fan palm ( chinensis) have Palms on the Edge: Species and Strategies been used in many commercial land- for Landscape Utilization scape projects, whereas these species were once foreign to landscapes out- side of the immediate Gulf Coast or Garry V. McDonald1,4, Michael A. Schnelle2, the Houston, TX, metropolitan area and Michael A. Arnold3 before 1990 (G.V. McDonald, per- sonal observations). fan palm ( filifera) and jelly ADDITIONAL INDEX WORDS. , Palmae, cold temperature tolerance, palm (Butia capitata) have been suc- microclimate cessfully planted in northeastern (Zones 7b and 8a) at several SUMMARY. An emerging niche in landscape design is the creation of exotic venues in commercial and residential settings using unusual materials. For instance, the sites since the early 1990s (G.V. creation of a tropical looking pool area at a hotel, a southwestern desert look for a McDonald, personal observations). Mexican restaurant or an oasis for the consumer at a shopping venue can all be in In fact, jelly palm usually exhibits part achieved by the addition of specific . Palms (Arecaceae) can be an poor growth in areas lacking distinct important component of this effort, even in temperate landscapes. This article cooler winter temperatures such as focuses on issues related to the incorporation of palms in temperate landscapes. southern and the lower Rio Although palms are signature plants of tropical regions, a surprising number of Grande Valley of Texas (Arnold, species can be grown in U.S. Department of Agriculture cold hardiness zones 8, 7, 2008; Riffle, 1998). Another impor- or colder via a combination of appropriate genotype selection, attention to tant factor in the expanded range in microclimates in design specifications, and/or special cultural practices to mitigate which palms are planted is the heat the impact of cold temperatures. Cold-tolerant palms can be a critical design element, especially when paired with other lush tropical-appearing plants, to achieve island effect associated with large the goal of creating the illusion of an exotic tropical locale in temperate-climate urban areas. Heat islands are highly landscapes. Genotypic and site specification, careful attention to establishment developed areas that are hotter than requirements, and modified maintenance practices are critical determinants for nearby rural areas. It has been success that will be addressed. reported that the annual mean tem- perature of a city with a population alms used in the landscape can tropical areas (Arnold, 2008). In exceeding one million people can be create a tropical ambiance and addition, many homeowners use 1to3C warmer than surrounding Pare commonly used around palms in the landscape to lend an areas (Englehart and Douglas, 2003). commercial, retail, and private prop- exotic feel to backyard entertainment An even greater effect can occur erties in the southern , areas or to stress the southern vernac- at night when temperatures can be particularly along the coast from Vir- ular (Riffle, 1998; Riffle and Craft, 12 C higher than rural areas (Oke, ginia to Texas and in the southwest- 2003). 1987, 1997). ern United States and in many parts of Studies investigating tempera- In addition to the california fan California. They are especially popu- ture trends in Texas and the south- palm, which is native to isolated can- lar around swimming pools and sun western United States from the years yons in California and , other deck areas at hotels and resorts 1941 to 2000 show an overall warm- species of palms are indigenous to the (Arnold, 2008). Palms are also exten- ing trend post-1970 (Englehart and southern and southwestern United sively used around many ethnic food Douglas, 2003). Warmer winters States and have varying cold tolerance restaurants, such as those with Mex- associated with this trend have (Table 1). Florida has the greatest ican or cuisine, to create encouraged palm plantings away from variety of native palm species in the an atmosphere indicative of more the immediate Gulf Coastal areas into United States. (Black, 2003; Little, areas as far north as the Dallas-Ft. 1977). The needle palm (Rhapido- Worth metroplex in Texas [U.S. phyllum hystrix), a native to the south- The authors wish to thank Dr. Brent Pemberton for the use of his images. Department of Agriculture (USDA) eastern United States, is listed as Plant Hardiness Zones 8a-7b (USDA, hardy to USDA zone 6b (McClendon Mention of a trademark, proprietary product, or vendor does not constitute a guarantee or warranty 1990)] central and eastern et al., 2007). Dwarf palmetto ( of the product by the authors or the University of (Zones 7a and 7b), central minor) has a wide geographic distri- Arkansas, Oklahoma State University, or Texas A&M University, and does not imply its approval to the (Zone 7a), and across the upper south- bution in the United States with exclusion of other products or vendors that also may ern United States to the Atlantic provenances and recognized horticul- be suitable. seaboard. For instance, in College tural variants from coastal North Car- 1Assistant Professor of Landscape Horticulture, 316 Station, TX, which is listed as USDA olina to McCurtin and LeFlore Plant Science Building, Department of Horticulture, University of Arkansas, Fayetteville, AR 72701 Zone 8b, plantings of queen palm counties of Oklahoma to the hill coun- (Syagrus romanzoffiana) and chinese try region of central Texas (Arnold 2Professor and Extension Ornamentals/Floriculture Specialist, Horticulture and Landscape Architecture Department, 358 AG Hall, Stillwater, OK 74078- 06027 3Professor of Landscape Horticulture, Texas A&M Units University, Department of Horticultural Sciences, 207 Horticulture/Forestry Sciences Building, Col- To convert U.S. to SI, To convert SI to U.S., lege Station, TX 77843-2133 multiply by U.S. unit SI unit multiply by 4Corresponding author. E-mail: [email protected]. (F – 32) O 1.8 F C(1.8·C) + 32

