172 Hodel et al.: Palm Root Growth ever, Tomlinson (1961) had noted that severed palm roots usually regrow from just proximal of the cut. Understanding how palm roots respond to cutting is critical to determining the optimal root ball size for success- ful transplanting (Broschat and Meerow 2000). In Florida on field-grown palms, Broschat and Donselman (1984a, 1984b, 1990) reported that the response of roots to cutting varies among species. In Phoenix reclinata (Senegal date palm), Roystonea regia (royal palm), Syagrus romanzoffiana, and Washingtonia robusta, some severed roots regrow from just proximal of the cut and, generally, the percentage of regrown roots increases the farther the roots are cut from the trunk. Thus, they advised that a relatively large root ball was important for root regrowth and successful establish- ment of these species. In contrast, they discovered in Cocos nucifera that about half of severed roots regrow regardless of the distance they are cut from the trunk, suggesting that root ball size was less important for this species. However, another interpretation of their data shows that a root ball extending 15 cm (6 in.) out from the trunk would be sufficient for nearly all these species because about half or more of all new roots are found within that distance. In Sabal palmetto, Broschat and Donselman (1984a, 1984b) stated that nearly all cut roots die back to the trunk and nearly all new roots grow from the RIZ. Because this species must grow an entirely new root system from the RIZ to establish successfully, they recommended a root ball only large enough to protect the RIZ. Phoenix reclinata, Roystonea regia, and Washingtonia robusta grow large numbers of new roots from the RIZ, while Cocos nucifera (coconut palm) and Syagrus romanzoffiana grow relatively few (Broschat and Donselman 1984a, 1984b). In the latter two species, however, the number of cut roots that regrow is large and surpassed the number of new roots that grow from the RIZ. Phoenix dactylifera offshoots responded similarly, with over two-thirds of new root growth originat- ing from roots severed during removal from the mother palm, all of which were cut only 2 to 10 cm (1 to 4 in.) long (Hodel and Pittenger 2003). Minimum recommended root ball sizes (measured by the distance out from the trunk) are 15 to 30 cm (6 to 12 in.) for Syagrus romanzoffiana, 30 to 60 cm (12 to 24 in.) for Washingtonia robusta, and at least 60 cm (24 in.) for Phoenix reclinata and Roystonea regia, because there is little regrowth of roots cut shorter than these lengths (Broschat and Donselman 1984a, 1984b, 1990). Root balls for the last three species are larger in order to encompass a sufficient percentage of regrown roots. Root pruning is recommended 2 to 3 months prior to transplanting for these species in order to stimulate a large number of new roots to grow from the RIZ so that a smaller root ball could be taken (Broschat and Donselman 1984a, 1984b, 1987, 1990). Perhaps the most important interpretation of Broschat and ©2005 International Society of Arboriculture Donselman (1984a, 1984b) was that most of a palm’s roots occur within 30 cm (12 in.) of the trunk. Root pruning prior to transplanting could benefit species that grow most of their new roots from regrown cut roots because it would shorten or eliminate the length of time for the cut roots to regrow after transplanting (Broschat and Donselman 1987). Later, Meerow (1992) and Broschat and Meerow (2000) placed less emphasis on root pruning but said it might be useful in some cases, although they presented no supporting data. More recently, the ability of cut palm roots to regrow as a determinate of root ball size has lost favor in transplanting guidelines. Meerow (1992), Meerow and Broschat (1992), and Broschat and Meerow (2000) now simply recommend a root ball extending at least 20 cm (8 in.) out from the trunk for single-stemmed species up to 5 m (15 ft) tall and at least 30 cm (12 in.) out from the trunk for larger or multi- stemmed species. In species where few or no severed roots survive, such as Sabal palmetto, they recommend a root ball 10 to 15 cm (4 to 6 in.) out from the trunk, just large enough to protect the RIZ. However, no supporting data are offered for these newer recommendations. In some cases, plant maturity may affect transplant success. In Chamaedorea elegans and Phoenix roebelinii, root growth is dependent on the presence of a RIZ that does not form until the palm stem attains its maximum diameter and begins to elongate vertically (Broschat and Donselman 1990). However, in Phoenix dactylifera (Hodel and Pittenger 2003) and Phoenix canariensis and Syagrus romanzoffiana (Hodel et al. 2003), the RIZ develops and becomes active at a much earlier stage, well before the stem attains its maxi- mum diameter and elongates vertically. These conflicting findings and interpretations raise ques- tions about the importance of root ball size, regrowth of severed roots, and growth of new roots from the trunk (RIZ) to successful transplanting. Perhaps time of year and total number of new roots produced, regardless of their origin, are as or more important than root ball size or the origin of new roots. The objective of this study was to assess the seasonality and distribution of palm root growth in 16 species of ornamen- tal palms in a Mediterranean climate to provide the landscape industry with clear, precise information about the optimal time of year and root ball size for transplanting palms. METHODS From June 1997 through December 2000, we conducted a field study with mature, established specimens of 16 palm species (Table 1) at The Los Angeles County Arboretum & Botanic Garden in Arcadia, California, U.S. This site is about 25 km (16.5 mi) east of Los Angeles and has a Mediterra- nean climate and a sandy clay loam soil. The palms ranged from 30 to 50 years of age according to arboretum acces- sion records and were 2 to 10 m (6 to 30 ft) tall.
July 2005
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