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· Vol. 44 No.1 Winter 1983-84 arno ~a Amoldia (ISSN 0004-2633) is published quarterly in Page spnng, summer, fall, and wmter by the Arnold 3 Walnuts for the Northeast Arboretum of Harvard University. Edward Goodell Subscriptions are $12.00 per year; single copies $3.50 20 Propagating Leatherwood: A Lesson in Second-class postage paid at Boston, Massachusetts. Humility Postmaster: Send address changes to: Peter Del Tredici Amoldia The Arnold Arboretum 25 When Is a Pine Not a Pine? The Arborway B. June Hutchinson Jamaica Plain, MA 02130 29 The River Birch Copyright © 1984 President and Fellows of Harvard Anne Carlsmith College. 32 COLLECTOR’S NOTEBOOK Eileen J. Dunne, Editor Peter Del Tredici, Associate Editor 34 BOOKS David Ford, Graphic Designer Front cover photo~ Cone of the umbrella pme (Sciadopi- tys verticillata). /. Back cover Needles of the same species. Al Bussewitz photos. Walnuts for the Edward Goodell Northeast The walnut has been held in high esteem of walnut trees is poison to all plants within since ancient times. The Romans considered its compass...." While it is true that walnut it preeminent among nuts and gave it the species (and some hickories) produce a toxic name jovis glans (Jupiter’s nut), from which chemical identified as juglone (5-hydroxy-l, the modem taxonomic name for the walnut 4-napthoquinone) the actual phytotoxic ef- genus, juglans, has been derived. Several fect varies considerably depending on / 1 ~ the species of walnuts are popular throughout different levels of tolerance among plants, (2) the Western World today, both for their fine whether or not the roots of adjacent plants wood and flavorful nuts. Large commercial contact the walnut root, and (3) the amount walnut industries exist in the Mediterranean of air present in the soil. Although the black region of Europe and on the west coast of walnut probably has the most notorious the United States (primarily California, with reputation for allelopathy (as destructive 200,000 acres). Although walnuts are grown chemical interaction between plants is throughout much of this country, they have known), this phenomenon has been increas- never been grown widely in the Northeast. ingly documented throughout the plant Several types can be grown here, however, world, and it is worth noting that tall fescue including the Persian walnut ( juglans regia grass (Festuca arundinacea Schreb.) exerts L.), the black walnut (J. nigra L./, the butter- a similar deleterious effect on walnuts (Riet- nut (f. cinerea L.~, and the Asian walnuts (J. veld 1981/.( . ailanthifolia Carr, j. mandshurica Maxim., Toxic juglone is oxidized from hydroju- and j. cathayensis Dode), all of which have glone, a nontoxic substance found in all nutritiou6, flavorful nuts. We will explore in parts of the walnut plant. Oxidation of the following the merits of these types and hydrojuglone occurs in the presence of air their cultivation in the northeastern United and during contact between walnut roots States and adjoining areas of Canada. and the roots of other plants that contain Any talk of walnuts must begin by ad- oxidizing compounds. Hydrojuglone is dressing the toxic effect walnuts are said to highly reactive, however, and in the con- exert on neighboring plants. Pliny the Elder, tinued presence of air quickly breaks down in the first century A.D., wrote, "the shadow into harmless substances. Root-to-root con- tact is therefore the only means by which damage can occur unless plants are in soil that is poorly drained, and therefore poorly aerated. Black walnuts / Juglans nigra/ ripening on the tree. Al Bussewitz photo. Tomatoes, apples, most ericaceous plants, 4 and many conifers are known to be adversely regia is often called the English walnut, pre- affected (MacDaniels 1976 and 1980). The ef- sumably because it was introduced here by fects of walnut toxicity are noticeable either English colonists. However, it is not grown as flagging, wilted leaves (in tomatoes) or extensively in England. Most horticul- more commonly as a long-term gradual turalists call it the Persian walnut in defer- stunting of the plant. On the other hand, ence to its origin. many plants are not affected at all, including The Persian walnut is native to the area numerous grasses and vegetable crops, many between the Carpathian Mountains in east- legumes, some Rosa and Vitis species, and ern Europe and the Himalayan Mountains in most native hardwoods. This immunity may northern India, a swath across the Balkan result from either a root zone that does not Peninsula, Turkey, Iran, Afghanistan, and coincide with that of the deep-rooting wal- the adjoining areas of the USSR. A geograph- nut or an actual tolerance to the toxic prop- ically disjunct population occurs in northern erties of juglone. China (Icongraphia Cormophytorum According to current information we can Sinicorum 1972 and Meyer 1911). Wild be optimistic about