How to Prevent Freeze Damage
Spring in the mountains is a wonderful time, with a succession of blooming shrubs and trees from late February into June. But unseasonably warm temperatures can cause plants to break dormancy early, making them vulnerable. A cold snap may kill the blossoms on fruiting shrubs and trees so that no fruit is formed that year. Frost may turn leaves an ugly black or brown, and new healthy foliage may take a while to appear. Or the plant is damaged beyond repair and dies.
Some of the reasons that freeze damage occurs are not as obvious as you might think. Here are some thoughts on how to keep your plants healthy this spring.
1. Freeze damage happens frequently because plants were purchased too early in the season. Many stores start selling tender annuals in early April when there is a virtual certainty that we’ll have another heavy frost. If you plant them too early, you might have to buy them again later.
Remember that our average last frost date is May 5th, and the 80% safe date is May 15th, so plan your purchases accordingly. (These dates are for Waynesville at an elevation of 2,658 feet.)
2. A considerable amount of heat is required to melt frost, and a large portion of this heat is taken from plant tissue. Many plants that can withstand temperatures below freezing can’t survive the melting process.
Keep frost off your plants by covering them with a blanket or bed sheet. Or wash any frost off with a fine water spray before the sun comes up and melts it. This will minimize damage down to about 27 or 28 degrees, depending on the plant. Be sure to drain the hose the night before so it doesn’t freeze solid.
3. Dormant plants usually handle normal winter temperatures without a problem. But once the juices have started to flow in stems and branches, a deep freeze can be catastrophic.
Remember the 2007 Easter freeze? An early warm spell brought many plants out of dormancy, so when the temperature dropped below 20 degrees devastation occurred. The plant clinic received many calls about Japanese maples, whose small diameter stems didn’t provide enough insulation against the cold. The fluids inside the stems froze solid, killing many trees.
When it gets this cold use water-filled plant protectors to protect small transplants, then cover the protectors with a blanket. For anything larger try heavy blankets or special frost protection covers available at home centers or online. Then cross your fingers.
4. Did you ever notice rhododendron leaves curling up in cold weather? Leaves of some broadleaf evergreens will dry out in cold winds, causing permanent damage. And if enough leaves are killed, the plant dies. So rhodos decrease the surface area exposed to the cold by curling their leaves.
To prevent plants from drying out, water the plant thoroughly in late fall so that a good supply of moisture is available. Build a windscreen out of burlap or other similar material to reduce the amount of drying winds that contact the plant (but don’t completely wrap the plant in burlap.)
Anti-desiccant sprays applied in fall can also help. But read the label carefully, because these sprays can harm as many plants as they protect.
I have a target date for each type of plant to go in the ground each spring. Prior to planting I always look at the long term forecast, and if unseasonably cold weather is predicted planting is postponed for a few days. Old bed sheets can cover larger plants like hydrangeas and dahlias. Water-filled plant protectors around tomatoes and peppers work well. If the forecast is for an overnight low of less than 35 degrees I get up before sunrise and hose down the entire garden to wash off any frost.
Jim Janke is an Extension Master Gardener Volunteer in Haywood County. For more information call the Haywood County Extension Center at 828-456-3575. © 2013 NC State University.