At the North Central Regional Plant Introduction Station in Ames, Iowa, I am responsible for a program to search out, propagate and distribute for long-term testing new woody landscape plants that are adapted to midwestern conditions (Widrlechner 1985). In this capacity, I work with trial site cooperators located in a wide range of environments throughout the central part of the United States at state experiment stations, botanical gardens and the USDA Soil Conservation Service. From my experience cooperating with these people, who generally have a good working relationship with their local nurseries, and by reviewing popular and professional journals in arboriculture, gardening, landscape architecture and nursery management from the last five years, I have identified four trends that influence the rise of new plants and the decline of some of the current offerings. These include: the increasing importance of low-input plantings as water and labor become more expensive; the desire to have plantings that are compatible with high-density urban development; the integration of food gardening and landscape plantings; and the refinement of tissue culture to propagate plants that are difficult to handle using conventional propagation methods.
A history of the xeriscape movement was outlined by Wellingham-Jones (1986), who documented its spread, beginning with a challenge issued by the Denver County Water Department in 1981 and going to 1986, when there were already 24 xeriscape programs in 60 western cities. She stated that the movement was drawing increased attention in the east and especially in Florida where fresh water will be in short supply due to the state's special geology and growing population.
Xeriscape requires special plants adapted to dry conditions, but that can thrive in common soils and with occasional periods of high moisture. Many woody plants that naturally grow under xeric conditions grow on very well drained or unusual soils and may not be widely adapted to artificial landscape conditions. Baetz (1988) suggested ways that design, installation and maintenance of xeriscape plantings could be modified to better fit the natural adaptations of these plants. Klett (1986) has coordinated a project to test herbaceous and shrubby xeriscape plants under both dryland and irrigated conditions to leam more about the adaptability of these ornamentals.
A review of the literature shows a great deal of interest in plants that require little water and many lists of recommended plants (Farmar-Bowers 1984, Hudak 1984, Mielke 1986a, b, c, Mitchell 1985, Pair 1986, Proulx 1984, Sacamano 1983, Simpson 1983). Wellingham-Jones (1986) commented, "a major problem has been finding new plant materialsin California demand exceeded nursery supply for a number of years." The interest is there but so far plants for xeriscape have not been easy to obtain. This may be because xeriscape species are difficult to propagate or to grow in the nursery using conventional production techniques, because many of them were new to the industry or because they were not considered as attractive or marketable as were moisture-loving ornamentals.
This situation should change as more nurseries develop methods needed to produce and market xeriscape plants. Anderson (1987), Baetz (1988) and Taylor (1988) have all described cases where western municipalities have been working with the local nursery industry to build markets for these plants. Proulx (1984) highlighted the progress of one of the growing number of new specialist nurseries, Plants of the Southwest, that can help supply future demand.
Two broad and convincing arguments can be made in support of regionalism. First is the esthetic argument that it is visually interesting for local character to be expressed in the landscape (Briggs 1984a). The other is that regional landscapes should be low-input landscapes. In other words, if plants are chosen that have been shown to be well adapted to local condifions, the costs of keeping plantings healthy and attractive will decrease.
Xeriscape is one strong example of a response to local needs. Another manifestation of regionalism can be found in increasing interest in naive trees and shrubs. The number of lists of useful native landscape plants is staggering. In the interest of space, I will cite just a sample of recent books and articles on naives, representing all regions of the United States (Diekelman and Schuster 1982, Farmar-Bowers 1984, Hightshoe 1988, Hubbard 1986, Jones 1987, Norris 1983, Penn 1982, Proulx 1984, Taylor 1988, Wasowski and Ryan 1985, Welch 1984).
It is sometimes suggested that native plants should be less costly to maintain because they evolved under local environmental stresses (Norris 1983, Taylor 1988), but the fact that the soils, microclimates, and arrangements of plants in artificial landscapes bear little resemblance to their nearby natural counterparts complicates the issue.
Overplanting of American elm (Ulmus americana L.) in our cities and the devastating spread of Dutch Elm Disease, caused by Ceratocystis ulmi (C. Moreau), ultimately made this attractive tree one of the most labor-intensive street trees (Gibbs 1978). Breeding programs at the University of Wisconsin (Smalley and Lester 1983) and the National arboretum [formerly at the USDA-ARS Nursery Crops Lab in Delaware, Ohio (Townsend 1983)] have been working to develop disease-resistant elms with good form that are adapted to American conditions, such as the new hybrid clones 'Regal', 'Pioneer' and 'Homestead'. These selections are just now getting into the trade and their impact on an industry cautious about elms is unclear.
The prairie region of the north central United States has some of the most challenging environments for trees and shrubs. For over twenty years, the Minnesota Landscape Arboretum has had a breeding and selection program to develop landscape plants adapted to such climatic and edaphic extremes. Releases from this program fit well with a desire to promote a regional landscape (Pellett and Luby 1985). Notable products include 'Princess Kay', an especially cold hardy ornamental plum (Prunus nigra Ait.) and a number of deciduous azalea selections with midwinter flower bud hardiness to at least -37°C, such as the hybrid cultivars 'Pink Lights', 'Rosy Lights', 'White Lights', and 'Orchid Lights' (Moe and Pellett 1986).
