Landscape Plant Selection Criteria for the Allergic Patient
2018; Elsevier BV; Volume: 6; Issue: 6 Linguagem: Inglês
10.1016/j.jaip.2018.05.020
ISSN2213-2201
AutoresBrett J. Green, Estelle Levetin, W. Elliott Horner, Rosa Codina, Charles S. Barnes, Warren V. Filley,
Tópico(s)Lichen and fungal ecology
ResumoPatients with pollen-related allergies are concerned about the species within their landscape that provoke their symptoms. Allergists are often asked for guidance but few information sources are available to aid patients in the recognition of allergenic plants and strategies to avoid personal exposure to them. Landscaping and horticultural workers also have few reliable guidance references, and what is available usually extols the virtues of the plants rather than their negative features. The aim of this article was to provide the results of the Landscape Allergen Working Group that was formed by the AAAAI Aerobiology Committee, which aimed to fill these existing knowledge gaps and develop guidance on producing a low-allergenic landscape. Within the context that complete pollen avoidance is unrealistic, the workgroup introduces selection criteria, avoidance strategies, and guidance on low-allergenic plants that could be selected by patients to reduce the overall pollen burden in their landscape environment. Specific focus is placed on entomophilous plants, which require insects as dispersal vectors and generally produce lower quantities of pollen, compared with anemophilous (wind-pollinated) species. Other biological hazards that can be encountered while performing landscaping activities are additionally reviewed and avoidance methods presented with the aim of protecting gardeners, and workers in the landscape and horticulture industries. The guidance presented in this article will ultimately be a helpful resource for the allergist and assist in engaging patients who are seeking to reduce the burden of allergen in their landscape environment. Patients with pollen-related allergies are concerned about the species within their landscape that provoke their symptoms. Allergists are often asked for guidance but few information sources are available to aid patients in the recognition of allergenic plants and strategies to avoid personal exposure to them. Landscaping and horticultural workers also have few reliable guidance references, and what is available usually extols the virtues of the plants rather than their negative features. The aim of this article was to provide the results of the Landscape Allergen Working Group that was formed by the AAAAI Aerobiology Committee, which aimed to fill these existing knowledge gaps and develop guidance on producing a low-allergenic landscape. Within the context that complete pollen avoidance is unrealistic, the workgroup introduces selection criteria, avoidance strategies, and guidance on low-allergenic plants that could be selected by patients to reduce the overall pollen burden in their landscape environment. Specific focus is placed on entomophilous plants, which require insects as dispersal vectors and generally produce lower quantities of pollen, compared with anemophilous (wind-pollinated) species. Other biological hazards that can be encountered while performing landscaping activities are additionally reviewed and avoidance methods presented with the aim of protecting gardeners, and workers in the landscape and horticulture industries. The guidance presented in this article will ultimately be a helpful resource for the allergist and assist in engaging patients who are seeking to reduce the burden of allergen in their landscape environment. AAAAI Position Statements, Work Group Reports, and Systematic Reviews are not to be considered to reflect current AAAAI standards or policy after five years from the date of publication. The statement below is not to be construed as dictating an exclusive course of action nor is it intended to replace the medical judgment of healthcare professionals. The unique circumstances of individual patients and environments are to be taken into account in any diagnosis and treatment plan. The statement reflects clinical and scientific advances as of the date of publication and is subject to change.For reference only. AAAAI Position Statements, Work Group