Journal of Arboriculture 31(4): July 2005 171 PALM ROOT GROWTH AND IMPLICATIONS FOR TRANSPLANTING By D.R. Hodel1 , D.R. Pittenger2 , and A.J. Downer3 Abstract. Palms need active root and shoot growth to establish quickly after transplanting. For many palm species, roots grow most abundantly during warmer months and occur within 30 cm (12 in.) of the trunk. Thus, palms can be transplanted year-round in warm regions where air and soil temperatures are nearly always sufficient to ensure adequate root and shoot growth. However, in regions with cool seasons, palms are best transplanted at the beginning of the warm season. For most species, root balls with a 30 cm (12 in.) radius from the trunk and 30 cm (12 in.) deep are adequate because they capture over half the roots. The few species that must grow nearly all their new roots from the trunk after transplanting need a root ball only large enough to protect the root initiation zone, about 15 cm (6 in.) out from the trunk. Species that grow about 30% or more of their roots below 30 cm (12 in.) deep would probably benefit from a deeper root ball, about 60 cm (24 in.) deep. Key Words. Palms; transplanting; time of year; root ball size; root growth; root initiation zone. installations fail to establish (Meerow 2000). Rapid growth of new roots from the RIZ and/or regrowth of roots cut during transplanting are critical for successful establish- ment. While there are many factors affecting palm root regeneration and successful transplanting, the time of year and root ball size are among the most important. There is limited research-based information on the seasonality of palm root growth and root ball size as they relate to root growth. Nearly all the information was developed for a few species in Florida, U.S., which has a warm, humid climate—unlike palm-growing regions with a Mediterranean climate where winters are cool and with little rain and summers are warm to hot and rainless. Palm trees are high-value and increasingly common compo- nents of landscapes wherever they can be grown. Large, older specimens are in great demand and command a premium price. They are usually dug and removed from existing landscape sites or from a commercial nursery field, transported, and replanted at another site, creating an instant mature landscape. Specimen palms are relatively easy to transplant com- pared to broad-leaved and coniferous trees. A relatively small root ball is necessary when transplanting most palms because, as monocots, they have an adventitious root system composed of numerous fibrous primary (first-order) roots that grow independently and periodically from the root initiation zone (RIZ), the area at the base of the stem or trunk near or at ground level from which new roots grow (Tomlinson 1990). In California and the southwestern United States, it is standard industry practice for a root ball to extend from the trunk only about 15 to 30 cm (6 to 12 in.) when transplanting palms up to 20 m (60 ft) tall with trunks 30 to 100 cm (12 to 39 in.) in diameter. Generally, transplanted palms will establish as long as they grow new roots in a timely manner. Despite the ease of transplanting palms, a significant number do not survive, or they require an inordinate length of time to establish. Thirty percent or more of palms in some PREVIOUS WORK Although palms are transplanted year-round in some areas, the traditional recommendation is to transplant them during the warmer times of the year (Donselman 1981; Hodel 1995, 1996, 1997; Broschat and Meerow 2000). Broschat (1998) reported that palm root and shoot growth are greatest when soil and air temperatures are highest. He suggested that palms could be transplanted at any season in tropical areas and southern Florida, because root and shoot growth occurred year-round. However, he suggested that winter planting was not advisable in cooler climates. In preliminary studies, Hodel et al. (1998) and Pittenger et al. (2000) reported that, in southern California, root growth of most palms tended to be highest during the warmer months from spring through fall. However, the landscape industry in southern California transplants especially hardy palms, such as Chamaerops humilis (European or Mediterra- nean fan palm), Phoenix canariensis (Canary Island date palm), P. dactylifera (date palm), Syagrus romanzoffiana (queen palm), Trachycarpus fortunei (Chinese windmill palm), Washingtonia filifera (California fan palm), and W. robusta (Mexican fan palm) year-round to keep pace with development and demand, although the success rate is variable. The optimal root ball size when transplanting palms has been the subject of much debate and speculation. Traditional recommendations range from nearly no root ball to one as big as possible. Broschat and Donselman (1984a, 1984b) reported that palm root balls commonly extend 45 cm (18 in.) out from the trunk because most roots cut during transplanting die and must be replaced by roots originating from the trunk. How- ©2005 International Society of Arboriculture
July 2005
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