Timothy K. Broschat

Hong Kong orchid tree is an outstanding flowering tree for tropical and subtropical areas, but in south Florida’s nutrient-poor sand soils, it typically develops moderate to severe K and Mg deficiency symptoms during the fall, winter, and spring months. A 3-year field experiment was conducted to determine if flowering was responsible for the development of these deficiencies and to determine if these deficiencies could be prevented by fertilization with medium or high rates of a 24N–0P–9.2K turf fertilizer (24–0–11) an 8N–0P–10K–4Mg plus micronutrients palm fertilizer (8–0–12) or a 0N–0P–13.3K–6Mg plus micronutrients palm fertilizer (0–0–16). Onset of deficiency symptoms coincided with the beginning of flowering, but leaf nutrient concentrations of N, P, K, and Mg continued to decline after flowering ceased in late January, presumably because of movement of these elements from the leaves to stem tissue. Leaf nutrient concentrations were poorly or negatively correlated with all measured plant quality variables and were poor indicators of plant quality or nutritional status. Although fertilization with a high rate of 24–0–11 or either rate of 8–0–12 increased tree height, caliper, and number of flowers, no treatment significantly decreased the severity of K and Mg deficiencies.

Broadleaf ornamental trees are known to vary widely in their responses to fertilization, depending on the species and soil and other environmental factors. Thus, it is important to study the responses of a wide range of tree species to fertilization, especially on nutrient-poor soils. Four species of temperate to tropical trees, live oak ( Quercus virginiana ), west indian mahogany ( Swietenia mahagoni ), black olive ( Bucida buceras ‘Shady Lady’), and beautyleaf ( Calophyllum brasiliense ), planted into a sandy native soil in south Florida were fertilized with a 24N–0P–9.3K turf fertilizer or an 8N–0P–10K–4Mg plus micronutrients palm fertilizer at rates of 10 or 20 g of nitrogen per tree four times per year. Tree height, width, caliper, and nutrient deficiency rating scores for nitrogen, potassium, and magnesium were determined at 1 year after planting (establishment period) and at 3 years after planting (maintenance phase). Data from these measured variables were subjected to principal component analysis to obtain a single measure of overall quality, namely, the scores for each tree on the first principal component. West Indian mahogany showed no response to fertilization during or following establishment. Either fertilizer type or rate improved live oak, black olive, and beautyleaf quality over that of unfertilized controls during both establishment and maintenance phases, but the high rate of the palm fertilizer was superior to either rate of the turf fertilizer for beautyleaf both during establishment and afterward. Leaf nutrient concentrations generally were poorly correlated with overall tree quality, but manganese concentrations differed significantly among treatments for all four species. Based on these results, fertilization of West Indian mahogany is not recommended, but live oak, black olive, and beautyleaf will benefit from fertilizer applied at the time of planting and after establishment.

Three species of tropical shrubs, bush allamanda ( Allamanda schottii ), ixora ( Ixora ‘Nora Grant’), and surinam cherry ( Eugenia uniflora ), were planted into a native sand soil and a calcareous fill soil in south Florida and were fertilized with a 24N–0P–9.2K (24–0–11) turf fertilizer or an 8N–0P–10K–6Mg plus micronutrients (8–0–12) palm fertilizer at rates of 10 or 20 g of nitrogen (N) per shrub four times per year. Two additional treatments using a 0–0–13.3K–6Mg plus micronutrients (0–0–16) palm fertilizer were applied at equivalent rates of potassium (K) (12.5 or 25 g/shrub of K) to that applied in the two 8–0–12 palm fertilizer treatments. Shrub size measurements, nutrient deficiency severity ratings, number of flowers, and shrub density ratings were determined at 6 months after planting (establishment period) and at 3 years after planting (maintenance phase). Data from these measured variables were subjected to principal component analysis to obtain a single measure of overall quality, namely, the scores for each plant on the first principal component. During the establishment period, ixora fertilized with the high rate of 8–0–12 had the highest quality on the sand soil, but there were no differences among treatments on the fill soil for this species or on either soil type for allamanda and surinam cherry. After 3 years of growth, ixora showed no differences in quality on either soil in response to the fertilizer treatments. On the sand soil, allamanda receiving the high rate of 24–0–11 or the low rate of 8–0–12 had significantly higher quality than unfertilized control plants, and the low rate of 8–0–12 produced the highest quality plants on the fill soil. Surinam cherry grown on sand soil had the highest qualities when fertilized with the high rates of either 24–0–11 or 8–0–12. In general, leaf nutrient concentrations were inversely correlated with overall shrub quality, with largest, highest quality plants having the lowest nutrient concentrations because of dilution effects. However, leaf manganese (Mn) concentrations were consistently within deficiency ranges for all species under most treatments, suggesting that Mn deficiency was stunting shrub growth on both soil types.