Reports, and Systematic Reviews are not to be considered to reflect current AAAAI standards or policy after five years from the date of publication. The statement below is not to be construed as dictating an exclusive course of action nor is it intended to replace the medical judgment of healthcare professionals. The unique circumstances of individual patients and environments are to be taken into account in any diagnosis and treatment plan. The statement reflects clinical and scientific advances as of the date of publication and is subject to change. For reference only. Plants produce reproductive propagules termed pollen that are aerosolized into local air masses. Although pollen deposition generally occurs in proximity to the source,1Molina R.T. Rodríguez A.M. Palaciso I.S. López F.G. Pollen production in anemophilous trees.Grana. 1996; 35: 38-46Crossref Scopus (153) Google Scholar there are examples of pollen transportation across regions, states, and even entire countries.2Levetin E. Buck P. Evidence of mountain cedar pollen in Tulsa.Ann Allergy. 1986; 56: 295-299PubMed Google Scholar, 3Rogers C.A. Levetin E. Evidence of long-distance transport of mountain cedar pollen into Tulsa, Oklahoma.Int J Biometeorol. 1998; 42: 65-72Crossref Scopus (42) Google Scholar, 4Levetin E. Van de Water P.K. Pollen count forecasting.Immunol Allergy Clin North Am. 2003; 23: 423-442Abstract Full Text Full Text PDF PubMed Scopus (23) Google Scholar, 5Wallin J.-E. Segerström U. Rosenhall L. Bergmann E. Hjelmroos M. Allergic symptoms caused by long-distance transported birch pollen.Grana. 1991; 30: 265-268Crossref Scopus (46) Google Scholar, 6Lorenzo C. Marco M. Paola D.M. Alfonso C. Marzia O. Simone O. Long distance transport of ragweed pollen as a potential cause of allergy in central Italy.Ann Allergy Asthma Immunol. 2006; 96: 86-91Abstract Full Text PDF PubMed Google Scholar, 7Šikoparija B. Skjøth C. Kübler K.A. Dahl A. Sommer J. Radišić P. et al.A mechanism for long distance transport of Ambrosia pollen from the Pannonian Plain.Agr Forest Meteorol. 2013; 180: 112-117Crossref Scopus (63) Google Scholar, 8Mohanty R.P. Buchheim M.A. Anderson J. Levetin E. Molecular analysis confirms the long-distance transport of Juniperus ashei pollen.PLoS One. 2017; 12: e0173465Crossref PubMed Scopus (29) Google Scholar Personal pollen exposure occurs in both urban and rural settings, and airborne levels vary among species, with highest concentrations usually reported during the spring (trees), summer (weeds and grass), and fall (weeds). The lowest concentrations occur during winter. Personal exposure to pollen grains, their associated fragments, and allergen released into the air can result in allergic sensitization, which can cause allergic rhinoconjunctivitis, and allergic asthma. Many allergic patients often seek guidance from allergists to assist in plant identification or enquire about avoidance strategies to minimize personal exposure. In response to questions and concerns that were voiced to the AAAAI Aerobiology Committee, the Landscape Allergen Working Group was formed and consisted of clinicians and researchers with expertise in allergy, occupational health, aerobiology, and botany. The aim of the workgroup was to address the existing knowledge gaps and provide guidance on strategies that patients and workers could use to reduce the burden of pollen exposure within their personal landscape or workplace. This approach focused on design strategies that would result in the selection of candidate plants with a low pollen yield. These design characteristics will ultimately assist the patient and the worker but also provide a new resource that clinicians could use to assist patients during the design phases of a low-allergenic landscape. Although eliminating personal pollen exposure is not completely feasible given regional and background sources, the guidelines and avoidance strategies developed by the workgroup that are outlined in this article could assist in the local reduction of pollen exposure and associated plant hazards. It is important to note that the design and production phases of a low-allergenic landscape require a basic understanding of plant biology concepts and botany. Understanding these concepts will further enhance the patient's ability