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Donald R. Hodel

Comprising a natural and distinctive group, palms (Arecaceae) differ from other woody plants in their structure and growth features that require or offer unique and sometimes advantageous landscape management opportunities. Although palms are a mostly tropical group that lacks dormancy and cold tolerance, there are numerous species possessing sufficient cool or cold hardiness to be suitable for landscaping in subtropical and even some temperate areas. The habit of palms is largely dependent on the number of stems and the length to which they elongate. There are solitary or multistemmed palms and tree or shrub palms. Regardless of habit, there is typically only one growing point or apical meristem per palm stem. Thus, multistemmed palms may be especially versatile landscape subjects because in many instances, one is able to control height and density by judicious removal of stems. The uniquely constructed palm stem, with growth restricted to its extremities (leaves and inflorescences distally, roots proximally, and wholly the product of primary growth), is composed largely of numerous, dispersed, hard, fibrous-sheathed, vascular bundles embedded in a matrix of water- and carbohydrate-storing parenchyma cells. Often likened to a steel-reinforced concrete column and offering tremendous strength and resiliency, palm stems lack a peripheral vascular cambium and, thus, capability for secondary growth, meaning they do not thicken much once they elongate vertically and there is no ability to repair damaged tissue. Thus, care should be taken when performing horticultural tasks to avoid making wounds (which are permanent, unsightly, and potential entry sites for pests and diseases) and damaging the sole apical meristem. A palm's total photosynthetic and reproductive efforts are concentrated into relatively few but large organs (leaves and inflorescences respectively), offering a unique opportunity to capture an entire year's worth of potential leaf, flower, and fruit litter before it falls into the landscape. The palm root system is adventitious and composed of numerous, small- to medium-sized, nonwoody roots. All primary roots are of a more or less constant diameter and arise independently from an area at or near the base of the stem called the root initiation zone. Because of these root system characteristics and the ability of their trunks to store water and carbohydrates, palms are relatively easy to transplant—even large specimens with small root balls—resulting in instant, mature landscapes.

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Donald R. Hodel and Dennis R. Pittenger

The objectives of this study were to investigate the responses of several groundcovers to periodic mowing and determine which ones benefit from mowing in terms of aesthetic quality, density, height and thatch development.

Eight species were transplanted on 30 cm centers in September. Five species had become fully grown in 10 months and were mowed to either a 5 cm or 10 cm height. Four species received a second mowing at 10 cm 8 or 11 months later. The remaining three species became fully grown in 18 or 21 months and were mowed at 10 cm at that time. Visual quality scores were recorded monthly, as were average overall plant and thatch heights. Lantana, Osteospermum and Verbena expressed little or no long-term loss in visual quality, while their height and thatch growth were controlled well when mowed in the spring-summer period. Height and thatch growth were controlled well in Drosanthemum and Aptenia, but visual quality was unacceptable. Spring mowing appears to produce reductions in height and thatch with no significant loss in quality of Myoporum and Baccharis but with significant loss in quality of Rosemarinus.

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Donald R. Hodel, A. James Downer, and Dennis R. Pittenger

The optimum time to transplant palms (Arecaceae) is at the beginning of the warm season in temperate climates or at the beginning of the rainy season in tropical climates if irrigation is unavailable. Careful and proper handling, including covering and protecting the leaves and root ball during transplanting to protect them from injury and drying out and immediate planting upon arrival at the new site, helps to ensure rapid and successful establishment. A root ball extending out from the trunk for 30 cm appears to be adequate for most solitary-stemmed species. Larger root balls may be necessary for multistemmed or unusually tall or large specimens. Tying up leaves facilitates handling during digging, transport, and planting, but it is best to untie them after planting. In most instances leaf removal during transplanting does not appear to be advantageous, and it is probably best to remove leaves only when they die and turn brown. Too deep or too shallow planting lowers transplant success and stresses palms, making them susceptible to diseases, disorders, and pests. Amending the backfill when transplanting palms is not beneficial in most cases. However, mulch applied around the base of the palm after transplanting can enhance growth. Keeping the soil, backfill, and surrounding site soil evenly moist helps to ensure successful establishment.

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D.R. Pittenger, Donald R. Hodel, and David A. Shaw

Non-turf ground-covers occupy a significant portion of the landscape, and understanding their water requirements is important when water conservationism being practiced. Six groundcover species (Baccharis pilularis `Twin Peaks', Drosanthemum hispidum, Vinca major Gazania hybrid, Potentilla tabernaemontani and Hedera helix `Needlepoint') representing a range of observed water needs were evaluated under different levels of irrigation based on percentages of real-time reference evapotranspiration.

Treatments of 100%, 75%, 50% and 25% of ETO were applied during 1989 while treatments of 50%, 40%, 30% and 20% of ETO were applied during 1990. Plant performance ratings in the first year indicated that 50% of ETO was the minimum treatment which resulted in acceptable plan aesthetics for all species except for Drosanthemum which performed equally well at each treatment. Significant differences in performance did occur among and within species at the different treatments. Results from 1990 will reveal which species might maintain aesthetic appearance at irrigation levels between 50% and 20% of ETO. These results will be presented and discussed in terms of their significance to species selection and total landscape irrigation management.

