Arguably the most cold-hardy of shrubby palms, the needle palm (Rhapidophyllum hystrix) is native to the southeastern United States' moist deciduous forests and lowland swamps. It is grow with relative ease across U.S. Department of Agriculture hardiness zones 7 through 11, although in zone 6 it can survive winters if sited out of winds and near a protected, warm face of a building or sunny wall. The key to successfully growing a needle palm is ample heat in the growing season. While the palm survives considerable cold, if the summer fails to provide prolonged temps above 85 degrees F, growth will be slow and weak.
Young seedling needle palms are less resilient to cold. According to the authors of "Hardy Palms for the Southeast," only large, established plants tend to survive winters where temps drop to 0 to 10 degrees F well. In fact, considerable frond damage occurs when temps get below zero, and expect temps below -15 degrees F to cause death.
Penetration of harmful cold temperatures into the needle palm causes tissue damage, drying of vascular stems and even freezing of the growing tips within the plant base. Excessive cold also dries and kills the roots. Killed fronds are replaced the next growing season, but if the palm's base is damaged or killed, it likely will not rejuvenate any fronds or a new growing tip (base) from the roots. Dry soil, especially during the summer also leads to the palm's demise.
Needle palm plants cease growth once soil temperatures drop below 60 to 70 degrees F, and air temps remain under 80 degrees F. If a damaging freeze or heavy snowfall breaks fronds, the plant will not begin regrowth until ample soil and air warmth remains in spring or early summer. Do not prune away damaged fronds until spring when no threat of temps below 40 degrees F exists. Leaving dead frond and stems on plants in winter helps to protect the integrity of the growing tips in the plant base, acting as insulation. Pruning cuts in winter also expose plant tissues to air and water, potentially allowing fungal spores to gain internal access. Remember that needle palm is always slow-growing even with perfect growing conditions.
Apply granular palm fertilizer around the palm root zone in mid-spring and keep the soil evenly moist at the start of the growing season. Do not over-apply fertilizers; the only way to know if the needle palm is going to recover from the winter cold exposure is to wait for newly emerging fronds from the base. Alan Meerow of the University of Florida comments that the extend of winter damage may not manifest its full effects until six months later, as seen by diminished frond re-growth or a weak plant overall. Heat is what encourages new growth. Once new growth is seen, prune away brown and damaged fronds from the winter.
Needle palms stressed by summer drought will benefit from immediate and repeated irrigation. Ample soil moisture alongside temps above 85 degrees F will encourage new frond growth. However, emergence of new fronds slows and halts by the time early fall arrives and doesn't resume until the next summer.
A container-grown needle palm more readily sustains winter cold damage since the roots and plant base are more exposed to cold air temps. Relocate potted plants indoors to an unheated building when temps below 15 degrees F are expected. Needle palms outdoors in the ground may benefit from covering the base with insulating coarse, dry leaves and then covering the entire plant with cloth sheets or frost cloth. Try to keep the palm base dry. Alternating thaws and freezing with moisture on the palm proves harmful, especially to tender new growth. To avoid sickly looking plants in summer, maintain a moist to wet soil rich in humus even during seasonal dry spells.