to participate in the selection of plant species that produce the least amount of pollen but are able to grow and persist in their landscape environment. An algorithm and selection guidelines that are intended to assist the patient in the selection of a nonallergenic plant landscape are also included in this article. Methods to reduce exposure to other plant-related hazards are additionally discussed. Many land plants disperse reproductive propagules through the air. Mosses and ferns produce spores that are spread by air currents, although in small amounts and usually over limited areas. In contrast, conifers and some flowering plants can produce large quantities of pollen that can reach high local concentrations1Molina R.T. Rodríguez A.M. Palaciso I.S. López F.G. Pollen production in anemophilous trees.Grana. 1996; 35: 38-46Crossref Scopus (153) Google Scholar, 9Traverse A. Paleopalynology. Unwin Hyman, Boston, Mass1988Google Scholar and can travel in air masses over regional and continental distances.2Levetin E. Buck P. Evidence of mountain cedar pollen in Tulsa.Ann Allergy. 1986; 56: 295-299PubMed Google Scholar, 3Rogers C.A. Levetin E. Evidence of long-distance transport of mountain cedar pollen into Tulsa, Oklahoma.Int J Biometeorol. 1998; 42: 65-72Crossref Scopus (42) Google Scholar, 4Levetin E. Van de Water P.K. Pollen count forecasting.Immunol Allergy Clin North Am. 2003; 23: 423-442Abstract Full Text Full Text PDF PubMed Scopus (23) Google Scholar, 5Wallin J.-E. Segerström U. Rosenhall L. Bergmann E. Hjelmroos M. Allergic symptoms caused by long-distance transported birch pollen.Grana. 1991; 30: 265-268Crossref Scopus (46) Google Scholar, 6Lorenzo C. Marco M. Paola D.M. Alfonso C. Marzia O. Simone O. Long distance transport of ragweed pollen as a potential cause of allergy in central Italy.Ann Allergy Asthma Immunol. 2006; 96: 86-91Abstract Full Text PDF PubMed Google Scholar, 7Šikoparija B. Skjøth C. Kübler K.A. Dahl A. Sommer J. Radišić P. et al.A mechanism for long distance transport of Ambrosia pollen from the Pannonian Plain.Agr Forest Meteorol. 2013; 180: 112-117Crossref Scopus (63) Google Scholar, 8Mohanty R.P. Buchheim M.A. Anderson J. Levetin E. Molecular analysis confirms the long-distance transport of Juniperus ashei pollen.PLoS One. 2017; 12: e0173465Crossref PubMed Scopus (29) Google Scholar Pollen from both conifers and flowering plants may be of allergenic clinical relevance at the community level. Conifers are gymnosperms, or plants that produce naked seeds that are not enclosed in a fruit. The pine, spruce, and fir cones are seed-bearing (female or megasporangial) strobili. These cones are aggregates of scales, with each scale, or bract, bearing 2 naked seeds on the axial surface. The smaller, more ephemeral-male (microsporangial) cones of pine, juniper, spruce, and fir are also aggregates of scales but bear pollen sacs on the axial surface of each scale. The male cones drop as soon as the pollen sheds, whereas the female cones are retained for the season to allow maturation of the seeds. Figure 1 shows the megasporangial and microsporangial arrangements of Loblolly pine (Figure 1, A) and microsporangial cones of mountain cedar (Figure 1, B), respectively. Flowering plants, or angiosperms, produce seeds from ovules encased in an ovary (fruit) rather than naked on a scale. Flowering plants include those with the ancestral “magnolia-type” flowers, inconspicuous flowers such as those produced by grasses, ragweed, and maple as well as the morphologically complex flowers of orchids and sunflowers. Examples of these flowering structures are depicted in Figure 1, C, D, E, F, and G. All flowering plants produce pollen and ovaries although there is a large array of variability in how this is achieved. For example, tulip, poppy, orchid, and magnolia flowers include all the reproductive and accessory parts in one flower called a “perfect” flower. These parts are arranged in concentric whorls. The outermost are the sepals, inside these are the petals, next is a whorl of stamens, and in the center are 1 or more carpels containing the ovaries and stigmas. The stamens produce pollen, and the stigmas receive pollen through a deposition process. In contrast, other angiosperms have separate male and female flowers