Radiational freezes or frosts occur on calm, clear nights when heat radiates from the surfaces of objects into the environment. When the air is moist, a radiant freeze results in frost on surfaces. Dry radiational freezes leaves no frost but can cause freeze damage.
Windy cold, like what’s predicted for tonight, results in an advective freeze. Advective freezes occur when cold air masses move from northern regions causing a sudden drop in temperature. Due to the rapid drop in temperature and prolonged duration, plant protection during advective freezes is often more difficult.
Coverings protect more from frost than from extreme cold. During freezes with windy weather, the covers tend to be less effective because the wind blows the heat away. During windy freezes or very cold nights, the addition of plastic sheeting over the cloth may be worth the effort on valuable plants.
Covers that extend to the ground and are not in contact with plant foliage can lessen cold injury by reducing radiant heat loss from the plant and the ground.
Foliage in contact with the cover is often injured because of heat transfer from the foliage to the colder cover. Some examples of coverings are cloth sheets, quilts, or black plastic.
WHAT TO DO THIS AFTERNOON?
- Move container plants inside or under protective covering like a lanai. Group containers together to increase their protection.
- Use windbreaks like fences, walls, tree canopies or other coverings to protect container plants that can’t come inside. Coverings include frost cloth, sheets and quilts, plastic, and even large cardboard boxes (these work great).
- A string of Christmas lights or a light bulb placed under a protective cloth may be enough to provide simple protection to ornamental plants.
- Most perennials are root hardy. Use mulch to protect the roots, and be accepting of foliage that dies back to the ground.
WHAT TO DO TONIGHT?
Agricultural operations use sprinkling for cold protection, which helps keep leaf surface temperatures near 32°F (0°C) because sprinkling utilizes latent heat released when water changes from a liquid to a solid state. Sprinkling must begin as freezing temperatures are reached and continue until thawing is completed. Water must be evenly distributed and supplied in ample quantity to maintain a film of liquid water on the foliage surfaces. Based on weather reports, that means running the sprinkler system tonight from 2 am to 8 am. WE DO NOT RECOMMEND THIS OPTION for home landscapes! Residential landscape sprinkler systems do not have the needed flow to protect plants in this manner. As a result, cold damage to plants from inadequate amounts of irrigation water may be more severe than if nothing was done at all.
Instead, prepare your landscape by moving plants to a protected area and covering with protective cloths before the sun goes down this evening. Then snuggle up on the couch with a cup of hot cocoa.
WHAT TO DO TOMORROW?
Freezing temperatures predicted again, so whatever you do tonight, you’ll probably have to do again tomorrow.
Remove plastic covers when the sun comes out and the temperatures rise above freezing or provide ventilation of trapped solar radiation. This will prevent your plants from “baking.”
By Wednesday, remove the protective coverings and check plants to make sure they have enough water. Wind causes evaporative loss, so plants may be suffering from both cold stress AND water loss. Do not overwater, though. Use your finger to check the moisture of the soil.
There is debate about pruning after a hard freeze:
- OPTION 1 – Wait until freeze risk has passed (about February 15th), check plants for living tissue, then prune back to remove dead/dying tissue. The damaged plant material helps to insulate and protect the still living parts of the plants.
- OPTION 2 – Prune all dead and freeze damaged tissue back immediately after a freeze. This will make plants easier to cover and protect if freezing temperatures return before the end of the season.