Potassium (K) deficiency is a widespread problem in palms growing in sandy or calcareous soils in southeastern United States. Its symptoms are highly conspicuous, reduce palm aesthetic appeal, are difficult to correct, and can be exacerbated by nitrogen (N) fertilization. The objective of this study was to determine the optimum fertilization rates and ratio for N and K in areca palm [ Dypsis lutescens (H. Wendl.) Beentje & J. Dransf. and Mexican fan palm ( Washingtonia robusta H. Wendl.) growing in a calcareous sandy fill soil. Both species had their highest quality when fertilized with 12.2 g·m ⁻² N and 12.2 g·m ⁻² K from controlled-release (2–3 month release) sources every 3 months. Actual N and K application rates were better predictors of palm quality than N:K ratio at the highest fertilization rates that would be recommended for field nursery production. However, at lower application rates more typical of those used for landscape palm maintenance, palm quality improved as the N:K application ratio was decreased.

Palms are an important component of landscapes in tropical, subtropical, and Mediterranean climates, but are anatomically very different from broadleaf trees. Very little is known about the movement and persistence of systemic fungicides into various parts of the palm canopy. This information is critical in selecting fungicides that may be effective against diseases that infect specific parts of the palm. In this study, potassium phosphite was injected into mature coconut palms ( Cocos nucifera ) at rates of 0, 30, 60, or 90 mL per tree. Various leaf tissue samples were collected periodically thereafter up through 60 weeks and were analyzed for phosphite concentrations. Phosphite moved quickly into leaflet tissue, but concentrations dropped off sharply between 1 and 5 weeks after injection. This drop in leaflet concentrations was balanced by a concomitant increase in spear leaf concentrations. Phosphite persisted at high concentrations in basal rachis tissue of both old and new leaves throughout the experiment. This suggests that this material may be useful for controlling diseases that infect spear leaves and petiole or rachis tissue, but not leaflets.

Palms are an increasingly important element in landscapes in the subtropical and warm temperate regions of the United States. Unfortunately, palms have very high nutritional requirements and rarely can be found without at least one nutrient deficiency, especially on the sandy and calcareous soils of the southeastern United States. These deficiencies are conspicuous and unsightly, reduce canopy size and vigor, and can become fatal. Current maintenance fertilizer recommendations for landscape palms in Florida growing in these soils entail four applications per year of an 8N–0.9P–10K–4Mg plus micronutrients palm fertilizer. However, because phosphorus (P) and nitrogen (N) are considered pollutants of ground and inland and coastal surface waters, it is important to apply only as much of these elements as necessary for palm health. This study showed that areca palms ( Dypsis lutescens ) can be grown in a native sand soil or in a calcareous fill soil without supplemental P and with no N applied during the rainy summer months of June through September when application of these elements may be legally restricted. It also demonstrated that the negative effects caused by high N:potassium (K) ratio turf fertilizers can be mitigated by adding a controlled release palm fertilizer that contains no N or P. Because of strong dilution effects in this study, leaf nutrient concentrations were found to be poor indicators of palm quality and nutritional status.