containing only stamens or carpels, respectively. These are known as “imperfect” (and incomplete) flowers. Staminate and carpellate flowers may occur on the same plant, which is called monoecious, or on separate plants, which are known as dioecious. Among wind-pollinated plants, petals and sepals usually are vestigial or absent, and many species are monoecious. Showy perfect flowers are almost always entomophilous (Figure 1), whereas imperfect, and inconspicuous flowers like grasses (Figure 1) and ragweed are typically anemophilous. In many anemophilous plants, the flowers occur in inflorescences (clusters of small flowers) rather than as solitary flowers. There are many different types of inflorescences, based on the arrangement of the flowers in the cluster. Many wind-pollinated trees including oaks, birches, and mulberries produce male catkins; each catkin is a long, slender inflorescence of staminate flowers (Figure 1, C). A list of anemophilous plants and common aeroallergens is presented in Table I. Most anemophilous plants produce large quantities of pollen grains; for example, an elm tree can produce roughly 1 billion pollen grains and an oak tree 500 billion.1Molina R.T. Rodríguez A.M. Palaciso I.S. López F.G. Pollen production in anemophilous trees.Grana. 1996; 35: 38-46Crossref Scopus (153) Google Scholar Most pollen released from these plants falls in proximity to the source, with estimates of 90% deposited from less than 100 m to 2.7 km.9Traverse A. Paleopalynology. Unwin Hyman, Boston, Mass1988Google Scholar The remaining percent can become entrained in the turbulent layer of the atmosphere and travel from hundreds to thousands of kilometers under certain meteorological conditions. A few known examples of long distance transport of relevant aeroallergens include mountain cedar pollen in North America,2Levetin E. Buck P. Evidence of mountain cedar pollen in Tulsa.Ann Allergy. 1986; 56: 295-299PubMed Google Scholar, 3Rogers C.A. Levetin E. Evidence of long-distance transport of mountain cedar pollen into Tulsa, Oklahoma.Int J Biometeorol. 1998; 42: 65-72Crossref Scopus (42) Google Scholar, 4Levetin E. Van de Water P.K. Pollen count forecasting.Immunol Allergy Clin North Am. 2003; 23: 423-442Abstract Full Text Full Text PDF PubMed Scopus (23) Google Scholar, 8Mohanty R.P. Buchheim M.A. Anderson J. Levetin E. Molecular analysis confirms the long-distance transport of Juniperus ashei pollen.PLoS One. 2017; 12: e0173465Crossref PubMed Scopus (29) Google Scholar birch pollen in northern Europe,5Wallin J.-E. Segerström U. Rosenhall L. Bergmann E. Hjelmroos M. Allergic symptoms caused by long-distance transported birch pollen.Grana. 1991; 30: 265-268Crossref Scopus (46) Google Scholar and ragweed pollen in various parts of eastern and central Europe.6Lorenzo C. Marco M. Paola D.M. Alfonso C. Marzia O. Simone O. Long distance transport of ragweed pollen as a potential cause of allergy in central Italy.Ann Allergy Asthma Immunol. 2006; 96: 86-91Abstract Full Text PDF PubMed Google Scholar, 7Šikoparija B. Skjøth C. Kübler K.A. Dahl A. Sommer J. Radišić P. et al.A mechanism for long distance transport of Ambrosia pollen from the Pannonian Plain.Agr Forest Meteorol. 2013; 180: 112-117Crossref Scopus (63) Google Scholar In fact, a recent molecular detection study has tracked mountain cedar (Juniperus ashei) pollen from Texas into London, Ontario, in Canada.8Mohanty R.P. Buchheim M.A. Anderson J. Levetin E. Molecular analysis confirms the long-distance transport of Juniperus ashei pollen.PLoS One. 2017; 12: e0173465Crossref PubMed Scopus (29) Google ScholarTable IRecognized allergenic plants according to US floristic zoneZone nameTrees/shrubsGrassesWeedsNorthern ForestAlder, aspen, birch, cedar, fir, juniper, maple, pine, willowLow levels of pasture grassesChenopods, mugwort, nettle, pigweedsEastern AgriculturalAlder, ash, beech, birch, box elder, elm, hawthorn, hickory, locust, maple, oak, pines, red cedar, sycamore, walnutNortheast states: brome, fescue, orchard, ryegrass, sweet vernal, timothySouthern states: Bermuda, JohnsonDock, lamb's quarter, mugwort, nettle, pigweed, plantain, sheep sorrel; Short, giant, southern, and western ragweedCentral PlainsAs in Eastern Agricultural; box elder, cottonwood, locust, willowAs in