Julia Laughlin: Winter protection for landscape plants
Plants that often experience winter damage are the broadleaf evergreens such as hollies and boxwood.
Water loss can cause severe damage to broadleaf evergreens during the winter when high winds or temporary warm weather causes a plant to give off an unusually high amount of moisture. If this water loss occurs at times when the ground is frozen, roots cannot take up moisture to replace lost water. The result is a browning or burning of the foliage.
To prevent this type of winter damage, make sure the plants enter the dormant season in a healthy and vigorous condition with adequate soil moisture. Check to see that the center of the plant is free of dead leaves and other debris. Be sure to continue watering during the dry winter months by watering during extended dry periods, so about one to two times per month if we are getting no rainfall. Water only when air temperatures are above 40 F. Apply water at midday so it will have time to soak in before possible freezing at night. Mulch also will protect the plant by preventing rapid temperature changes at the soil surface.
Watering in the winter maintains good soil moisture and protects plants from cold damage. When the soil has adequate moisture, it can freeze as the temperature drops below 32. Small amounts of heat are given off as the water changes from a liquid to a solid state. This small amount of heat helps to keep soil temperatures near the freezing point, even though the air temperature may drop much lower. When soils are dry, the soil temperature can go much lower resulting in damage to plant roots.
Mulching is an important cultural practice for many reasons, and winter mulching also can protect plants from severe temperatures during cold weather. Applying 2-3 inches of organic mulch acts like a blanket to hold in soil heat. When used in combination with winter watering, mulch provides prolonged plant protection.
Mulching in winter also can prevent the repeated freezing and thawing of soil that cause plants to “heave”, or lift, out of the ground. Winter mulch can keep the soil frozen even during thaws. However, the key is to wait until after the first freezing winter frost. This is important for plants that drop their leaves such as roses. Dropped leaves provide cover for harmful insects that may attack your plants in the spring. Once the ground is frozen hard, it is time to apply mulch over perennials and rose beds.
You may use mulches such as pine needles, straw, leaves or shredded bark. Compost is an excellent choice to mulch with since it provides good insulation and helps improve garden soils in the years to come. Again, it is best to apply a layer of 2 to 3 inches around each plant. After you’ve laid it down, gently pull it away from the trunks and stems to give plants room to breathe. This helps prevent disease problems.
The OSU Extensions Service has several great fact sheets to help you with winter protection. HLA-6404 “Winter Protection for Landscape Plants” and HLA-6005 “Mulching Garden Soils” and many other useful fact sheets can be found at osufacts.okstate.edu.
Email Julia Laughlin, Oklahoma County Extension Horticulture educator, at firstname.lastname@example.org.