Regrowth Of Severed Palm Roots

Materials and Methods

Five 5-8 m tall field-grown palms each of royal palm (Roystonea regia), coconut palm (Cocos nucifera), queen palm (Arecastrum romanzoffianum), and cabbage palm (Sabal palmetto) were used in the experiment. A trench 1.5 m long, 30 cm wide, and 60 cm deep was dug on 23 Aug 1982 with a mechanical trencher through the root system of each palm, tangent to the trunk of the palm. This exposed cut roots varying in length from 5 to 100 cm. The trench was then refilled with moist perlite and was covered with a porous polypropylene fabric. Palms were irrigated periodically to keep the perlite zone moist. Eighteen to 30 weeks after trenching, depending on the species, the perlite was removed, the roots rewashed, and root growth response recorded for 4 different length classes (0-15 cm, 15-30 cm, 30-60 cm, and 60-90 cm) of root stubs.

Results and Discussion

The severed queen palm roots were re-exposed after 18 weeks of regrowth. Among 88 roots cut to a length of less than 15 cm, only 3% branched and continued growth (Fig. 1). Percentage of cut roots branching increased to 41% for roots 15-30 cm long and continued increasing with increasing root length to 57% for roots 60-90 cm long. Thus 15-30 cm appears to be the minimum threshold length for branching of cut queen palm roots. Since relatively few new roots initiated from the trunk (mean = 23) during the 18 week regrowth period, emphasis should be on taking as large a root ball as possible to increase the percentage of roots branching and continuing growth.

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Fig. 1.

Average percentage of severed queen palm roots branching in 4 different root length classes after 18 weeks.

After 20 weeks the root-pruned royal palms were re-excavated. As with queen palms, the percentage of branched-cut roots increased as root length increased (Fig. 2). Less than 1% of roots cut to 15 cm or less branched whereas 24% of the 30-60 cm-long roots and 36% of the 60-90 cm-long roots branched. Large numbers of new roots (mean = 97) emerged from all royal palm trunks following root pruning. This suggests that if roots must be cut to a length of less than 30 cm where branching is minimal, root pruning should be performed 2-3 months prior to actual moving of the palm. In this way active new roots will already have emerged by the time the palm is moved and transplanting shock should be minimized. If handling large root balls with royal palms can be accomplished, then root pruning prior to digging such that 60-90 cm-long roots remain, should result in considerable branching of old roots as well as extensive production of new roots and would be the preferred method of handling these large palms.

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Fig. 2.

Average percentage of severed royal palm roots branching in 4 different root length classes after 20 weeks.

Coconut palms were redug 23 weeks after root pruning. Cut root length had no effect on branching in this species and roots of all lengths branched about 50% of the time (Fig. 3). Relatively few new roots (mean = 20) were produced from the trunk, but that may have been due to the rather young age (ca. 5 yr) vs 7-12 yr for the other species) of the coconut palms used in this experiment. Root ball size in coconut palms thus appears to be less important for root branching than in queen or royal palms.

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Fig. 3.

Average percentage of severed coconut palm roots branching in 4 different root length classes after 23 weeks.

Root regrowth in cabbage palms was much slower and more irregular than in the other 3 species. The severed roots were re-examined 30 weeks after trenching and even then branching of cut roots averaged only 1 % for all root lengths (Fig. 4). New root production was extensive (191-393 new roots/palm) on 3 of the 5 palms, but in 2 plants few (5-26) new roots were produced. Since cabbage palm roots generally do not branch at any length, root pruning 8-10 weeks prior to digging for stimulation of new root production is important for successful transplanting of this species.

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Fig. 4.

Average percentage of severed cabbage palm roots branching in 4 different root length classes after 30 weeks.

In summary, cut root branching response varies among species of palms. Percentage of branched roots increases with increasing root length in queen and royal palms so that larger root balls are desirable for branching and continued growth of old roots. Root length had no observable effect on branching in coconut and cabbage palms with half of all cut roots branching in coconuts and virtually none branching in cabbage palms. New roots were initiated from the trunks of all four palms at a rate inversely proportional to the ability of that species to regenerate severed root tips. Root pruning 2-3 months prior to moving the palms is important for species such as royal and cabbage palms in which root branching may be minimal and new root production extensive, but is also helpful for coconut and queen palms which produce some new roots following root pruning.