Dennis Pittenger^1,3, A. James Downer², and Donald Hodel¹
University of California Cooperative Extension
¹Los Angeles and ²Ventura Counties
³Department of Botany and Plant Sciences, U. C. Riverside

ature palms are important and conspicuous elements in the landscapes of California, Florida, Hawaii, Arizona, and other warm-climate areas. They are the signature plant and are emblematic of Mediterranean and tropical landscape settings. Palms are woody monocots characterized by the production of simple, fibrous, adventitious roots from the base of the trunk (6). Large mature specimens can often be transplanted successfully with a relatively small root ball. The ease with which many palm species can be transplanted gives them an advantage in landscape development and installation over the transplanting of mature conifers and dicot trees, which are characterized by large, multi-branched, woody root systems. Despite these advantages, many large specimen palms do not survive transplanting or they require an inordinate length of time to reestablish. Transplanting failures resulting in replacement can be 30% or more in some installations (4). While palm species are transplanted year-round in some climate zones, the common recommendation, and general practice in the landscape industry, is to transplant palms in the late spring or early summer as soil temperature is increasing (2, 4, 8). There is no generally agreed upon recommendation for the optimal root ball size when transplanting large field-grown palms. It is common practice for the root ball to be less than 45 cm in radius (3), since it is widely believed that cut palm roots die back to the trunk and are replaced by new roots originating from the trunk (1). However, Tomlinson (6) states that palm roots usually branch and continue growing from a point behind the apex if it is removed.

There has been limited study on the distribution and growth of palm roots in relation to the survival and reestablishment of large, transplanted, field-grown trees. Research on field-grown trees in Florida found that the branching response of cut roots varies among palm species (7, 9). In Sabal palmetto nearly all of the roots cut during transplanting died completely, and this species essentially generated a new root system from the base of the trunk to survive. The investigators’ interpretation was that for species with this response, root pruning a few months prior to transplanting should enhance development of new roots from the base of the trunk. In Syagrus romanzoffiana, Roystonea regia, Washingtonia robusta, and Phoenix reclinata, some cut roots branched and continued to grow, and the percentage that branched increased the farther the roots were cut from the trunk. In addition, these species varied widely in the number of new roots generated from the base of the trunk. Roystonea and Washingtonia generated high numbers of new roots per tree (97 and 144, respectively), while Syagrus and Phoenix generated moderate numbers of new roots per tree (23 and 62, respectively). Based on these findings, the author recommend that species responding in this manner should be transplanted with as large of a root ball as possible (minimum 30 cm radius), because more roots will be cut at a greater distance from the trunk, thereby increasing the number of roots that branch and survive the transplanting operation. Cocos nucifera exhibited an intermediate response to root cutting. About half of the cut roots of this species branched and continued growing regardless of how far from the trunk the roots were cut. The data in these studies also showed that the palm species had most of their roots close to the trunk (<30 cm), and that the actual number of cut roots that regrew per tree was relatively small (80 or less).

These findings and interpretations raise further questions about the importance of root ball size and regrowth of cut roots to successful transplanting. Perhaps total numbers of new roots generated from any source and the time of year that transplanting is done are as or more important than the root ball’s size or the number of cut roots that branch and continue to grow.

Material and Methods

In June of 1997, we began a field study with mature specimens of 16 palm species (Table 1) at the Arboretum of Los Angeles County in Arcadia, California to assess the distribution and regrowth of roots after they were cut and to evaluate how roots respond to cutting at different times of the year. A trench 15-cm wide x 60-cm deep x 100-cm long was dug on a tangent from the base of three single-tree replicates of each species. The tree-side face of each trench was divided into 4 distance zones that were subdivided into 2 depths. An initial distribution of roots found along the trench wall was recorded immediately after the trenches were dug, and the trenches were then backfilled with perlite. Trees were well irrigated throughout the study to ensure the perlite was kept moist. At three-month intervals from the original trenching, each trench was re-excavated and the roots growing into it in each of the 8 zones were harvested, counted, and weighed. No attempt was made to discern whether harvested roots were branched from previously cut roots or were new roots originating from the base of the trunk.

Preliminary Results and Discussion

As expected, we found that species typically produced most of their roots within 30 cm of the trunk, and cut roots of many species were frequently observed to branch and regrow. Species varied greatly in the time lag and maximum amount of root production that occurred after roots were cut. Root regeneration was minimal for nearly a year in Trachycarpus fortunei, Phoenix reclinata, Archontophoenix cunninghamiana, Serenoa repens, Sabal minor, Rhapidophyllum hystrix, and Butia capitata. New roots in the trench during any 3-month period (season) were lowest, never exceeding 70 in the trench, for Archontophoenix, Serenoa, Rhapidophyllum, Butia, and Brahea. Relatively high numbers of new roots were produced (100+ in the trench) in one or more 3-month periods in both Trachycarpus species, both Phoenix species, Syagrus, Washingtonia, Chamaerops, and Livistona decipiens.

Species also varied in the time of year that maximum root growth occurred. Livistona chinensis, and Washingtonia grew maximum numbers of roots in spring through summer, Brahea and Rhapiophyllum grew maximum roots in summer through fall, and all other species had maximum root growth during the summer months. Also, the number of roots produced each year increased for most species. Only Livistona chinensis, Caryota, and Brahea demonstrated decreasing numbers of roots produced over time, while Chamaerops, Rhapidiphyllum, and Syagrus followed a pattern of increasing then decreasing root production.

If total numbers of roots regenerated, regardless of their origin, is an important factor in successful palm transplanting, then these preliminary findings and those from further analysis of the study's data should provide guidance for proper transplanting procedures.

1. Harris, R. W. 1992. Arboriculture: integrated management of landscape trees, shrubs, and vines. Englewood Cliffs, NJ: Prentice Hall.
2. Hodel, D. R. 1996. Planting palms correctly for vigorous, attractive growth and fewer problems. Turf Tales Magazine 3(1): 10-11. Southern California Turfgrass Council.
3. Hodel, D. R., A. J. Downer and D. R. Pittenger. 1998. Palm root regeneration. Proc. Landscape Below Ground II Workshop, March 5-6,1998, San Francisco. D. Nealy and G. Watson, eds. Champaign, IL: International Soc. Arboriculture.
4. Meerow, A. W. 2000. Betrock’s guide to landscape palms. Hollywood, FL: Betrock Information Services.
5. Meerow, A. W. and T. K. Broschat. 1996. Transplanting palms. Gainesville, FL: University of Florida IFAS, Cooperative Extension Service Circular 1047.
6. Tomlinson, P. B. 1990. The structural biology of palms. Oxford: Clarendon Press.
7. Brochat, T. K. and H. M. Donselman 1984. Regrowth of severed palm roots. J. Arboric. 10: 228-240.
8. Donselman, H. M. 1991. Planting a palm tree. Gainesville, FL: University of Florida IFAS, Cooperative Extension Service Fact Sheet ENH-46.
9. Broschat, T. K. and H. Donselman. 1990. Regeneration of severed roots in Washingtonia robusta and Phoenix reclinata. Principes 34(2): 96-97.