Hardy Asian Alders

Hardy Asian Alders Kim E. Tripp The alder, whose fat shadow nourisheth- Each plant set neere him long flourisheth. -William Browne, 1613, Brittania’s Pastorals, Book I, Song 2 The search for new and unusual plants with About thirty-five species of Alnus are found handsome ornamental character and reliable around the world, all of them in the northern landscape performance is as old as horticulture hemisphere with the single exception of A. itself. The peak period of plant exploration may acuminata, which extends below the equator have passed with the likes of E. H. Wilson in the into Andean South America (Furlow 1979a, b). earlier part of this century, but at the Arnold Among the alders are some of the most cold Arboretum new plants from around the world hardy of broadleaved trees, including shrubby are still being added to the living collections and species in subarctic regions as well as numerous evaluated as ornamentals. species adapted to cool mountain climates. Potential new ornamentals must meet a They are most often found growing on poor demanding set of criteria before being declared soils, especially in wet conditions, and will good landscape plants. They should thrive in thrive where many other woody plants cannot.’ diverse landscapes under conditions of low maintenance, with minimal supplemental The Genus Alnus water and fertilizer. They should be reasonably Alders are of interest biologically, botanically, drought tolerant and suffer no significant pest or and ecologically. The genus comprises an an- disease problems. They must be easy to propa- cient group of deciduous trees and shrubs in the gate using standard nursery techniques and Betulaceae (birch family), of which the closely grow rapidly enough to be commercially profit- related Betula and Alnus may be considered the able. Obviously they must also possess attrac- most primitive members (Furlow 1979a). The tive ornamental features, preferably several for Latin name Alnus is variously believed to derive multiseason interest-showy flowers, fruit, fall from the classical Latin verb alo (to nourish, color, attractive winter habit or bark color, probably referring to its usual close association or handsome foliage with useful shade or ever- with water); or from the Celtic al (near) and green characteristics. lan (riverbank) (Furlow 1979a). Indeed, Alnus The great wonder of the woody plant world is are most often found growmg in moist or wet the number of new plants that are continually habitats-in or near streams, rivers, ponds, being found to meet these criteria-either as lakes, swamps, wetlands, and on moist slopes- brand-new finds from the wild or from breeding but some species inhabit moderately dry upland programs, or as rediscoveries from the forgotten sites, and others can grow in a range of environ- corners of gardens and landscapes. In the latter ments from very wet to relatively dry. They are category, one neglected but fascinating genus most often found at low to middle elevations, holds extraordinary promise for demanding but a few notable exceptions climb nearly to modern landscapes-the genus Alnus, or alder. timberline. This pair of Manchuman alders (Alnus hmsuta) along Willow Path at the Arnold Arboretum makes a handsome feature m the wmter landscape 188 which bears nodding pistillate catkins.) One related group of alders, the subgenus Clethropsis (A. maritima, A. nepalensis, A. mtidaJ, flowers in the fall, while all the others flower in the spring. As seed develops, "female" catkins mature into small dry infructescences, oval in shape, with many woody scales enclosing single- seeded, narrowly winged nutlets that are later dispersed by wind and water. These infructescences re- semble those of birches when young, but whereas the infructescences of Alnus are woody and persist after the seed matures, those of birches are papery and fall apart. Alders’ pistillate catkins can develop into mature infructescences even when seed has not been fertilized; in that case, the seed develops into an empty, shrunken nutlet. There is some evi- dence of limited development of viable seed without fertilization through a process known as apo- mixis (Furlow 1979a, Santamour 1995). Whether filled with viable seed or not, mature alder infructescences look very much like the diminutive true cones of a conifer. On most alders of flowering age (at least two years old), the spent "cones" are con- after the The refmed silhouette of Alnus japonica is lovely in all four spicuously persistent long seed has a delicate seasons of the year. fallen, producing ornamental feature as well as a dis- Like other members of the Betulaceae, alders tinctive field-key character. These attractive are monoecious, bearing separate staminate infructescences are sometimes gold plated and ("male") and pistillate ("female") catkins, or sold as jewelry. aments, on the same tree. (Catkins are compact Perhaps the most fascinating aspect of alder aggregates of individual flowers in a single biology is the ability of all species to "fix" atmo- structure, like the staminate "tassles" of oak.) spheric nitrogen in a process analogous to that Both staminate and pistillate catkins develop of leguminous plants like beans and peas. Fixing in the axils of the leaves or as the terminal bud. nitrogen is the process of converting atmo- Staminate flowers mature on pendent catkms spheric nitrogen into a form usable by plants similar to those of birches, while the small pis- and other biota. This unusual ability enables tillate catkins are for the most part relatively alders to pioneer successfully on sites of low fer- erect and less conspicuous at time of flowering. tility, where over time they contribute signifi- (An exception is Alnus firma var. multinervis, cant nitrogen to the soil, principally as leaf litter 19 (Binkley et al. 1994). As a result, Alnus is often one of the first species to establish itself after fire, clear cutting, volcanic activity, or other disturbances to forest environments (Furlow 1979a~. Its vigorous growth can prevent or sig- nificantly inhibit colonizing by other plants, both wild and cultivated. For example, follow- ing the eruption of Mt. St. Helen’s, vigorous hybrid poplars (PopulusJ were planted on mudflows caused by the eruption. Alnus rubra (red alder) seedlings rapidly established them- selves naturally on these sites, however, out- growing the Populus and other species to the extent that after six years the stands on the mudflows were 93% Alnus (Binkley et al. 1994). Alders’ nitrogen-fixing ability has also been used to advantage for many years in mine spoil reclamation (Tarrant 1968).(. Their ability to fix nitrogen combined with their rapid growth rate also permits alders to outperform other species in managed environments, sometimes in an undesirable way. For example, in the northwestern United States, The graceful fohage and catkms of Alnus japonica are lllustrated in this drawmg by C E Faxon. From the Archives of the Arnold Arboretum native Alnus rubra was historically eradicated from recently harvested and disturbed sites by commercial forestry managers who considered it a weed in competition with high-value coni- fers. More recently, red alder’s rapid growth and its ability to produce biomass on marginal sites is receiving the recognition it deserves, and its use-both as a "nurse crop" to provide nitrogen for higher value crops and as a primary crop whose wood value is itself on the increase-is now a major subject of research in United States forestry science (Hibbs et al. 1994). Hardy Asian Alders of Ornamental Potential From a horticultural standpoint, the alders’ ability to fix nitrogen and to thrive in wet soils makes them a natural choice for many difficult sites with low fertility. The horticultural merit of ornamental alder has been far more The bold foliage and catkms of Alnus hirsuta are species drawn m fme detail by C. E. Faxon. From the widely appreciated in Europe, Asia, and western Archives of the Arnold Arboretum. North America than in the eastern United 20 States, possibly because the alders native to hardmess. Rather than attempt to treat all of the those areas include handsome, full-sized trees. approximately thirty-five alder species in an In contrast, the alders native to eastern North article of this scope, I have chosen to discuss America are generally a shrubby, disheveled only hardy Asian alders of particular horticul- lot-biologically tough and ecologically impor- tural merit-plants which, unlike their Euro- tant, but less than aesthetically pleasing. This pean cousins, have been neglected as specimen, has given the entire genus an undeservedly poor shade, and street trees. Herein, "Asian" refers to reputation in the eastern half of the United the regions inclusive of China, Japan, Korea, States, where the vigor and attractiveness of Sakhalin, the Kuriles, Kamchatka, and the Alnus could make an important contribution to eastern and central regions of the former USSR gardens and other landscapes. (that is, Russia’s non-European regions); while European species like Alnus glutinosa and A. "hardy" refers to plants that will survive and cordata are already widely grown in Europe and grow in areas with winters at least as severe as the United States and each has notable culti- those of USDA hardiness zone 6. vars. The Asian alders, on the other hand, have Many alders fall into that nebulous category received little attention, although they include of "large shrubs or small trees," depending on some of the most beautiful taxa of the genus- where they are growing and on whether they taxa with rapid growth rates, no significant pest have been pruned to one or a few main trunks. or disease problems, and useful degrees of cold Many species that grow at both low and high The Other Asian Alders* Two categories of Asian alders are not in- are often nearly round with a notched apex cluded in this article: the ornamental but and can be 4 to 5 inches in diameter, the not hardy, and the hardy but not ornamental.

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