The recent cold weather that followed our late-winter, record-breaking warm spell has gardeners anxious about potential damage to their plants. Before getting too stressed, let us talk a little about winter hardiness.
As the days become shorter and the weather starts to cool in late summer, the growth of woody plants slows to a stop and they gradually acclimate or become ready for winter’s cold temperatures. Acclimation is a physiological process that takes place within plant tissues. As temperatures decline, the process continues until a plant achieves its full dormancy and maximum hardiness in mid-winter. Due to their genetically dictated tolerance to cold, different species and even varieties of plants vary in their maximum potential hardiness.
Periods of warm weather during winter can cause a plant to de-acclimate, making it more vulnerable to damage from severely cold temperatures. However, plants can re-acclimate and reclaim their hardy condition if the decline in cold temperatures is gradual.
Near the end of winter when the days start to lengthen and temperatures start to rise, plants begin the process of de-acclimating and start losing their tolerance to freezing temperatures. If unseasonably warm weather occurs during late winter, plants will become increasingly less hardy and more vulnerable to freeze damage. While they can reclaim some cold hardiness with cooler weather, they will not revert to their full mid-winter hardiness.
Our mild winter and recent warm spell appear to have area gardeners and plants ready for spring. How can we protect plants with swelling buds that seem ready to open? Here are a few things that might help.
▪ Check the soil for moisture. One gardener told me this week that the soil beneath the evergreen hedge that divides her yard and the neighbor’s is bone dry. I recommend watering, even though it means getting a hose out to do the job. Dr. Linda Chalker-Scott, WSU Extension horticulturist, points out that while the top of your tree and shrub may be dormant, the roots are not dormant. She adds that a moist soil provides better insulation from cold temperatures than dry soil does and can help protect the smaller roots from damage by cold temperatures.
While not related to cold weather concerns, keeping soil moist when it is not frozen is important to overall plant health. Drought conditions lead to the death of fine roots and impair the plant’s ability to take up water and nutrients to support top growth in the spring.
▪ On frosty nights, small plants or flowering perennials that have started to grow can be covered at dusk with a blanket or burlap to hold in heat radiating from the soil. The fabric should completely cover the plants down to the ground. Any openings will let heat escape. A frame or some type of support can help keep a heavy blanket from damaging plants. One or more five gallon buckets filled with hot water placed underneath the covering will provide additional heat. Remove any coverings in the morning after it warms up.
Some spring flowering bulbs are already peeking out of the soil. Depending on the situation, cover these at night with a blanket or use inverted five-gallon buckets to cover individual plants or clumps. Remove them during the day. This can be tedious if your bulbs get a very early start, so an alternative is to cover them with some extra wood chips, shredded bark or compost mulch that will need to be removed when spring arrives.
Hopefully, our plants will be OK.
Marianne C. Ophardt is a retired horticulturist for Washington State University Benton County Extension.