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Dennis R. Pittenger, Donald R. Hodel, and A. James Downer

Successful reestablishment of transplanted palms [members of the Arecaceae (Palmae)] depends on rapid regeneration of roots, avoiding injury and desiccation of the trees during transit and handling, and maintaining sufficient soil moisture around the root balls after transplanting. Since landscape contractors and nurserymen spend considerable resources and labor transplanting specimen palms, understanding the seasonality of palm root growth, how palm roots respond when trees are dug, and the effects of canopy manipulation during transplanting will enable them to adopt effective and rational transplanting practices. This manuscript provides a review of research findings that can be applied to maximize reestablishment of transplanted specimen palms.

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A. James Downer, Donald R. Hodel, and Maren J. Mochizuki

Landscape palms (Arecaceae) are pruned (i.e., leaves removed) to avoid the hazard of falling fronds, to remove diseased or brown leaves, and, in some cases, to minimize growth by diminishing photosynthetic capacity. In studies at two California locations (Long Beach and Irvine), even complete leaf removal every 3 to 4 months for 18 to 21 months resulted in similar new leaf production by queen palm (Syagrus romanzoffiana) or windmill palm (Trachycarpus fortunei) compared with no pruning or “10 and 2” pruning (industry standard pruning referring to the palm canopy visually beginning at the 10 o'clock position and ending at the 2 o'clock position on a clock face). By contrast, complete leaf removal reduced the number of new leaves of california fan palm (Washingtonia filifera), young mexican fan palm [MFP (Washingtonia robusta)], and taller, more mature MFP by 30%, 23%, and 21% compared with no pruning and “10 and 2” pruning. Leaf petiole length, leaf blade length, leaf blade width, and total palm height were also reduced 19% to 43% after complete leaf pruning compared with no and “10 and 2” pruning of young and more mature MFP. Although “10 and 2” pruning did not reduce growth of any palms, pruning all but the four newest leaves reduced leaf petiole length by 21% for the taller MFP. An important consideration for palm disease control is that tools used for pruning may harbor pathogen inoculum. Flaming pruning saws with a propane torch for 10 s reduced total fungal colonies and palm pathogenic fungi recovered on a selective medium by 95%. Increasing our understanding of palm response to leaf removal and how to minimize unintended consequences of pruning, such as the spread of disease, is an important part of improving palm maintenance.

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Dennis R. Pittenger, A. James Downer, Donald R. Hodel, and Maren Mochizuki

The responses of five landscape palm species [king palm (Archontophoenix cunninghamiana), mediterranean fan palm (Chamaerops humilis), queen palm (Syagrus romanzoffiana), chinese windmill palm (Trachycarpus fortunei), and california fan palm (Washingtonia filifera)] to three levels of irrigation [50%, 25%, and 0% (no irrigation) of reference evapotranspiration] were evaluated in a coastal mediterranean climate in Irvine, CA. Cumulative leaf production varied greatly among the species, but only king and chinese windmill palms produced more leaves with additional irrigation. All species maintained at least minimally acceptable visual quality at the no-irrigation treatment. Mediterranean fan and california fan palms expressed near optimum performance with no irrigation. Many established landscape palms can maintain at least minimally acceptable appearance for an extended period with little or no supplemental water in coastal mediterranean climates. However, when rainfall plus irrigation is less than 50% of reference evapotranspiration, sensitive landscape palms could be expected to appear less attractive and grow less. Responses of palm species in this study were similar to those of many other landscape tree and shrub species, but the water needs of landscape palms are considerably less than those of commercial date palm (Phoenix dactylifera), oil palm (Elaeis guineensis), or coconut palm (Cocos nucifera).

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D.R. Pittenger, Donald R. Hodel, David A. Shaw, and D.B. Holt

A previous field study had shown that Baccharis pilularis, `Twin Peaks', Drosanthemum hispidum, Vinca major, Gazania hybrid, Potentilla tabernaemontanii and Hedera helix, `Needlepoint', express no loss in relative aesthetic appearance when irrigated for one season at 50% of reference evapotranspiration (ETo), but three species did not perform acceptably at 25% of ETo. In this study these six species were grown in the field for 16 months under treatments of 50%, 40%, 30% and 20% of real-time ETo to more closely determine their minimum irrigation needs.

Analysis of seasonal plant performance ratings indicates that for Vinca, Gazania and Potentilla there is no significant increase in relative performance when irrigated at more than 30% of ETo. Baccharis, Drosanthemum and Hedera exhibited no significant improvement in performance when irrigated above 20% of ETo. A general decline in aesthetic appearance and performance was observed during the study in Gazania and Potentilla at all treatments, suggesting that their long-term minimum irrigation need may be more than 50% of ETo.

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Dennis R. Pittenger, David A. Shaw, Donald R. Hodel, and William E. Richie

The performance of six landscape groundcover species was evaluated when irrigated at 30% of ET0 at irrigation schedules of three times per week, once per week, once every 2 weeks, and once every 4 weeks. Potentilla tabernaemontani could not be sustained under any of the treatments. For the other species (Baccharis pilularis, Drosanthemum hispidum, Vinca major, Osteospermum fruticosum, and Hedera helix) there were no season-long differences in a species' performance or density due to irrigation frequency, but there were significant differences among species across irrigation treatments. Drosanthemum and Osteospermum provided good overall appearance and density consistently through the season. Baccharis maintained acceptable performance most of the irrigation season, while Vinca and Hedera became unacceptable in appearance in mid-season. Soil moisture content differed among species, but was not consistently different between irrigation treatments.