The Eastern Hop William E. Fehrenbach, jr. Hornbeam: Its Natural History and Landscape Potential

The eastern hop hornbeam, Ostrya vir- very short petioles. All are monoecious (bear- giniana (Mill.) K. Koch, is a tree with an or- ing flowers of both sexes on the same tree). namental value that has largely gone unrec- Staminate catkins are borne in groups of two ognized. This ubiquitous native understory or three at the tips of the previous year’s tree is one of the least-studied trees m North twigs, and the fruits are bome in clusters of American forests: literature searches turn bladderlike sacs. All species exhibit rough, up very little reliable information on its cul- scaly bark. tural requirements or ecology. In this article Three species are found in the United I will bring together the available horticul- States and Canada. Of these, two are de- tural information and point out areas where scribed in horticultural literature: Ostrya more research is needed. Lack of publicity virginiana and0. knowltonii Coville. Un- may be the only reason that this tree has not like the widespread O. virginiana, O. become more popular. knowltonii is a rare tree, with an extremely Ostrya virgmiana is the most widespread limited range at altitudes of 1500-2100 m in species in its genus. It is hardy to USDA canyons of southwest Texas, New Mexico, zone 4 and is found from Nova Scotia to Arizona, and southern Utah. It differs from Minnesota and south to eastern Texas and its relative in its lesser height, nearly always Florida. The genus name, Ostrya, is derived under 9 m. Its limbs are slender and crooked, ’ from the Greek word ostrua, designating a forming a rounded crown. Its leaves are tree with very hard wood /Vmes 1960). Many somewhat similar to those of O. virginiana, of its common names also refer to the hard- but they are smaller and more broadly ovate. ness of its wood: ironwood, leverwood, hard- Both its leaves and petioles tend to be hack, and hornbeam. Two other common somewhat more pubescent than those of O. names are deerwood and Indian cedar. virginiana, a characteristic that is typical of The genus Ostrya belongs to the Be- some desert species. The flowers and fruits tulaceae, or birch family. According to T. S. are also smaller than those of the other Elias (1980), it is a genus of only eight species but similar in structure. Bark and species, which are quite similar to each twigs are similar. other. All are short deciduous trees or Elias (1980) mentions the Chisos hop shrubs, with simple and alternate leaves and hornbeam (Ostrya chisosensis/, which he 24

desirable species in the farm woodlands of Ohio and Indiana. In many areas it domi- nates the understory so completely that the reproduction of the more valuable trees is often suppressed...."" The wood of this species is light brown, tinged with red or white. The specific grav- ity of Ostrya wood is 0.83 (Young 1933),), which ranks it among the hardest of our na- tive woods. Its use is limited to such items as tool handles, golf clubs, mallets, fence posts, miscellaneous woodenware, and fuelwood. It is reported to take a very fine fimsh and probably would be a very valuable wood if the tree were larger. The buds and catkins of the hop hornbeam are a preferred winter food of the ruffed grouse, especially in New York, Pennsyl- vania, and Wisconsin (Hamilton 1974), and the fruit is a secondary fall food. Other ani- mals that feed on hop hornbeam are the Trunk of a mature eastern hop hornbeam (Ostrya bobwhite, rmg-necked pheasant, downy vmgmiana) at the Arnold Arboretum woodpecker, mockingbird, purple finch, red, gray, and fox squirrels, deer, and cottontail (DeGraaf and Whitman 1979; Hamilton 1974). describes as a shrub, rarely a tree, found in Undoubtedly, the hop hombeam’s greatest the same region as O. knowltomi. No other assets are its ornamental qualities. At one horticultural information is available on this time it was recommended for use as a street obscure species. tree. Restricted root space and the fact that Historically, the eastern hop hombeam many city trees are planted with bare roots has been labeled a "weed tree" of no com- may have been major factors in the hop mercial value. Most management prescrip- hornbeam’s poor performance in the few tions for commercial forest land classify the cities where it has been tried. Because of this hop hombeam as a competitor to more adverse experience, many horticultural profitable species and recommend its re- writers unjustly removed the species from moval. In fact, one of the few published re- their lists of recommended trees. In cities search papers available on hop hornbeam where the tree has been located and planted (Diller and Marshall 1937) deals exclusively properly, its performance is reported to be with techniques of cutting the tree to reduce excellent. Nurseries in Buffalo, New York, chances of resprouting. The authors ob- cannot grow enough of the trees to satisfy served that "hop hornbeam is one of the less demand (personal communication 25