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Table 1. Palm species native to the continental United States with associated USDA plant rating and native range. Common name Scientific name Zone rating Native range Everglades palmetto Accoelrrhaphe wrightii 8b Florida Florida silver palm argentata 9b Florida Keys thatch palm morrisii 9b Florida Needle palm hystrix 6b , Florida, Florida royal palm Roystonea regia 9a Florida Scrub palmetto Sabal etonia 7b Florida Texas palmetto 8b Texas Dwarf palmetto 7a Alabama, Arkansas, Florida, Georgia, , , North Carolina, Oklahoma, Texas, Cabbage palm 8a Florida, Georgia, South Carolina, North Carolina Saw palmetto repens 8a Alabama, Florida, Georgia, Mississippi Florida thatch palm radiata 9 Florida California fan palm Washingtonia filifera 9 Arizona, California

Table 2. Palm species commonly used in cool or temperate landscape plantings practical nor recommended for typi- and associated USDA plant hardiness zone ratings. cal landscape situations. Other Common name Scientific name Zone rating species may prove to have cold toler- ance, but because of lack of seed, poor Blue hesper palm Brahea armata 8a germination, slow growth, or other Jelly palm Butia capitata 8b factors such as expense, are hard to Mule palm · Butyagrus nabonnandii 8b find commercially and are usu- Bamboo palm Chamaedorea spp. 8b ally limited to palm enthusiasts’ Mediterranean fan palm humilis 8a collections. Chilean wine palm Jubaea chilensis 8a The use of palms in landscape Mazari palm ritchiana 8a designs is often grouped according to Canary island date palm canariensis 8b morphology or growth habit Date palm Phoenix dactylifera 8b (Arnold, 2008). Typical groupings Needle palm Rhapidophyllum hystrix 7a by leaf morphology would include Texas palmetto Sabal mexicana 8a feather palms (pinnate ) versus Dwarf palmetto Sabal minor 7a fan palms (palmate or costapalmate Cabbage palm Sabal palmetto 8a leaves). Among the most commonly Saw palmetto Serenoa repens 8a grown cold-hardy feather palms read- Queen palm Syagrus romanzoffiana 9a ily available in the U.S. nursery trade Windmill palm fortunei 7b are jelly palm, mule palm, date palm, Mexican fan palm Washingtonia filifera 8a and canary island date palm (Tables 1 9a and 2; Fig. 1). Generally speaking, the jelly palm is the most cold tolerant of these feather palms (Arnold, 2008). 2008; Little, 1977; McClendon et al., hardiness are provided in Table 2 Among the most cold hardy of the 2007; Vines, 1960). The increased (Arnold 2008; McClendon et al., commonly available fan palms are the use in palms grown outside traditional 2007). This species list is not exhaus- needle palm, dwarf palmetto, wind- areas has generated inquiries about tive, but represents palm species that mill palm, cabbage palm, mediterra- cold-hardy species, and their manage- are being grown commercially and nean fan palm, and texas palmetto ment and incorporation into landscape thus available to the landscape indus- (Tables 1 and 2; Fig. 2). Some of design. This article describes species try and homeowners. Microclimates, the most cold-hardy palms, such as that are cold hardy to at least USDA heat islands, or protected areas needle palm and the dwarf palmetto, Zone 8 and gives recommendations on may allow some species to be grown have only short trunks or fronds that management and cold protection in outside their stated hardiness zones. arise at or near ground level (Arnold, marginal climatic areas. Other species may be found to 2008). These short palms have their have cold tolerance greater than apical meristems (growing points) Cold-hardy landscape species reported due to heat island effects or near the soil, which is presumed to It has been estimated that as local microclimates. ‘‘Heroic’’ efforts, help in insulting them from cold many as 100 species of palms will such as portable greenhouses and temperatures compared with exposed tolerate temperatures as low as 20 F heating tapes, are also employed to buds positioned high above the (Frankco, 2003). Selected palm spe- grow palms far outside the normally ground in taller palms. However, cies grown in warm to cool temperate accepted range for a given species, there is no correlation of trunk regions and their reported cold but these efforts are not always height with hardiness as arborescent

706 • October–December 2009 19(4) when root uptake of water is mini- mum during cold temperatures.