Eastern AgriculturalAs in Eastern Agricultural; chenopods, pigweed, waterhempRocky MountainsAsh, aspen, box elder, cottonwood, maple, mountain cedar, oak, pines, willowBrome, fescue, ryegrass, orchard, timothyAmaranths, chenopods, dock, giant and western ragweed, sagebrush, sageNorthwest CoastalAlder, birch, cedar, fir, hazel, pineBrome, fescue, orchard, sweet vernal, timothy, ryegrass, velvetAmaranths, chenopods, dock, plantain, short ragweed, sheep sorrelCalifornia LowlandAlder, elm, eucalyptus, olive, sycamore, walnut, willowBermuda, Johnson, velvetAmaranth, baccharis, nettle, scales, tumbleweeds,Arid SouthwestAsh, acacia, cedar, mesquite, Mountain mulberryBermuda, grama, witchgrassAmaranth, scales, tumbleweedsGreat BasinAs in Rocky Mountains and Arid SouthwestBermuda, Johnson, witchgrassMarshelders, rabbitbrush, scales, tumbleweedsSoutheastern CoastalAs in Eastern Agricultural; ash, bald cypress, box elder, elm, hackberry, hickory hornbeams, oak, pecan, sweet gumAs in Eastern Agricultural; bahia, dallisgrassAs in Eastern Agricultural, baccharisSubtropical FloridaBald cypress, eucalyptus, mulberry, oak, palm, palmetto, pineBahia, Bermuda, dallisgrass, Johnson,Baccharis, goosefoots, pigweed, short ragweedAdapted from Weber.10Weber R.W. Floristic zones and aeroallergen diversity.Immunol Allergy Clin North Am. 2003; 23: 357-369Abstract Full Text Full Text PDF PubMed Scopus (13) Google Scholar Open table in a new tab Adapted from Weber.10Weber R.W. Floristic zones and aeroallergen diversity.Immunol Allergy Clin North Am. 2003; 23: 357-369Abstract Full Text Full Text PDF PubMed Scopus (13) Google Scholar Although there are some exceptions, allergenic plants tend to be anemophilous (Table I), whereas most entomophilous plants are not a risk at the community level. So, a general rule of thumb for landscaping is to use plants with showy, insect-pollinated flowers. However, allergic sensitization and symptoms are a matter of whether or not exposure occurs. Induction of pollen allergen symptoms has the further consideration of exposure to cross-reactive allergens. Pollen-allergic subjects can suffer symptoms from allergens specific for a plant to which they have become sensitized. In other cases, specific IgE can cross-react to homologous proteins produced by other taxonomically related and even distant species. Examples of cross-reactive allergens produced by plants have been reviewed by Ferreira et al11Ferreira F. Hawranek T. Gruber P. Wopfner N. Mari A. Allergic cross-reactivity: from gene to the clinic.Allergy. 2004; 59: 243-267Crossref PubMed Scopus (225) Google Scholar and Weber.12Weber R.W. Patterns of pollen cross-allergenicity.J Allergy Clin Immunol. 2003; 112: 229-239Abstract Full Text Full Text PDF PubMed Scopus (136) Google Scholar Polcalcins illustrate the potential breadth of this.13Kleine-Tebbe J. Jakob T. Molecular allergy diagnostics: innovation for a better patient management. Springer, Cham, Switzerland2017Crossref Scopus (0) Google Scholar From different parts of the plant kingdom, homologous allergens from alder and ragweed (families in different orders) and from juniper and olive cross-react (classes in different phyla/divisions). Much information has appeared recently and continues to appear on this topic. In addition to using plants with showy, insect-pollinated flowers, a patient's allergen reactivity profile and potential sources of cross-reactivity should be considered. Selecting plants for landscaping can be a challenging task. Lists of recommended plants are included in gardening magazines and for every category or planting. There are recommended lists for ground covers, annual and perennial flowers, trees, shrubs, and turf grass species, sometimes considering particular purposes, for example, swimming pool plantings and landscaping. These lists are driven by popularity and influenced by what is new and available at garden centers. However, considerations about hardiness and floristic zones are important parameters in the selection of the most appropriate landscape plants for a particular location. Climate gradients and microclimates exist in particular zones, and often the same botanical species grow in different geographic locations under natural conditions.14Bridwell F.M. Landscape plants: their