown, and then become green again in spring. This is a natural defense against cold. The color comes from chemicals that accumulate in the leaves to protect it them from drying out. Not all individual plants of this species will do this, and the Emerald Green Arborvitae, a popular hedging plant in colder parts of the country, was specially selected partly because it stayed green in winter, and didn’t bronze. If you live in colder places, this is the ideal hedging plant, exactly because it will give you a great green background to your garden all winter long. Perhaps the reason this plant stays green is because it has a thicker cuticle on the leaves, and so doesn’t need those bronzing chemicals to protect it from drying out.
Winter Bronzing and Thuja Green Giant
Another Thuja plant that will not turn bronze in the winter is Thuja Green Giant. Although not as hardy as Emerald Green Arborvitae, it is the number one hedging and screening choice all across milder areas. This vigorous hybrid plant is hardy all through zone 5, where winter temperatures can fall to minus 20 (Fahrenheit) – plenty cold enough to bring about bronzing in a wide range of evergreens. Thuja Green Giant will stay a beautiful green color through all freezing weather, bringing all-year-green to your garden.
The only time you may see bronzing in this plant is if it is water stressed. This is seen in younger plants, especially in the first year after planting. Plants may green up again in spring, but if you have neglected them you may find some dead branches too. The best protection is the methods we looked to earlier – deep watering just before freeze-up, and mulching.
So if you have planted new Thuja Green Giant this summer or fall, or perhaps even if you planted them last year, late fall is a good time to take care of these simple tasks. You can apply mulch at any time, so this should be your first job. Ideally, use a rich organic material, like compost, rotted leaves or manure, that will improve the soil and feed your plants as well as preventing water loss. If not, then bark chips, shredded wood, gravel or pebbles will all reduce water loss. The deep watering is best done a couple of times in the fall, so that can be done almost any time too. Smart gardeners keep an eye on the weather forecast, and when the first really cold nights, with temperatures well below freezing, are predicted they pull out the hose and give their hedge one last deep watering, before packing up for the winter.
Once your plants are well-established, these precautions are not necessary, although they never hurt. Thuja Green Giant is one evergreen that is virtually guaranteed to give you a beautiful rich-green backdrop to your garden 365 days of the year. This is just another reason why this plant remains the number one hedging and screening choice across the country.