from R. Walkowiack, June 23, 1983).( . and may perhaps be the basis for developing The hop hornbeam is small in stature, a cultivar in the future. usually attaining only 10.5 m. As such it is The most notable ornamental features of useful for smaller properties or locations this tree are its fruits and flowers. The hop with limited overhead space. The largest hop hornbeam gets its name, in part, from the hornbeams are found in Arkansas and Texas, similarity of its fruits to the true hop fruits where some specimens reach 18 to 21 m high /Humulus sp./. The compound fruit is ob- and have trunk diameters of 45-60 cm. In long. It is made up of a cluster of bladderlike the Northeast 10.5 m is the average maxi- sacs, each containing a single, ovoid, faintly mum height, and crown spread is usually striated nutlet about 6 mm long. Often, the equal to two-thirds of the height. Trunk di- smaller sacs at the bottom of the cluster are ameters are seldom greater than 15 cm and empty. The fruit clusters become conspicu- rarely reach 30 cm. The habit of the hop ous in July, when their color is an attractive hornbeam is graceful, with many horizontal pale green in contrast to the darker green of or drooping branches. Few trees its size can the leaves. The fruit ripens from late August match the hop hornbeam in fineness of tex- to early October, and the sacs turn tan to ture, from the narrow shaggy strips of gray hght brown. The sacs are covered with fine, bark on its trunk to its slender reddish stiff hairs that are irritating to the skin when brown twigs. handled. The shape of the tree is distinctly irregu- The hop hornbeam’s flowers are not dra- lar, ranging from conical to oval to irregu- matic or showy but are interestmg nonethe- larly rounded. Understory trees are often ir- less. The tree is monoecious and bears its regularly shaped, because of the various di- flowers in catkins much the same way as rections of the hght penetrating the forest other members of the birch family. The canopy. Its status as an understory tree also staminate catkins are from 2.5-5 cm long means that the hop hornbeam tolerates and reddish brown. They begin to expand dense shade. It is not restricted to shady slightly m March and then more rapidly spots, however, and grows well in full sun. It until fully open in April, when they polli- is m full sun that the tree develops its most nate the pistillate catkins emerging with the desirable rounded form. leaves from beneath the bud scales. The leaves of the hop hornbeam are alter- The bark of the new shoots, twigs, nate, egg-shaped, and 7-12 cm long and branches, and trunk varies considerably and 3.8-5 cm wide. The margin of the leaf is ser- is quite attractive. The newest shoots are rate. The top surface of the leaf is glossy reddish green with minute brown lenticels. green in summer. The lower surface is pale Larger twigs, and branches less than 5 cm m green and somewhat hairy, especially along diameter, are smooth and purplish brown to the veins and midrib. Various fall colors red-brown in color. On these smooth stems have been observed, from a poor yellow or the lenticels are tan to gray and lengthen yellowish brown to red and even purple. horizontally, so that the young bark looks Some trees retain their leaves well into the much like that of birch or cherry. Stems 6 winter, though leaf retention is not a very re- cm and greater in diameter have the charac- liable or widespread trait within the species teristic gray, narrow, striped bark that sets 26