the prospects of success- plants are normally found in mixed broad- fully growing a wide range of plants in as- leaf forests, along stream banks, in valleys, sociation with walnuts. A general precau- and on mountain slopes (Komarov 1936). In tion is advised: locate susceptible plants sheltered valleys, where it thrives, Juglans with a long life span outside the eventual regia may grow 20-35 m high with a root zone of walnuts. This is urged espe- straight, upright trunk and a large, spreading cially in wet soils, where the toxic effect is crown. Normally the wood is purplish more likely. However, during the initial pe- brown, hard, and satin-smooth and shiny riod of growth walnuts can be grown near when polished. The nuts of wild trees gener- even susceptible plants (as root spread is nar- ally have thin shells and large kernels. Vari- row and contact therefore avoided). For ex- ous subspecies have been proposed based on ample, at an orchard spacing of 6 meters on- nut shape, growth habit, and geographic ori- center black walnut can be interplanted gin (Komarov 1936 and Rehder 1940). with vegetable crops, small fruit, and/or Persian walnuts have always been valued Christmas trees for at least 10 years. trees within their native and naturalized range. The timber is used in Europe for fine woodwork and veneer. Trees that are suc- Persian Walnut cessful in the northeastern United States have come from colder regions of the The common commercial walnut is the species’ range, Romania, Hungary, Czecho- Persian walnut, fuglans regia. This was the slovakia, Poland, USSR, and Germany, where only walnut known to the Ancient Romans. they have traditionally been popular yard juglans regia came to Rome from Persia via trees. Outstanding selections are currently Greece, and Roman conquerors spread it being propagated and tested for commercial throughout southern Europe to England. orchards in these countries (Shreve 1981/. On the North American continent Juglans Most selections of Persian walnuts cold 5 Carpathian walnuts, /Juglans regia) above, origi- nated m the Mountams of southern Carpathian called walnut trees and known Poland. Carpathian for their cold hardiness, became widely es- tablished from British Columbia to Nova hardy enough for North America derive from Scotia and are still being selected for their seed collected in the Carpathian Mountains hardiness, nut qualities, and yields. of southern Poland. Reverend Paul Crath, a Reverend Crath’s introductions were not native of the Ukraine, worked as a Presbyte- the first cold-hardy Persian walnuts brought rian missionary in that region between 1924 to North America, however. Since the late and 1936, after having been ordained a 1700s German immigrants had been bring- minister in Canada. Reverend Crath was ing hardy Juglans regia seed from their keenly interested in the Juglans regia trees homeland to Pennsylvania and farther west. he found commonly growing in Poland, The descendants of these seedlings are on where minimum winter temperatures could the average less tolerant of cold than the reach -40°F. He believed that some of these Carpathian walnuts, but several are among trees would succeed in cold areas of North the most hardy and productive cultivars. America. Through arrangements with Other promising introductions have been members of the Northern Nut Growers’ As- made from the Russian Ukraine, the sociation and the Wisconsin Horticultural Himalayan Mountains, and northern China. Society, he sent back several tons of seed A great deal of potential still exists for col- (Devitt 1953). The resulting trees, often lecting genetically superior seed from the 6 1 cold-temperate areas where the Persian wal- (personal communication from L. H. Wil- nut has been grown since antiquity. moth, July 21, 1982). Regardless, the overlap In a 1936 letter Reverend Crath described in staminate and pistillate flowering times the trees of the Carpathian region: would rarely be enough to ensure a good We examined 79 walnut trees in and around Cos- crop. Several cultivars are notable for their seev. The age of the trees varied from 15-100 self-fruitfulness, but even these seem to years. Trees 30 years old and over were from 60 to benefit significantly from cross-pollination 75 feet tall, and from I to 3 feet in diameter. Of with another cultivar. the 79 trees, 3 trees were the only damaged by In the North the most common cause of frost of 1929 (45 degrees below zero) (Rahmlov 1962). crop damage among Persian walnuts is their tendency to produce leaves and flowers be- Experience on this continent has shown that fore the possibility of late frosts has passed. with site conditions a Per- good cold-hardy In Ontario, Persian walnuts are most suc- sian walnut tree will rapidly grow to the size cessful within 20 km of the Great Lakes, and of a tree. shape very large apple The where the cool spring climate delays vegeta- canopy is globe shaped and dense, the bark tive growth.