Protecting Plants During Extreme Cold

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Row cover

Row covers will not be enough to protect plants from record breaking cold.

Record setting cold is on the way, prompting the National Weather Service to urge North Carolinians to “prepare now for the potentially life threatening cold expected Wednesday night into Saturday.” In addition to the danger posed to pets, livestock, and people, this cold snap has the potential to seriously damage ornamental and edible plants due to its extreme nature and duration.

Nighttime temperatures of 5 degrees or lower are predicted for much of the piedmont. If this occurs, even plants rated hardy to our area’s USDA hardiness zone 7b, which has an expected average minimum of 5 degrees, could be damaged. The potential for damage is magnified by the duration of this cold event, with temperatures not expected to rise above freezing for a 65 hour period beginning Wednesday evening.

What’s a Gardener to Do?

Unfortunately there is very little that can be done in the face of this bitter cold other than snuggle up and wait for spring. In less extreme events, covering plants with row covers or blankets can be enough to protect them from cold injury but that will likely not be the case this week.

Perennials are less likely to be damaged because they are still dormant and hopefully tucked in beneath a 2”-3” layer of mulch. Woody plants, especially non-native varieties hardy to zone 7 or 8 such as gardenia, figs, and loropetalum, are more at risk. Any swollen buds or open blossoms on fruit trees will be killed, though the trees themselves should survive.

For vegetable plants, any exposed tissue will likely be killed. Seeds in the soil may be okay if they have not yet started to germinate. Any seeds that have started the germination process will be more susceptible to cold damage. Forty degrees is the minimum soil temperature at which cool season vegetable seeds such as garden peas and lettuce will begin to germinate. Most other cool season crops require warmer soil temperatures to sprout. Soil temperatures in central NC are currently in the mid-30’s, though had reached the mid-40’s earlier this month.

Protecting Prized Plants

For a prized plant, such as an heirloom fig bush or grafted rose, it may be worth the effort to provide extra protection. One option is to surround each bush with a wire cage and fill it with loose, dry mulch or straw. Strawberry plants and vegetable crops can be completely covered with loose mulch or straw then covered with row cover for extra protection. Be sure to remove the mulch as soon as temperatures moderate, which hopefully means Saturday.

Another option is to completely cover a plant with frost protection cloth or a blanket, ensuring covers reach all the way to the ground and are well secured, and to place a heat source underneath. Old-fashion incandescent bulbs or Christmas lights are often used, but they have to be the older, less energy efficient types to be effective. Newer LED bulbs give off little heat – that’s part of what makes them so energy efficient. One Chatham County gardener uses a slow cooker filled with water as a heat source to keep her greenhouse just above freezing. No matter what you use, take extreme care to prevent fire and electrical malfunctions.

When spring does arrive, don’t be too quick to give up on cold damaged plants. Even if the entire top is frozen, some woody plants will recover by sprouting from the base or roots. This is common with figs and gardenia, though it may be May or even June before new growth emerges.

LEARN MORE

Extension Fact Sheets on Protecting Landscape Plants from Cold:

Additional Resources:

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