Time of the year vs. Establishment of Palms
Palms are subtropical or tropical plants. Their rate of growth and establishment in the landscape is directly related to air and soil temperatures. The growth of the roots of most palms begin to slow dramatically when the soil temperature drops to 65 degrees F., and new growth almost totally stops below 60 degrees F. My friend and colleague at the University of Florida, Dr. Alan Meerow writes in his 1992 book, Guide to the Landscape Palms, "Many palms exhibit reduced root function at soil temperatures below 65 degree F, thus winter planting should be avoided." He also states, "Palms establish more quickly if transplanted during the spring and early summer when soil temperatures are on the increase."
From work by Dr. Timothy K. Broschat and myself, we know that when palm roots are severed when the palm is dug, that the majority of the roots die back to the trunk and have to be replaced with new roots that develop from the root initiation zone at the base of the trunk (that is, the cut roots don't branch and continue on growing). Anything that we can do to hasten the replacement of these roots will speedup the establishment of the transplanted palm. Note, this does not apply to containerized or boxed palms.
From our palm research we have found that there is a window of time for planting palms. This "window" is when the soil temperatures are beginning to warm-up in the spring (mid-March to April in southern California) and extending through the fall (mid-October to November in southern California). In Central California the window is about a month shorter overall. This same planting window is valid in Las Vegas, but I also try to avoid the month of August and the first two weeks of September, when air temperatures are routinely above 100 degrees F. Florida and Texas have similar "windows" depending on the latitude in these respective states.
The planting window is the "best" time to plant, but doesn't mean that all palms planted in the winter will die. It just means that the palm rootball will set in the planting hole for an extended period of time with little or no new root development. This cool time of the year is also our rainy season in California, compounding the problem with the palm roots being inundated with water and cold soil. These are perfect conditions for secondary root pathogens like Pythium to develop and further weaken the root system.