hop hombeam apart from other native trees tree is found in moist bottomland soils, near with rough bark. streams and rivers, and on dry, gravelly The hop hornbeam is a slow-growing tree, ridges, with oaks and hickories. It does not reaching, on average, 6 m in 20 years. Some tolerate flooded or heavily compacted soils. speculate that this slow growth is responsi- Average garden soils will adequately support ble for the remarkable strength and tough- hop hornbeam, provided that drainage is ness of the wood. The strong flexible twigs good. Slightly acid soils are best but not cru- and excellent branching structure make the cial. In general, adding generous amounts of hop hombeam almost impervious to damage peat to average soils is the only site prepara- by ice, wind, or heavy snow. Popular opinion tion that should be necessary when planting also maintains that the species is extremely a hop hornbeam. pest resistant; however, this cannot be sub- Planting sites for this tree must allow ade- stantiated until the tree is studied further, quate space for rooting and branching. Ade- especially in stressful situations. quate space is essential for the development of a root system that will provide necessary moisture and nutrients. Lawns less than 3 m or within 5 m of a wall or Growing the Hop Hornbeam wide, building, will not provide adequate rooting space. Pro- Propagating the hop hornbeam is a chal- tection from wind is not necessary except for lenging task. The seeds exhibit a double staking the tree during the first year or dormancy that requires lengthy stratifica- two to provide support while new roots are tion in a moist medium in order to germi- being formed. nate. The regimen most often recommended Most people who know this tree agree that for stratification is to place seeds in moist it is difficult to transplant successfully, es- sand or peat for 60 days at 20°-30°C and pecially if specimens are large. Wild trees 140 days at 5°C. An alternative regimen is tend to have exceptionally irregular root sys- 6’/z months at 10°-22°C and 90 days at 5°C tems, which penetrate deeply in loose soil, (Schopmeyer 1974). Despite the low percent- so it is best to move them when they are age of germination, propagation by seed young. Hop hornbeam should always be seems to be the only practical method for balled and burlapped when planted in order nursery production. Grafting hop hornbeam to improve its chances for establishment and on rootstock of the same species, or perhaps preserve the mycorrhizal relationships the on rootstock of another species in the birch tree relies upon (Hamilton 1974). family, may be attempted in the future at A new approach to container growing ap- the Holden Arboretum. If the attempt proves parently improves establishment of hop successful, cultivars chosen for improved hornbeam and other species with "difficult" fall color, leaf retention, or improved growth root systems. This approach (called the rate may be developed. Tissue culture may "Minnesota System") was developed by Dr. also be an avenue to explore if cultivars Harold Pellett 1981 and involves the use of prove to be commercially promising. bottomless containers arranged in trays of The soil and moisture requirements of the standing water. This prunes the strong tap- hop hornbeam are not rigid. In nature the root and produces a more fibrous, compact 27

References Braun, E. L. 1961. The Woody Plants of Ohio. Colum- bus, Ohio: Ohio State University Press. DeGraaf, R., and G. Whitman. 1979. Trees, Shrubs, andd Vmes for Attractmg Bmds. Amherst: University of Massachusetts Press.s. Diller, O. D., and E Marshall. 1937. "The Relation of Stump Height to the Sproutmg of Ostrya vir- gimana in Northern Indiana." /ournal of Forest- ry, 35: 1116-19. Elias, T. S. 1980. The Complete Trees of North Amer- ica. New York: Van Nostrand Reinhold. Hamilton, T. S., Jr. 1974. Shrubs and Vmes for North- eastern Wildhfe. Forest Service General Techm- cal Report, NE-9. Washmgton, D.C.: U.S. Gov- ernment Prmtmg Office. Pellett, H. 1981. "The Mmnesota System: A New Ap- proach to Tree Production." Amemcan Nursery- man, 153 ( 10/: 14,38,40,44. Schopmeyer, C. S. 1974. Seeds of the Woody Plants m the Umted States. Washmgton, D.C.: U.S. Gov- ernment Prmtmg Office. Vmes, R. A. 1960. Trees, Shrubs and Woody Vmes of the Southwest. Austm, Texas’ University of Texas Press. Young, L. J. 1933. "The Growth of Ostrya vmgimana." Papers of Michigan Academy of Science, Arts and Letters, 19: 341-44. Eastern hop hornbeam (Ostrya vmgmiana ~ grow- ing in a sunny location. root system that is better able to support the newly planted tree. The Holden Arboretum is currently promoting the use of the hop hornbeam by providing both the grower and the buyer with reliable information about this little- used tree. It is currently in the process of locating seed sources and determining the best methods of germination. Next, soils, transplanting techniques, and plantmg sites will be tested and evaluated to determine optimum cultural practices for growing the tree. The staff hopes to stimulate public interest in this tree by increasing awareness of its utility, ease of maintenance, and William E. Fehrenbach, ~r., is assistant supenntendent understated charm. of the Holden Arboretum in Kirtland, Ohio.