A plant selection program designed to support the xeriscape movement is directed by Benny Simpson at Texas A&M University His releases of Leucophyllum frutescens (Berland.) I. M. Johnst., 'White Cloud' and 'Green Cloud', L. candidum I. M. Johnst., 'Silver Cloud' and Salvia regia Cav., 'Mount Emory' were specifically chosen for their performance under xeric conditions (Simpson 1983).
Three recent articles give an excellent overview of the special stresses faced by urban vegetation (Bassuk 1985, Kozlowski 1985, Moffat 1987). These stresses include poor, compacted, irregular soils that are often saline and/or alkaline (Steiner 1980); extreme microclimates with little natural buffering; low or irregular light intensities and photoperiods; and air pollution. Whitlow and Bassuk (1987) published a detailed analysis of extreme temperatures and water stress on New York City street trees and Pfeiffer et al. (1987) reported on stresses faced by plants around King County, Washington parking lots. Experimental data have also been published on the performance of trees and shrubs under controlled soil compaction (Alberty et al. 1984, Gilman et al. 1987).
It becomes obvious that most plants do not perform well under such stresses and furthermore the plants that do survive under these conditions, such as Ailanthus altissima (Miller) Swingle (Pan and Bassuk 1986), may not necessarily have other characteristics that make them desirable. Publications from Pennsylvania State University's School of Forest Resources have pointed out the lack of data on the performance of urban trees and have made suggestions on how systems could be designed for cities to do their own testing (Gerhold 1985, Gerhold and Bartoe 1976). Without such data, it is not possible to know how well new landscape plants will thrive under urban stresses and, just as importantly, how well they will fit into urban space constraints caused by utility lines, narrow parkways and the need for traffic visibility.
One approach is used by breeders who include tests for urban stresses in their selection criteria, such as the salt tolerance system suggested by Townsend (1980), or who are otherwise selecting for plants that have the size requirements to fit into the urban landscape. This approach has led to some interesting selections, such as the low-growing Weigela hybrids released by Agriculture Canada, 'Rumba', 'Samba' and 'Minuet' (Svejda 1982, 1985, 1986), and the pollution-tolerant, non-invasive, triploid Hibiscus syriacus L. selections from the National Arboretum (Egolf 1988).
The other approach has best been explained in a series of presentations by George Ware of the Morton Arboretum (Davis 1984, Ware 1983,1984, 1985). In the spirit of Briggs' (1984b) statement that, "the first thing to remember in selecting urban plants is that there is a direct correlation between a plant's adaptability and maintenance costs," Ware posed an interesting hypothesis. He suggested that a detailed analysis of urban soils and climates could be used to direct a search for trees and shrubs growing under analogous conditions in nature. These plants should be better candidates for adaptation to urban stresses than the present array of species.
Ware has spent many years studying the problems of trees in the Chicago region and stated (1984) that many of the problems faced there were a result of poorly drained, alkaline clay soils. By exploring regions with poorly drained, calcareous soils that have even more extreme fluctuations in climate than the area around Chicago, such as certain parts of the Great Plains and Rocky Mountain states, one could find candidate plants for propagation and evaluation.
Diverse populations of many of our common native landscape trees have been evaluated for performance and adaptation by foresters and horticulturists. The results of such evaluations,, known as provenance tests, for the following common landscape trees have been published: Acer rubrum L. (Townsend et al. 1979); Cercis canadensis L. (Donselman and Flint 1982); Fraxinus americana L. (Alexander et al. 1984, Clausen 1984, Wright 1944); Picea pungens Englm. (Van Haverbeke 1984); Pinus ponderosa Dougl. ex P. Laws. & C. Laws., (Kopp et al. 1987); Pinus virginiana Mill. (Warlick et al. 1985); and Quercus rubra L. (Flint 1972).
A true test of Ware's hypothesis, provenance testing of selected populations under urban conditions, remains to be accomplished. It might be best done by first evaluating many provenances, chosen in part by Ware's criteria, at a field site near an urban area or with certain artificially controlled stresses. One could then test the best populations directly under urban conditions using designs of the sort outlined by Gerhold (1985).
This integration of food gardening with an attractive landscape, called edible landscaping, has been championed by a California landscape designer, Rosalind Creasy. Her book, "The Complete Book of Edible Landscaping," (1982a) has generated a great deal of interest in the topic and re-examination of many fruit and nut plants for their ornamental characteristics.
Since the publication of this book Creasy has given "how-to" examples to gardeners and the landscape industry (1982b, 1988) and many other reviews oriented toward edible landscape plants have appeared in popular publications (Goodell 1983, Hill 1986, McKinnon 1984). Certain uncommon trees and shrubs have been singled out for praise, because of their fruit and nut production. These include thorough reviews on Asimina triloba (L.) Dunal. (Mansell 1986, Nichols 1986), nut-producing species of Pinus (Reich 1988), and the use of Corylus in garden hedges (Benowitz 1986).