Mitigating cold damage to palms Proper species selection is the best way to ensure survival of palms in colder climates, and the correct species may vary with regions that have the same USDA plant hardiness zone rating (Arnold, 2008). Of sim- ilar importance is the selection of the correct site and microclimate in the landscape, which can affect palm sur- vival in marginal areas (Francko, 2000). Placing plants on the south side of a wall or in courtyards can trap radiant heat from the sun and there- fore reduce the amount of time a palm is exposed to subfreezing tempera- tures. Walls, evergreen hedges, or overhead tree canopies can provide windbreaks to minimize wind expo- sure and reduce leaf desiccation dam- Fig. 1. Commonly grown cold-hardy feather palms readily available in the U.S. nursery trade are jelly palm (A), date palm (B), and canary island date palm (C). age. Avoid planting palms in low areas Images copyrighted by M.A. Arnold. where poor air drainage may form frost pockets. Active winter protection techni- cold-hardy palms such as the windmill Factors affecting cold ques can include simple shelters, wind palm, cabbage palm, or texas pal- tolerance barriers, natural insulation materials, metto that can be grown in USDA trunk and/or foliar wrapping, tem- plant hardiness zones 7 or 8 (Tables 1 Many factors can influence cold porary greenhouses, and various heat- and 2). There are even cold-hardy hardiness in palms (Ingram and ing devices (Frankco, 2003). Simple palms with multiple trunks, such Yeager, 1982, Ingram et al., 1985). shelters include placing a blanket or as mediterranean fan palm and These factors include acclimation, plastic bubble sheeting over the palm. mazari palm (Nannorrhops ritchiana) cool down conditioning, and mineral Wind barriers, such as burlap or land- (Fig. 3). nutritional status. Duration of cold scape fabric supported around a palm Selection of provenances from may be as important as the absolute can reduce desiccating winds. Wind colder portions of marginal species minimum temperature exposure in barriers or simple shelters can be ranges or little-known species that determining survival. Certain palm augmented by placing loose mulch may prove cold hardy are beginning species may be able to survive low or other loose insulating materials to enter the U.S. landscape trade, temperatures of a short duration. around the palm. When such a tech- such as the kumaon palm (Trachycar- Humidity and precipitation levels nique is used, some sources cite spray- pus takil) from the Himalayan re- can also have an effect, with many ing the stem and bud with a broad gion of India. These genotypes may palms from semiarid regions suc- spectrum fungicide and/or bacteri- offer even greater diversity of cold- cumbing to subfreezing temperatures cide to reduce chances of bud rot hardy selections in the coming years in the humid, higher rainfall south- (Frankco, 2003). Layering accumu- for landscape designers. There are eastern United States, while remain- lated pruned dead palm leaves in an many examples of trees grown for ing unaffected at similar subfreezing overlapping thatched-roof or ‘‘tiki forestry purposes where prove- temperatures in semiarid climates hut’’ pattern can help to divert rainfall nances from high latitudes or high such as northern and central Texas and snow or ice melt from collecting elevations have been used to extend and eastern Oklahoma (McClendon in crevices in the bud and immature the use of species to colder envi- et al., 2007). The mazari palms spear leaves where repeated freezing ronments than is typical for the spe- (Frankco, 2003) and california fan and thawing may damage tissues lead- cies (Zobel et al., 1987). There palms (Arnold, 2008) are examples ing to crown rots. would appear to be good potential of palms that survive winters better More elaborate cold protection for this strategy to be employed to in drier climates. A site with good methods such as trunk and foliar identify genotypes of native and air drainage with the avoidance of wrapping or heating trunks or even exotic species of palms for which their frost pockets can mitigate cold tem- the whole plant using a temporary reliable use could be extended to peratures. Another factor affecting greenhouse can be effective for im- more northern locations in U.S. survival is the amount of leaf desic- portant specimen palms, but will re- landscapes. cation from high winds that occurs quire much more effort and expense

