Submitted to LNLA by Dr. Raj Singh, LSU AgCenter Plant Diagnostic Center
Palms are signature ornamental plant species planted in home gardens, public parks, and roadside medians in southern Louisiana. Twenty-one species of palms are known to exist in Louisiana. Cabbage palm, Canary Island date palm, Chinese windmill palm, edible date palm, queen palm, and silver date palm are among the most prevalent palm species.
Palms are monocots, or grass-like flowering plant. Their trunks, or stems, have a central growing point called apical meristem, commonly known as bud or heart at the top of the plant from which all new fronds (leaves) emerge. If for some reason (biotic or abiotic issues), this central growing point gets killed, the palm will die. This can happen even to fully grown, well-established palms.
Palms are native to tropical and sub-tropical climates, although cabbage and windmill palms are considered relatively cold-tolerant. The winter weather that the Bayou State experienced the week of Mardi Gras severely damaged a large community of palm trees. Extended low temperatures and freezing conditions affected, Canary Island date palm, edible date palm, queen palm, silver date palm and some windmill palms. Cabbage and Chinese windmill palms in some locations suffered hard freeze injury as well but the damage is not as severe as on queen and silver palms. Generally the freeze damage begins to show up a week or 10 days later.
If you are now wondering what to do with a freeze-damaged palm, it’s important to first understand the different types of injury they can sustain in low temperatures. First, chilling injury from temperatures above freezing results in browning of leaves commonly known as necrosis, or death, of the palm fronds. Chilling injury results from sudden drop in the temperatures in the 40-to-45 degree range. This type of injury is not deadly, and affected palms recovers fully from the damage.
The second type of injury is frost injury that occurs when the leaf temperature drops to 32 degrees or below. The damage is similar to chilling injury, but affected palms may take longer – a year or more – to fully recover.
The third and deadly type of injury results from hard freeze. Extended temperatures below freezing not only affect the exposed fronds, but may also kill the base of the spear leaf (newest leaf in the palm canopy) in the apical meristem. This dead tissue is subsequently colonized by decomposing fungi and bacteria, resulting in the death of the apical meristem. Severe freezing temperatures may also kill the apical meristem. The spear leaf turns brown and can easily be pulled from within the palm canopy. Once the meristem is dead, it will not produce another one.
Freezing temperatures also can cause longitudinal splits on the stems of queen and silver palms. These splits are later colonized by decomposing organisms, resulting in softening of the stem. As the decomposition progresses, vascular channel tissue rots, interrupting the water and nutrient supply. In some instances, palms break in the middle at the affected area.
When it comes to caring for freeze damaged palms, be patient. Do not rush into removing the affected (brown) fronds immediately after the damage becomes visible. If a portion of a frond is still green, leave it on the tree as long as possible. It may look unsightly, but will benefit the palm during the recovering phase. The green portion will aid in photosynthesis necessary for production of sugars to support the palm growth.
Palms usually start their season’s growth long after other shrubs and trees start their spring growth flushes. Wait for the affected palm to produce new growth, and do not remove the affected fronds until the danger of additional hard freezes is over.
Affected palms should not be irrigated or fertilized now. Fertilize palms during active growing season from as early as late spring to early fall. Water palms adequately to avoid any drought stress, especially during summer months.
Wrapping a palm trunk in burlap or frost-protection blanket to prevent cold damage does not protect the apical meristem. It may provide some protection to the palm trunk from getting damaged.
If a cold-damaged palm does not produce new growth and has to be replaced, plan to plant a new one in early summer. Root growth in Louisiana is best in June, July and August, and new palm will also benefit from annual fertilization in early summer. When selecting new palms, think about native, cold-hardy species, including cabbage and windmill palms.