Journal of Arboriculture 31(4): July 2005 179 findings support the concept that the total number of new roots that grow and their density, regardless of their origin, are the key factors in determining root ball size. Because rapid growth of a large number of new roots from the RIZ and/or from regrowth of severed roots is most critical to transplant success, in most cases root balls extending 30 cm (12 in.) out from the trunk are sufficiently large to ensure that this growth occurs. Even if the ability of severed roots to regrow is critical to determining root ball size, the total number of roots that regrow might be more important than the percentage of roots that regrow. For example, Broschat and Donselman (1984a, 1984b) reported that Cocos nucifera generally had 50% of cut roots regrow regardless of distance from the trunk. However, there were so few roots beyond 30 cm (12 in.) from the trunk in their study (only 14 out of a total of 160 regrown roots in all zones combined) that the high rate of regrowth in that area is irrelevant. In C. nucifera, Roystonea regia, and Syagrus romanzoffiana, the distance from the trunk at which 30% of roots regrew actually did encompass nearly a third or more of the total number of regrown roots in all the zones combined. Unfortunately, Broschat and Donselman (1990) reported only percentages of regrown roots, not actual numbers, for Phoenix reclinata and Washingtonia robusta. In many instances, the ability of severed palm roots to regrow might be more apparent than real. While we observed that most severed roots did not regrow at the cut point, neither did they always die. Some severed roots remained alive and appeared to be functional. Also, some severed roots that appeared to die did so only for a short distance and actually regrew into the trench from several centimeters proximal of the cut. When these regrown severed roots entered the trench, they appeared like new roots from the RIZ. Unless each root entering the trench is traced back to its origin, it is impossible to determine whether the root grew from the RIZ or from a severed root. Hodel and Pittenger (2003) showed that most severed roots of Phoenix dactylifera offshoots regrew, often within 2 cm (0.8 in.) of the trunk, but appeared as new roots from the RIZ when they emerged from the root ball. Thus, many of the roots reported as growing from the RIZ in earlier studies likely regrew from severed roots. Root Pruning Although they provided no supporting data, Broschat and Donselman (1984a, 1984b, 1987, 1990) stated that root pruning 1 to 3 months prior to transplanting would grow a new root system faster because it stimulates new roots to grow from the RIZ prior to moving the palm, thus minimiz- ing transplant shock. However, palm root growth is greatest during the warmer times of the year when soil and air temperatures are highest and, that in tropical areas and southern Florida, root growth is frequent and regular year- round (Broschat 1998). Thus, it is likely that the root growth from the RIZ in their studies was in response to the warm soil and air temperatures rather than the cutting of roots. Broschat and Donselman (1987) also concluded that root pruning would benefit even species that produce most of their new roots from regrowth of severed roots because it would shorten or even eliminate the length of time for the cut roots to regrow after transplanting. More recently, Meerow (1992) and Broschat and Meerow (2000) placed less emphasis on root pruning but said it might be useful in some cases. Harris et al. (2004) also supported the practice. Even if root pruning does provide these positive benefits, there is still a serious concern with this practice because any new roots that grow from the RIZ or regrow from roots severed during pruning will be highly susceptible to damage during the digging, transporting, and replanting processes. After root pruning, transplanting too early might lose the purported benefit of this practice, while waiting too long to transplant might damage any new roots that had grown. Thus, we feel that root pruning palms prior to transplanting has little merit. SUMMARY AND CONCLUSIONS For many palm species, most roots occur within 30 cm (12 in.) of the trunk and, in Mediterranean climates, are produced most abundantly during the warmer months, especially in the summer. Palms need active root and shoot growth to establish quickly after transplanting. Palms can be transplanted year-round in warm regions where air and soil temperatures are nearly always sufficient to ensure ad- equate root and shoot growth. In contrast, palms are best transplanted at the beginning of the warm season in regions where air and soil temperatures are insufficient for ad- equate growth for several months of the year. Thus, the best time to transplant palms in Mediterranean climates is the late spring to early summer because this time provides a long, warm period that promotes root and shoot growth to ensure quick establishment. Root balls with a 30 cm (12 in.) radius from the trunk and 30 cm (12 in.) deep are adequate for most palms when transplanting because this size captures well over half the roots. The few palms that must grow nearly all their new roots from the RIZ after transplanting need a root ball only big enough to protect the RIZ, about 15 cm (6 in.) out from the trunk. Palms that grow about 30% or more of their roots below 30 cm (12 in.) deep and exceptionally large specimens would probably benefit from a deeper root ball, about 60 cm (24 in.) deep. ©2005 International Society of Arboriculture
July 2005
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