In practice, however, xeriscape often has been reduced to the landscaping idea of maximizing the efficient use of water in gardens and landscapes by creating areas which group plants by their water requirements. Typically this has been interpreted as growing the plants with the greatest water need nearest the home and transitioning to very low to no irrigation at the zone furthest from the home. This zoning of the landscape according to water use may also have additional benefits pertaining to fire breaks, wildlife value, and in enhancing the diversity of plants that may be grown in the garden.
Although there is always a market for colorful annuals, biennials, and short-lived perennials, these are generally more difficult groups to depend upon in landscapes and gardens. Longer lived perennials, subshrubs, shrubs and trees are the backbone of landscapes and gardens and will constitute the primary nursery market.
Under current conditions, I am convinced that a greater diversity of plants which flourish under existing climatic conditions across the country are needed. These changes should be viewed as potential opportunities and require the search for, and development of, perennials, subshrubs, shrubs and trees in the following categories: dwarf or compact forms of existing plants; new plants of small stature; plants that are all season performers; low maintenance plants; long, or continuous, seasons of color, be it flowers, foliage, fruits, stems or bark; good-looking, water-thrifty plants; and plants that attract wildlife.
Native plants which fall into any or all of these categories will be particularly desirable as they have the additional benefit of fitting into the local and regional conditions both visually and ecologically. This opportunity creates a new, relatively unexploited regional niche market for the nursery industry, and should eventually lead to a more striking regionalism in the American landscape vernacular.
A good place to begin the investigation of new native plants for drier portions of our landscapes is to look at the large plant families and genera in the region you are serving, and to concentrate the search on those plants inhabiting drier habitats. In California and the southwest, these would likely include some of the following: Agavaceae (Agave, Nolina, Yucca), Alliaceae (Allium), Amaryllidaceae (Brodiaea, Dichelostemma, Triteleia), Asclepidaceae (Asclepias), Asteraceae (Artemisia, Erigeron, Senecio, Haplopappus--Ericameria, Hazardia, Isocoma), Cactaceae (Opuntia), Crassulaceae (Dudleya, Sedum), Cyperaceae (Carex), Ericaceae (Arctostaphylos), Fabaceae (Astragalus, Lupinus), Fagaceae (Quercus), Grossulariaceae (Ribes), Iridaceae (Iris, Sisyrinchium), Hydrophyllaceae (Phacelia), Lamiaceae (Lepechinia, Monardella, Salvia), Liliaceae (Calochortus, Lilium), Malvaceae (Malacothamnus, Sidalcea, Sphaeralcea), Poaceae (Festuca), Polygonaceae (Eriogonum), Ranunculaceae (Aquilegia, Delphinium, Ranunculus), Rhamnaceae (Ceanothus, Rhamnus), Rosaceae (Adenostoma, Cercocarpus, Heteromeles, Holodiscus, Prunus, Purshia, Rosa, Spiraea), Saxifragaceae (Heuchera), and Scrophulariaceae (Penstemon).
The majority of these large genera and families have yet to be tested or evaluated for horticultural purposes in a serious systematic fashion including most perennial members of Phacelia, Monardella, Eriogonum, and Astragalus. A number of these large genera have well known propagation problems (such as commercially viable method for asexual propagation of Quercus and Calochortus) or are known to be especially difficult to grow and/or maintain under nursery conditions (like Calochortus and many native Delphinium and Lupinus species).
The selection of an individual plant for possible introduction is generally accomplished by finding a desirable feature or set of features that are deemed desirable. These individuals can be found in the wild or in the garden and often represent extremes of the natural variation: compact growth habit, different foliage, albinos, and unusual color forms. Additional factors such as mutations including witch's brooms, variegated foliage, and natural hybrids also contribute to the possible pool of plants to choose from. Traditionally these matters of chance have been the most frequently reported sources of many excellent cultivars: Heterotheca (Chrysopsis) villosa 'San Bruno Mountain', a sterile, free-flowering dwarf selection; Artemisia pycnocephala 'David's Choice', a heat tolerant compact selection; Erigeron 'W.R.', a compact, free-flowering, heat and cold tolerant chance wild hybrid between Erigeron glaucus and an unknown species; and Acer macrophylla 'Seattle Sentinel', a fastigiate selection.