Many of the tree and shrub genera that are useful for edible landscaping for humans are also good for wildlife plantings [cf. species lists in reports by Burley (1987) and Thomas et al. (1973)]. There has been increasing interest in plants that feed and shelter backyard wildlife as is evidenced by the growing number of gardens enrolled in the National Wildlife Federation's Backyard Wildlife Habitat Program (Harrison 1983, Johnson 1987).
An increase in the number of edible landscapes, for both humans and wildlife, should increase demand for certain new introductions, such as three new selections of Amelanchier (Dirr 1977) made by Tom Watson a Wisconsin nurseryman. These three all have tasty fruits, the largest borne on A. canadenis (L) Medic., 'Prince William', a suckering shrub with orange-red fall color. A. x grandiflora Rehd., 'Princess Diana', is extremely floriferous and can show excellent fall color; the third, A. laevis Wieg., 'Prince Charles', was selected for its upright habit.
Vaccinium crassifolium Andrews, 'Wells Delight' and V. sempervirens Rayner and Henderson, 'Bloodstone' are two new groundcovers with edible fruit (Kirkman and Ballington 1985). These selections of little-used, native species were made for high-quality, evergreen foliage and adaptation to southeastern conditions. The black fruits of these two cultivars can be used by people and wildlife alike.
Tissue culture propagation has continued to expand both in terms of the number of species that have workable production protocols and in the total number of propagules produced (MacCàrthaigh 1986). This expansion has not come about without some problems, however. In a provocative opinion article, Mezitt (1988), one of the major tissue culture producers featured four years earlier by Smucker(1984), expressed concern that lack of quality control, problems with epigenetic variation a poor understanding of the market potential for tissue cultured products, and an insufficient commitment by the nursery industry to fund long-term research and development on in vitro propagation methods would hamper the future success of tissue culture to produce, high-quality, true-to-type plants economically. Many of the technical limitations to the in vitro propagation of woody plants are discussed in more detail by McCown (1986).
Even with such cautions to temper one's judgment about the future, there are already examples of useful new plants that would not be in the marketplace without tissue culture. It is my belief that the number of these examples will continue to grow.
Current success stories include the cold-hardy, deciduous azaleas released by the University of Minnesota (Moe and Pellett 1986) and Kalmia latifolia L. cultivars such as 'Ostbo Red' and 'Shooting Star' (Cross 1984). These plants would be limited to an extremely small market without tissue culture, as efficient cutting propagation systems have never been developed. Likely candidates for commercial success in the near future include the three Amelanchier cultivars mentioned in the previous section (Dirr 1987) and Betula platyphylla Sukachev var. japonica (Regel) Nakai, 'Whitespire'. This birch was selected from a Japanese plant introduction, PI 235128. It is a narrow-pyramidal white-barked tree well adapted to midwestern conditions that is highly resistant to bronze birch borer (Agrilus anxius) (Hasselkus 1987, Reed 1985). Commercial production is currently mostly from open-pollinated seed from the University of Wisconsin's Longenecker Gardens, but clonal propagation would be much more desirable. The seed comes from a somewhat isolated population of only three trees. The bulk of the seed is true but may be inbred and the possibility for outcross contamination does exist. Tissue culture is now being used to bring the original, superior genotype into the trade directly (Hasselkus 1987).
Acer campestre L. (European selections with good street-tree form)
Acer rubrum L. x A. saccharinum L.
Actinidia Lindl. (Hardier selections with desirable fruit)
Amelanchier Medic. (Small, non-invasive selections with desirable fruit)
Betula L. (Selections from wild populations subject to severe summer stresses)
Ceanothus L. Selections from wild populations growing on less wen-drained soils)
Cercocarpus HBK. (Selections from wild populations growing on less well-drained soils)
Conifers (Salt-tolerant selections for urban roadsides and screening)
Corylus colurna L.
Crataegus L. (Rust-resistant cultivars)
Fothergilla gardenii J. Murr.
Fraxinus ornus L. (Hardier selections)
Hibiscus syriacus L. (Sterile, cold-hardy cultivars)
Juniperus virginiana L. (Rust-resistant cultivars with good winter color)
Pistacia chinensis Bunge
Rhododendron L. (Cultivars selected for tolerance to higher pH soils)
Rhus copallina L. (Low-growing selections)
Sapindus drummondii Hook & Arn.
Sorbus alnifolia (Siebold & Zucc.) C. Koch
Ulmus parvifolia Jacq. (Hardier selections)
Vaccinium L. (Hybrids involving V. corymbosum L. and a stress-tolerant, low-bush parent, such as V. angustfolium Ait. or V. vacillans Torr.)
Weigela Thunb. (Cold-hardy, dwarf cultivars)