Following the temperatures we’ve had recently, there will likely be questions about how to deal with cold-damaged plants.
Parts of herbaceous (non-woody) plants that are brown and obviously dead can be cut back soon, although there doesn't need to be a rush to do so. (On a related note that I find interesting, research in North Carolina has found that pollinators and other beneficial insects often live in perennials’ dead stems that have been allowed to remain in place for the following growing season. Based on this, it’s been suggested to leave the first 1 to 2 feet of dead stems for beneficial insect habitat.)
For woody plants and palms, waiting is advised.
On citrus trees, you may not be able to tell the full extent of cold damage until sometime in the summer. Branches or trunks with injured shoots may end up putting out new growth. Likewise, a cold-damaged tree may put out flushes of new growth that later collapse. Waiting until July or August, after the second annual flush of growth has occurred, gives you more time to see the full extent of cold damage on citrus. At that time, you can remove dead wood.
Also remember to remove rootstock growth from below the graft union as citrus plants recover. Most rootstocks have trifoliate leaves (leaves with three leaflets), so their foliage looks different than that originating from the scion (top part of the grafted tree).
When the time comes to fertilize citrus trees in late winter or early spring, if it appears that you have cold injury, you can reduce the amount of fertilizer in proportion to the part of the tree that appears damaged and, instead of applying it all at once, divide the total amount of fertilizer up into several smaller applications. Be sure not to fertilize after the end of June, though, since late fertilizer application can predispose citrus trees to cold damage the following winter.
We’ll likely see cold injury on palm species that are marginally hardy for this area. On cold damaged palms, it’s recommended to let brown leaves remain until no further hard freezes are likely to occur. Survival of palms depends on survival of the growing tip (meristem) at the top of the plant. Dead or injured leaves can still help protect the growing tip from remaining cold events.
Since palms do not put out new growth until later than many other plants, wait until July to determine whether or not a palm is dead.
When you buy new plants, remember to choose ones that are cold hardy in your area. As you may have heard, the USDA Hardiness Zone map was updated recently based on temperature data from 1991 to 2020. While most of Tangipahoa and Washington Parishes used to be in USDA Hardiness Zone 8B, most of this area is now in Zone 9A, reflecting warmer average minimum temperatures. There are still some patches of Zone 8B in northern Tangipahoa Parish and western and north-central Washington Parish.
Hardiness Zone 9A indicates average minimum temperatures of 20 to 25 degrees F. Since these hardiness zones are based on average minimums, we occasionally experience temperatures lower than that range, as we did this month. If you want to play it safe, you can continue to select plants that are cold hardy enough for Zone 8B.
Let me know if you have questions.
Dr. Mary Helen Ferguson is an Extension Agent with the LSU AgCenter, with horticulture responsibilities in Washington and Tangipahoa Parishes. Contact Mary Helen at email@example.com or 985-277-1850 (Hammond) or 985-839-7855 (Franklinton).