identification, culture and use. Delmar Publishers, College Park, Md1994Google Scholar The United States Department of Agriculture, Agricultural Research Service has published a selection of plant hardiness zone maps that date back to 1960 and are freely available online to the public.15United States Department of Agriculture Agricultural Research Service Plant hardiness zone map.http://planthardiness.ars.usda.gov/PHZMWeb/Date: 2012Google Scholar In addition, anthropogenic alterations associated with landscaping are important elements to consider when designing a low-allergenic landscape. Regardless of whether or not plants are allergenic or can grow within a particular hardiness or floristic zone, those considered invasive or toxic should not be planted. The United States Department of Agriculture has prepared a list of noxious plants,16Federal noxious weed list.https://www.aphis.usda.gov/plant_health/plant_pest_info/weeds/downloads/weedlist.pdfDate: 2016Google Scholar which should be used as a reference for plants to avoid. In addition to the aesthetic value of candidate plant species and the general landscape design, the allergic patient should evaluate plants for reduced allergenicity. Because of the climatic and biologic diversity within the continental United States, specific recommendations are beyond the scope of this article. Instead, a brief list of low-allergenic plants often recommended is provided in Table II (written communication, D.L. Edwards, PhD, 2015).22Barrick W.E. American Horticultural SocietyAmerican Horticultural Society 75 great American garden plants. Oxmoor House, Birmingham, AL1998Google Scholar If a desired plant is not given within Table II, it could still be vetted in the same fashion to determine its suitability for the intended location. To assist the clinician and patient to make such a selection, the workgroup proposes the algorithm shown in Figure 2 to facilitate the plant selection process. As part of the selection process, known allergenic plants should not be planted, especially if the individual is sensitized to them, as determined by in vivo and/or in vitro tests.17Lewis W.H. Vinay P. Zenger V.E. Airborne and allergenic pollen of North America. Johns Hopkins University Press, Baltimore, Md1983Google Scholar, 23Jelks M. Allergy plants. World-Wide Printing, Tampa, Fla1986Google Scholar, 24Solomon W. Mathews K. Aerobiology and inhalant allergens.in: Allergy: principles and practice. Mosby Co, St Louis1998: 367-403Google Scholar, 25White J.F. Bernstein D.I. Key pollen allergens in North America.Ann Allergy Asthma Immunol. 2003; 91: 425-435Abstract Full Text PDF PubMed Google ScholarTable IIA selection of insect-pollinated trees, deciduous shrubs, and herbaceous perennials commonly used in the US landscape industry∗This list presents candidate nonallergenic plants that can grow within a broad range of hardiness zones. This list was adapted from several online and peer-reviewed sources14,17-21 and is intended to be a guide of candidate plants that could be used in the design of a nonallergenic plant landscape.TypeSpeciesCommon nameFamilyHardiness zone†Reference to plant hardiness zones presented by the United States Department of Agriculture, Agricultural Research Service.15TreeAmelanchier arboreaDown ServiceberryRosaceae4-9Cercis canadensisEastern redbudFabaceae4-8Chionanthus virginicusFringe treeOleaceae3-9Cornus floridaFlowering dogwoodCornaceae5-9Crataegus phaenopyrumWashington hawthornRosaceae3-8Diospyros virginianaPersimmonEbenaceae4-9Ilex opacaAmerican hollyAquifoliaceae5-9Kalmia latifoliaMountain laurelEricaceae4-9Liriodendron tulipiferaTulipMagnoliaceae4-9Magnolia × soulangeanaSaucer magnoliaMagnoliaceae4-9Sassafras albidumCommon sassafrasLauraceae4-9Sophora japonicaJapanese PagodaFabaceae4-8Deciduous shrubsAesculus parvifloraBottlebrush BuckeyeSapindaceae4-8Hydrangea arborescensSmooth hydrangeaHydrangeaceae3-9Ilex verticillataWinterberryAquifoliaceae3-9Spiraea × bumaldaBumald SpireaRosaceae4-8Viburnum × burkwoodiiBurkwood viburnumAdoxaceae5-8Herbaceous perennialAmsonia tabernaemontantaBluestarApocynaceae3-9Asclepias tuberosaButterfly weedApocynaceae3-9Aster tataricusTartarian asterAsteraceae3-9Baptisia australisBlue false indigoFabaceae3-9Boltonia asteroidesFalse asterAsteraceae3-10Coreopsis