Natural distribution patterns of boron (B) among leaves within a canopy, among leaflets within a leaf, and within single leaflets were determined for coconut palm ( Cocos nucifera L.) and within leaves for paurotis palm [ Acoelorrhaphe wrightii (Griseb. & H. Wendl.) Becc.]. Leaf B concentrations did not vary significantly among leaves within the canopy or among leaflets within a single leaf for coconut palm, but basal leaflets of paurotis palm had higher B concentrations than central leaflets. Boron concentrations were significantly higher toward the tips of individual leaflets in both species. Application of Solubor to the soil significantly increased leaf B concentrations in all leaves of coconut palm after 2 months as well as in new leaves produced up to 6 months later. Application of Solubor as a leaf axil drench was much less effective in increasing foliar B concentrations than soil treatment.

Coconut palms ( Cocos nucifera ) in a field planting that were fertilized with a 8N–0.9P–10K–4Mg fertilizer four times per year or were never fertilized experienced chilling injury (CI) temperatures in 2008, 2009, and 2010. Fertilized coconut palms had significantly less foliar necrosis in each year than unfertilized palms and also retained more of their fruits. The number of leaves supported by each palm, a measure of potassium (K) deficiency severity, was improved by fertilization and was negatively correlated with percentage of necrosis of the foliage caused by cold temperatures. Nitrogen and K concentrations in leaf 1 of coconut palms were also negatively correlated with CI severity.

The roots of container-grown ornamental plants primarily are concentrated within the original container substrate root ball during the establishment period following transplanting into the landscape. Plants growing in container substrates containing pine bark or peatmoss have higher nitrogen (N) requirements than in most landscape soils due to microbial immobilization of N by these organic components. However, use of high-N fertilizers, such as those used in container production of ornamentals, can cause imbalances with potassium (K) and magnesium (Mg) when used on palms in sandy landscape soils. Areca palm ( Dypsis lutescens ) and chinese hibiscus ( Hibiscus rosa-sinensis ‘President’) that had been growing in containers were transplanted into a landscape soil to determine if high N fertilization during the establishment period could accelerate the rate of establishment without exacerbating K and Mg deficiencies. Although plants of both species had the darkest green color and largest size when continuously fertilized with high N fertilizer, this treatment did induce Mg deficiency in both species. Plant size and color for both species were highly correlated with cumulative N application rates, but also with initial N application rates, suggesting that high N fertilization during the first 6 months affected plant quality at 12 and 24 months after planting, even if high N fertilization was discontinued. However, continued use of a moderate N landscape palm maintenance fertilizer ultimately produced areca palm plants as good as those receiving high N during the establishment period.

Palms (Arecaceae) growing in containers have similar nutritional requirements as other tropical ornamental plants and grow well with fertilizers having an elemental ratio of 3N:0.4P:1.7K. However, palms growing in the landscape or field nurseries have very different nutritional requirements from dicotyledonous plants. Whereas nitrogen (N) is the primary limiting nutrient element in container production, potassium (K), manganese (Mn), magnesium (Mg), boron (B), and iron (Fe) deficiencies are more widespread than N deficiency in most landscape soils. Because palms have a single apical meristem, deficiencies of K, Mn, or B can be fatal. In addition to insufficient nutrients in the soil, palm nutrient deficiencies can be caused by high soil pH, certain types of organic matter, deep planting, poor soil aeration, cold soil temperatures, and nutrient imbalances. Correction of nutritional deficiencies in palms can take up to 2 years or longer and therefore prevention of deficiencies by proper fertilization is important. Research has shown that high N:K ratio fertilizers applied directly, or indirectly via application to adjacent turfgrass in a landscape, can exacerbate K and Mg deficiencies in palms, sometimes fatally. For sandy Atlantic coastal plain soils in the southeastern United States, an analysis of 8N–0.9P–10K–4Mg plus micronutrients has been recommended.