Don’t Forget Your Evergreen Friends

Winter can be tough on landscape plants in general, but especially on evergreens. Needle-leafed plants like pines, yews and arborvitae and broadleaf evergreens, such as boxwoods and rhododendrons, can suffer serious winter injury when exposed to cold, dry winter winds. Newly planted evergreens are at the greatest risk. Although most of us have no control over the weather, there are a few things we can do to help our evergreen friends endure those wintery blasts.

The most important thing to remember is that an evergreen plant loses water to the atmosphere from its foliage all winter long through the process called transpiration. This is not a problem if the roots can take up sufficient water from the soil to replace that being transpired. However, if the soil is dry or the stems are frozen, water loss from the leaves can exceed uptake, leading to drying (desiccation) of the foliage. In many cases, this desiccation is not obvious because the leaves of evergreens may be rigid and resistant to wilting. The first evidence may be browning of the leaf tips or margins some time after the damage has occurred. Desiccation of evergreen foliage can occur when the temperature is below freezing as water in the leaves changes from the solid (frozen) to the vapor state without ever going through the liquid phase through the process called sublimation. Since cold air holds very little moisture, strong wind during a cold period can greatly accelerate desiccation of evergreen foliage.

So, what can be done to help evergreens get through the winter unscathed? The first line of defense is good planting technique. In general, evergreens require a well-drained, slightly acidic soil to thrive. A raised bed or berm, amended with organic matter and adjusted to a pH below 6.0 will get root growth off to a good start, allowing the plant to take up more water in the winter. Perhaps the most important practice in preventing winter injury is to make sure that the soil is moist as winter approaches. If the fall is dry, a thorough watering in November is good insurance. Make certain, however, that soil drainage is sufficient so that water can never collect around the roots for more than a few hours during wet periods over the winter and spring. The quickest way to kill an evergreen is to drown it. During dry winters like the last one, drag the hose out and do some watering during warm spells. Again, this is particularly important for newly planted evergreens with limited root systems. Mulching is also important because it conserves soil moisture and keeps the soil from freezing deeply so water is more available. Delaying mulching until the soil surface begins to freeze can help move plants into dormancy faster, thus reducing the chances of winter injury somewhat.

For evergreens planted within the previous two years, wind protection of some kind can reduce desiccation injury significantly. A simple screen made with burlap strung between fence posts can be very effective in protecting broadleaf evergreens from north and west wind. Although it is not particularly attractive itself, a burlap wind screen may prevent your evergreen from turning everbrown.

Chris Starbuck, Woody Ornamental Horticulture, UMC (573) 882-9630