Avoiding Cold Injury to Plants

(News article for November 7, 2020)

Each year, we face situations during which some of our plants could be injured by cold weather.

We can minimize the number of plants we have to protect by primarily choosing plants that tolerate the lowest temperatures we typically encounter. Plants hardy to USDA Hardiness Zone 8B will withstand most cold events we experience in Tangipahoa and Washington Parishes. In some parts of southern Tangipahoa Parish, temperatures typically stay warm enough for plants that are only hardy to Zone 9A.

Among commonly grown citrus, kumquats are the least likely to be injured by cold temperatures, with satsumas next in line on the cold-hardiness spectrum. Sabal or cabbage palms and pindo or jelly palms are some of our more cold-tolerant palm species.

If you like cold-sensitive tropicals lik bougainvillea and plumeria, one option is to plant them in a container large enough to prevent them from becoming root-bound but small enough to move into a protected area. Rolling planter caddies are available to place under larger pots.

The location of a plant in the landscape is important, since this affects the temperature of the air around it. Plants close to brick walls, concrete patios, and parking lots, for example, will likely experience warmer temperatures than those growing in the open yard. Likewise, cold air settles in low areas, so try to avoid placing plants that are marginally cold hardy in low areas of the landscape.

Things that we do to plants during the growing season can affect how cold tolerant they are. Healthy plants tend to be more cold-hardy than ones that are nutrient deficient or stressed by disease or insect issues.

Plants that are pruned hard or fertilized with nitrogen-containing fertilizer late in the growing season are likely to be less hardened off than they otherwise would be. Try to do any needed pruning or fertilizing of perennial plants by early to mid-September. (Pruning of plants like azaleas and blueberries that flower on previous season’s growth needs to be completed earlier than this. This early-to-mid-September rule of thumb is just for preventing cold injury.)

When a freeze is expected, make sure that plants have adequate water. Moist soil retains more heat than dry soil. Also, if water freezes in a pot, the plant can experience desiccation.

While mulch is desirable in many situations, bare soil heats up more during the day and so can provide more heat at night. For marginally hardy plants like citrus trees, one thing you can do is remove weeds, grass, and mulch under the plant. The soil needs time to heat up, so don’t wait until late afternoon on the day before the freeze event to do this.

When temperatures are anticipated to get colder than a particular plant can withstand, covering plants is sometimes an option. The cover should extend to the ground. A variety of materials can be used, but try to avoid having direct contact between the plant and the cover, especially if it’s made of plastic. Also, if you use a plastic cover, make sure to take it off or vent it during the day so that the plant won’t get too hot.

For additional cold protection, an incandescent light bulb can be placed under the cover. Make sure the light won’t get wet, and avoid direct contact between the light and either the tree or the cover.

For citrus trees in particular, a final thing you can is to wrap the trunk from the base to some point above the graft union. Most citrus trees are budded onto a rootstock, and the fruit that we want comes from the “scion,” or the part of the tree above the graft union. If some of the scion wood survives, the tree is likely to be able to regrow from it.

Let me know if you have questions.

Contact Mary Helen Ferguson.

Hammond Research Station landscapeFrost is visible on grass in low areas, as the sun rises at the Hammond Research Station (November 2019). (Photo by M.H. Ferguson)

11/24/2020 10:01:10 PM
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