Some landscape companies have been pushing “protection” of potentially cold-injured palms with bud sprays or trunk injections of nutrients and fungicides. As far as I know, sprays of copper or fungicides, or injections after the damage has occurred are of little to no benefit. To me, it would be similar to trying to resurrect a head of lettuce that was frozen in the refrigerator.
However, the updated information suggests if there is a dead-appearing leaf, treatments may help protect the heart or apical meristem, which in some palm species can tolerate colder temperatures than the tender, emerging spear leaf.
The University of Florida fact sheet, “Treating Cold-damaged Palms,” ENH-92, by Alan Meerow (see edis.ifas.ufl.edu/pdffiles/MG/MG31800.pdf), calls for treatment before the freeze to possibly minimize ice-nucleating bacteria which induce the formation of ice crystals (frost). In that document, under “Disease Control No. 4” and “cryptic cold damage,” is a very important recommendation: “Wait on palms despite how dismal they may look. Even if an emerging spear leaf dies, wait through the summer as they may recover.”
New information from the palm researchers, Monica L. Elliott and Timothy K. Broschat, at the UF Research and Education Center in Fort Lauderdale released this summary on Jan. 31 (with slight modifications from me here) on palms and cold weather: Be wary of salespeople strongly recommending palm treatments at this time. Any treatments are of unknown value and probably not needed.
“Trying to grow tropical palms in subtropical and temperate climates means cold damage is inevitable. This year’s cold weather is not atypical for Florida. For example, in the late 1970s and early 1980s, a series of hard freezes destroyed much of the citrus industry located north of Orlando, which is why that industry pushed south to Immokalee.
“During that same time period, severe freeze damage occurred on palms throughout the state. The only difference between then and now is that there are a lot more palms planted in communities that did not even exist in the early 1980s. So, for many people, this is their first experience with severe cold damage.”
This paper is in response to your many questions about cold damage on palms and expands upon some themes outlined in the EDIS document “Treating Cold Damaged Palms.”
As described in the EDIS document “Cold Protection of Ornamental Plants” (edis.ifas.ufl.edu/mg025), tropical and subtropical plants can be damaged at temperatures above freezing. Furthermore, while some years experience only one cold event the entire winter season, in other years, such as the current one, you may have an extended cold season.
Patience is essential with cold-damaged palms. Avoid the temptation to immediately trim damaged leaves. In most cases, the petiole and rachis will still be green. As long as any green tissue remains, the leaf should not be removed. Damaged leaves may provide some protection during subsequent cold events, plus green tissue is photosynthetic tissue.
Even a completely dead leaf may provide some insulating protection, especially against a radiational freeze or frost. Once the palm has produced substantial new growth (2 to 3 new leaves), damaged leaf tissue can be removed.
If trunk damage is observed externally due to a freeze, it is likely that there is substantial internal damage to the vascular and structural trunk tissue. These palms should be removed as they can pose a structural hazard in the landscape.
All new leaves of a palm develop from the apical meristem (bud), so that is the primary tissue that needs to be protected. Since leaf bases provide insulating protection to the apical meristem, this is one reason to not overtrim palms at any time of the year. Furthermore, good fertilization practices, including routine applications of 8-2-12-4 Mg (100 percent slow-release N, K and Mg), have been shown to greatly enhance cold tolerance.
Copper fungicides are recommended as an attempt — not a guarantee — to protect the apical meristem and developing leaves from secondary microorganisms that may attack damaged spear leaf tissue. There is no research to confirm if this is effective or not.
If the spear leaf does rot and can be easily pulled from the bud, it should be removed immediately, followed by a copper fungicide spray or drench of the bud region, which is now exposed. Still, you will not know if the apical meristem has survived until new growth emerges, which may be 4 to 7 months later. Hence, the need for patience! The new growth may be severely malformed or damaged, but the emergence of any living leaf tissue is a sign the palm is alive. Subsequent leaves will gradually improve in quality, but it may take as long as a year before normal leaves emerge.”
Doug Caldwell, Ph.D., is the commercial landscape horticulture extension agent and landscape entomologist with the University of Florida Collier County Extension Service. E-mail firstname.lastname@example.org ; phone, 353-4244 x203. Web site: collier.ifas.ufl.edu