Abstract
Paclobturazol is a plant growth regulator which reduces cell elongation and growth. It is commonly used in reducing the growth of herbaceous plants and woody dicots. It has also shown activity against young palms, such as oil and date palms. The effects of basal drench treatments applied annually or as a single application on mature Royal palms (Roystonea regia) were studied over a three-year period. No significant effects were noted on lateral trunk growth. Paclobutrazol treatments resulted in slower vertical growth than in palms not receiving a treatment. However, even at 36 MAT, the average reduction in vertical growth (approx. 50 cm) was still not horticulturally significant relative to the overall palm height of approximately 7 m. Royal palms may attain heights up to 24 m when mature. It is recommended that they not be planted within 7 m of utility lines especially considering regulating their vertical growth is not a practical option.
The Royal palm (Roystonea regia) is a valued specimen in Florida, U.S. landscapes. Part of their appeal is the smooth, grey trunk topped by a glossy green crown shaft and a dark green canopy consisting of feathery fronds up to 3.3 m long. This stately palm can grow quickly under the appropriate conditions of sun, soil moisture, and fertility, and reach heights of 24 m. Occasionally, these palms may be found near or growing under utility lines. Slowing their vertical growth rate is desirable in those circumstances.
Paclobutrazol is a plant growth regulator which inhibits gibberellin synthesis in apical meristematic tissue, thus reducing cell elongation and growth (Blaedow 2003).
This growth regulator has been available for decades and is commonly used in floriculture as well as in reducing growth of various woody dicots (Barrett and Nell 1983; Ruter 1994; Bai et al. 2004; Watson 2006). Paclobutrazol has been investigated for reducing plant growth near utility lines and in Rights-of-Way as well, in order to minimize power interruptions (Bowles 1985). Various application methods have been reported including foliar treatment, bark banding, trunk injection, and basal soil drench (Tamsberg 1990).
Carvajal et al. (1998) showed that paclobutrazol reduces height, rachis length, and leaf area when applied as a soil drench to nursery stock oil palm (Elaeis sp.). El-Hodairi et al. (1998) evaluated paclobutrazol effects on the date palm (Phoenix dactylifera). They showed that sequential crown drenches directed at the growing bud at 30 day intervals, reduced tree height in thee-year-old date palms. Hensley and Yogi (1996) investigated the effects of paclobutrazol on selected tropical species. They reported reduced height growth as a result of basal soil drench in Queen palm (Syagrus romanzoffiana) and Manila palm (Adonidia merrillii), 11 months after treatment.
The effects of other growth regulators on palms have also been reported by various researchers. Broschat and Donselman (1986) found monthly sprays of Ethephon retarded elongation and reduced the incidence of flowering in one-year-old Chamaedorea seifrizii. Similarly, Fisher (1980) reported leaf size reduction with Ethephon sprays or leaf tip treatments in young Caryota, Chamaedorea, Chrysalidocarpus, Phoenix, and Rhapis. Fisher and Theobald (1989) evaluated long-term effects of trunk injection with gibberellin or cytokinin in mature coconut trees. Neither product induced vegetative proliferation intended for mass propagation of coconuts. The multi-year study reported herein was conducted to evaluate the effects of paclobutrazol on mature Royal palms.
MATERIALS AND METHODS
Mature palms growing in a well-maintained landscape setting in Naples, FL, were selected for this study. The palms were approximately 7 m tall with an average diameter at breast height (DBH), 1.4 m above ground level, of 38 cm. Soil texture was sandy with pH 7.2 and 3.2% organic matter. Paclobutrazol (Cambistat® 2SC, Rainbow Treecare Scientific Advancements, Minnetonka, MN) treatments were as follows: 1) Cambistat 2SC at 1.6 gm ai/cm trunk DBH basal soil drench applied once on February 6, 2007; 2) Cambistat 2SC at 1.6 gm ai/cm trunk DBH basal soil drench applied annually on February 6, 2007; February 5, 2008; and February 6, 2009; 3) Untreated Check not receiving any treatment. Paclobutrazol was mixed according to label instructions, and treatments were applied as a basal drench with a watering can. Drench volume was 7.6 L/tree. Mulch (pine needles) at the bases of palms were pulled back prior to drench application then replaced afterwards. The trees were fertilized three times per year with 103 gm each of a granular, slow-release nitrogen fertilizer (7-2-14), applied near the base.
Palm height (clear trunk height) was determined with a clinometer (Bunton Clino Master model CM 66 LA). The reference point was the base of the crown sheath (most recent visible frond scar). Determinations were made from the same fixed distance (10 m) and direction from each palm. In addition, digital images were used to monitor tree height. Palms were photographed with a permanent reference point (i.e., building) in the background. Trunk size (DBH) was determined with a measuring tape (Keson diameter tape rule). Evaluations of height differences and DBH were made pre-treatment, 12, 24, and 36 months after treatment (MAT) during February each year.
The study was initiated in February 2007 and completed February 2010. Experimental design was a randomized complete block design with 10 replicate palm trees per treatment. Data were analyzed with ANOVA and mean separation via Student-Newman-Keuls test at P = 0.05.
RESULTS
No noticeable differences in lateral trunk growth (trunk diameter) were found among treatments (Table 1). Mature palms normally grow from the top without additional increase in girth. Any minute differences in reported data are likely due to measurement error. Significant differences were found in tree height at 24 and 36 MAT (Table 2). Palms receiving annual applications of paclobutrazol showed a trend for slower growth (though not statistically different) than those receiving a single application. Both paclobutrazol treatments resulted in slower vertical growth than in palms not receiving a treatment. However, even at 36 MAT, the average reduction in vertical growth (approx. 50 cm) was still not horticulturally significant relative to the overall palm height of approximately 7 m.
Effects of paclobutrazol basal drench on mature Royal Palm lateral trunk growth, Naples, FL, 2007–2010.
Effects of paclobutrazol basal drench on mature Royal Palm vertical growth, Naples, FL, 2007–2010
DISCUSSION
Annual basal drenches with paclobutrazol slightly reduced vertical growth of mature Royal palms over a 36-month period. Even though it is an effective growth regulator on woody dicots (Barrett and Nell 1983; Ruter 1994; Blaedow 2003), paclobutrazol did not result in an appreciable effect on mature Royal palms throughout the duration of this study. In order to prevent power interruptions, it is recommended that Royal palms be planted at least 7 m away from power lines. Planting these palms directly underneath the lines will result in future problems as the trees attain heights of 24 m or more.
Acknowledgments
We wish to express gratitude to Chris Andrews with Bay Colony Community Association for providing a site for this study. This work was supported, in part, by a grant from Rainbow Treecare Scientific Advancements.
- © 2010, International Society of Arboriculture. All rights reserved.
Jump to section
Related Articles
Cited By...
- No citing articles found.