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(Frankco, 2003). It is critical to pro- tect the trunk and apical meristem point, as most palms species have a single growing point per trunk. Once this growing point is damaged or killed, palms do not typically produce the adventitious buds that allow many dicotyledonous trees and shrubs to recover from severe cold damage (Arnold, 2008). Trunks and growing points can be wrapped with a variety of insulating materials (Fig. 4, A–C). Palm leaves are often tied up in a tight bunch around and above the growing point to help insulate the apical mer- istem and to prevent desiccation of the leaves (Fig. 4C). Heating pads, heat tape or pipe wraps, low wattage heat cables, or strings of lights can be used for high value or prominent specimens. Frankco and Wilhoite (2002) report that strings of C9 lights alone wrapped around palms can add 1to2C to the ambient air temper- ature and that using them under insulating wrap increases the temper- atures 6 to 9 C. Of course, temporary green- houses ranging from plastic covers with a light bulb to more elaborate portable glass or plastic structures can be highly effective in sheltering palms during cold temperatures. One advantage of such structures is that they may also provide some heat to the root zone (Frankco, 2003). Avoiding frozen soils will help to ensure moisture flow to the leaves, Fig. 2. Commonly grown cold-hardy fan palms are the needle palm (A), windmill palm (B), mediterranean fan palm (C), dwarf palmetto (D), cabbage palm (E), reducing the chances for desiccation and texas palmetto (F). Images copyrighted by M.A. Arnold. damage. Future opportunities for incor- poration of palms in cooler climate landscapes appear favorable as new introductions, from underused spe- cies to more cold-hardy provenance selections from more commonly grown species, are introduced to the trade. Various maintenance practices can be implemented to expand these design possibilities even further. Nearly all palms can be successfully established in at least one cold hardi- ness zone lower (colder) through securing a microclimate and utiliza- tion of other techniques described above. Cold-hardy palms add a unique character to landscapes than cannot be duplicated with other species, resulting in a greatly ex- panded plant palette for landscape designers and architects, and creating Fig. 3. Cold-hardy palms with multiple trunks, such as mediterranean fan palm (A) a new niche market for the nursery and mazari palm (B). Images copywrited by M.A. Arnold. industry.

708 • October–December 2009 19(4) Ingram, D.L., T.H. Yeager, and R.L. Hummel. 1985. Cold protection for nursery crops. Inst. Food Agr. Sci., Univ. Florida, Bul. 201. Little, E.L., Jr. 1977. Atlas of United States trees. Vol. 4. Minor eastern hard- woods. U.S. Dept. Agr. Misc. Publ. 1342. McClendon, T., W. Roberds, and J. LeVert. 2007. Hardy palms for the Southeast. Southeastern Palm Soc., Apison, TN. Oke, T.R. 1987. Boundary layer climates. Routledge, . Oke, T.R. 1997. Urban climates and Fig. 4. Mexican fan palm trunks (A) and apical meristems (B) can be wrapped with global environmental change, p. 273– a variety of insulating materials. Tying the leaves up around the apical meristem 287. In: R.D. Thompson and A. Perry (C) can also be used to help protect the growing point and minimize desiccation (eds.). Applied climatology: Principles damage. Images courtesy of H.B. Pemberton. and practices. Routledge, London. Riffle, R.L. 1998. The tropical look: An hardy palms in southwestern . Palms encyclopedia of dramatic landscape Literature cited 44(1):37–46. plants. Timber Press, Portland, OR. Arnold, M.A. 2008. Landscape plants for Frankco, D.A. 2003. Palms won’t grow Riffle, R.L. and P. Craft. 2003. An ency- Texas and environs, 3rd ed. Stipes Pub- here and other myths: Warm-climate clopedia of cultivated palms. Timber lishing, Champaign, IL. plants for cooler areas. Timber Press, Press, Portland, OR. Black, R.J. 2003. Native Florida plants for Portland, OR. U.S. Department of Agriculture. 1990. home landscapes. Inst. Food Agr. Sci., Frankco, D.A. and S. Wilhoite. 2002. USDA plant hardiness zone map. U.S. Univ. Florida, Dept. Environ. Hort. Fact Cold-hardy palms in southwestern Ohio: Dept. Agr. Misc. Publ. No. 1475. Sheet ENH-25. Winter damage, mortality, and recovery. Vines, R.A. 1960. Trees, shrubs, and Englehart, P.J. and A.V. Douglas. 2003. Palms 46(1):5–13. woody vines of the Southwest. Univ. Urbanization and seasonal temperature Ingram, D.L. and T.H. Yeager. 1982. Texas Press, Austin, TX. trends: Observational evidence from a Cold protection of ornamental plants. Zobel, B.J., G. van Wyr, and P. Stahl. data-sparse part of . Int. Inst. Food Agr. Sci., Univ. Florida, Dept. J. Climatol. 23(10):1253–1263. 1987. Growing exotic forests. Wiley, New Ornamental Hort. Fact Sheet OH-1. York. Francko, D.A. 2000. Effect of microcli- mate variation on cultivation of cold-

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