Another especially rich area for horticultural selection can be found in nearly all taxonomically confusing groups at either the generic [example: Arctostaphylos and Zauschneria (Epilobium)] or specific [example: Mimulus aurantiacus or Mahonia (Berberis) aquifolium] level. Taxonomic chaos is often indicative of the extreme plasticity found naturally in the group and by inference the potential horticultural availability of variation to select from. Similarly, there are a number of extremely variable species that are nearly as ripe for selection as the taxonomically complicated groups listed above. These species include Rhamnus californica, Erigeron glaucus, Ceanothus maritimus, Quercus chrysolepis, and Juniperus communis.
Peripheral populations of a desirable plant are another source of native plants for landscapes and gardens. Plants from these populations may be more tolerant of heat or cold, higher or lower elevations, drought or wetness, may exhibit resistance to diseases or pests, or may be adaptable to a different soil type or condition. An excellent example of this phenomena is Arctostaphylos uva-ursi 'Point Reyes', a selection from the fog-bound headlands of the Point Reyes peninsula in Marin County that is extremely tolerant of heat and drought in comparison to typical members of the species. The small relictual San Bernardino Mountains population of Populus tremuloides is remarkably well adapted to growing conditions at low elevations in southern California. When plants from these peripheral populations are not themselves horticulturally desirable, they may carry genes which may be useful in a breeding program.
Another particularly rich source of new plants to investigate are the genera that have been dependably used in landscapes and gardens before, and look for a different species with desirable characteristics. Cultivars of the "new" species can be selected directly or the "new" species may be used to create hybrids with the "established" species. The named Heuchera maxima x Heuchera sanguinea hybrids: 'Genevieve', 'Opal', 'Santa Ana Cardinal', 'Susanna', and 'Wendy', created by Dr. Lee W. Lenz are excellent examples of this approach.
Breeding programs involving controlled crosses and hybridization between species are relatively rarely encountered in the native flora. There are, however, a few exemplary programs which have yielded many fine plants: the pacific coast hybrid irises, the Mimulus hybrids (of the section Diplacus), the Lewisia cotyledon complex, and Lilium hybrids.
All new cultivars should be named, described, published and registered with the proper registration authority. A list of all currently accepted registration authorities for ornamental plants is available from the American Association of Botanical Gardens and Arboreta (AABGA).
Establishing a set of desirable traits and characteristics of plants which the public actively wants or desires is a critical first step in the development of a new plant introduction strategy. Consulting with focus groups from the nursery industry, the gardening public, native plant enthusiasts, realtors, horticultural groups and others should provide the researcher with plenty of input on a variety of desirable plant traits. As progress is made in the selection process, the new plants should be planted out in a number of test sites. Test plantings should then be evaluated by a number of outside reviewers. Plants surviving through the testing and evaluation process are then ready to be introduced and marketed to potential user groups. The most recent successful example of a western native plant to go through such a thorough program is Arctostaphylos uva-ursi 'Vancouver Jade', an introduction out of the University of British Columbia's well known plant introduction program.
Nevin's Barberry [Mahonia (Berberis) nevinii], an extremely rare plant in nature, is found in widely distributed and ecologically varied small populations in southern California. It is, however, a relatively commonly seen landscape plant throughout the state. A molecular level study of these plants showed that there is little to no variation present in the gene pool. Therefore, the seed source of plants to be planted in the vicinity of the remaining individuals in the wild is not of concern--there will be no adverse consequences to the gene pool.
The California dandelion (Taraxacum californicum) from the wet meadows in the San Bernardino Mountains is a rare plant whose continued existence is threatened by ongoing hybridization with the common European dandelion (Taraxacum officinale), a weedy pest plant that is common throughout the range of the rare species.
The Monterey pine (Pinus radiata) is known from five geographically isolated populations (three are from central coastal California and two are on islands off Baja California). This tree is a significant forest tree in the southern hemisphere, particularly in New Zealand. Material from all five populations has been grown together on a massive scale in New Zealand, such that the primary seed source of Monterey pine (even in California) is from New Zealand. The issue of primary concern here is that the "mongrel" Monterey pines of New Zealand mixed origin will genetically pollute the "pure" native stands of these trees, such that the unique genetic character of the California populations will be lost due to homogenization of the gene pool over time (the Baja California populations are not threatened in this way at this time).