verticillataThreadleaf coreopsisAsteraceae3-9Echinacea purpuraPurple coneflowerAsteraceae3-8Helleborus orientalisLenten roseRanunculaceae4-9Heuchera micrantha var. diversifolia “Palace Purple”Coral bellsSaxifragaceae4-9Lobelia cardinalisCardinal flowerCampanulaceae3-9Penstemon digitalisBeardtonguePlantaginaceae3-8Phlox divaricateWild sweet WilliamPolemoniaceae3-8Polygonatum odoratum var. pluriflorumSolomon's sealAsparagaceae3-8Rudbeckia fulgida var. sullivantii “Goldstrum”Black-eyed SusanAsteraceae3-9Sedum ternatumThree-leaved stonecropCrassulaceae4-8Symphyotrichum novae-angliaeNew England asterAsteraceae4-8∗ This list presents candidate nonallergenic plants that can grow within a broad range of hardiness zones. This list was adapted from several online and peer-reviewed sources14Bridwell F.M. Landscape plants: their identification, culture and use. Delmar Publishers, College Park, Md1994Google Scholar, 17Lewis W.H. Vinay P. Zenger V.E. Airborne and allergenic pollen of North America. Johns Hopkins University Press, Baltimore, Md1983Google Scholar, 18Missouri Botanical Garden. Plant finder.http://www.missouribotanicalgarden.org/plantfinder/plantfindersearch.aspxDate: 2017Google Scholar, 19Perennial encyclopedia from Walters Gardens, Inc.http://www.perennialresource.com/encyclopedia/2017Date: 2017Google Scholar, 20Cronquist A. Vascular flora of the southeastern United States. UNC Press Books, Asteraceae2001Google Scholar, 21Gleason H.A. Cronquist A. Manual of vascular plants of northeastern United States and adjacent Canada. van Nostrand, New York, NY1963Google Scholar and is intended to be a guide of candidate plants that could be used in the design of a nonallergenic plant landscape.† Reference to plant hardiness zones presented by the United States Department of Agriculture, Agricultural Research Service.15United States Department of Agriculture Agricultural Research Service Plant hardiness zone map.http://planthardiness.ars.usda.gov/PHZMWeb/Date: 2012Google Scholar Open table in a new tab A combination of native and ornamental anemophilous species typically make up the vegetation profiles of urban environments.26Levetin E. Buck P. Non-allergenic native and cultivated plants in Oklahoma: landscaping without hay fever.Ann Allergy. 1984; 52: 166-171PubMed Google Scholar, 27Cariñanos P. Casares-Porcel M. Urban green zones and related pollen allergy: a review. Some guidelines for designing spaces with low allergy impact.Landsc Urban Plan. 2011; 101: 205-214Crossref Scopus (191) Google Scholar In many cities, the selection of uniform species and dioecious male trees has eliminated fruit and litter production. This approach has resulted in homogeneous pollen profiles with a high community prevalence of allergy.1Molina R.T. Rodríguez A.M. Palaciso I.S. López F.G. Pollen production in anemophilous trees.Grana. 1996; 35: 38-46Crossref Scopus (153) Google Scholar In the United States, examples of common anemophilous trees planted in urban environments include maples (Acer rubrum and A saccharum), American sweetgum (Liquidambar styraciflua), pine (Pinus taeda and P ponderosa), beech (Fagus grandifolia), aspens and poplars (Populus species), Douglas-fir and fir (Pseudotsuga menziesii and Abies balsamea), white oak (Quercus alba), and sycamore (Platanus species). The selection and design phases of a low-allergenic plant landscape should avoid anemophilous species, as shown in the algorithm presented in Figure 2. Minimizing personal pollen exposure to allergenic plant species should also be a major aim to select appropriate plants in a landscape setting. However, eliminating seasonal pollen exposure is not completely feasible and can present some landscape design challenges. Although removal of anemophilous species from an existing landscape should ultimately reduce but not eliminate pollen exposure, this option is not always possible due to cost-benefit considerations. Although the removal of existing anemophilous species should reduce the immediate pollen load, pollen sources from surrounding areas cannot be eliminated. Reducing the pollen concentration in the immediate vicinity to the level of the regional background would likely reduce allergic symptoms in sensitized subjects
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