While discussing nurseries and gardens with a recent acquaintance, he happened to mention his desire to plant a few black walnut trees, so that his offspring could reap the benefits sometime down the road. This is a tale I have heard several times; if you want to get rich, plant a few black walnuts and stand back. I used to believe this myself a few years ago, when I first moved here.

I live on a farm that has been used for growing Christmas trees since the 1950s. The Christmas trees are still here, as well as a small nursery I started for other interesting deciduous plants. The house itself is surrounded by gardens that my wife and I tend. Outside the immediate vicinity of the house are hundreds of black walnut trees. The main driveway entrance is lined with them; the 2 acre horse pasture is a monoculture of walnuts down to the pond; the old abandoned scotch pine plantation on the hill to the north is slowly, inexorably, changing to a black walnut wood, as the remaining pines die and are blown over; the mixed hardwoods to the west are also converting to walnut. All of these are the offspring of five trees planted in the pasture several decades ago by a previous owner who presumably thought black walnuts were a good investment. At that time the pasture was an apple orchard, long since extinguished by the walnuts. I have made some inroads over the years, cutting down walnuts that were too close to the house, and opening up an area for growing grasses, perennials and cut flowers. But there is a lot more work to be done.

Black walnut (Juglans nigra) is a member of the Juglandaceae family, which includes walnuts (Juglans spp.), hickories (Carya spp.) and wingnuts (Pterocarya spp.). Walnuts are native to America and Eurasia, hickories are American, and wingnuts are Asian. The familiar English walnut (Juglans regia) is actually native to southeast Europe and Asia, and is also more appropriately called Persian walnut. All of these species have certain features in common. They all have catkins for male flowers, compound leaves of many leaflets which are often fragrant (or smelly, depending on your nose) along stalks up to a foot long , prominent buds above pronounced leaf scars, and nuts. The walnuts are distinguished from hickories by the nuts and the twigs. Hickory nuts are small and encased in 4-valved husks, which usually split open at maturity, while the walnuts are large and encased in a fleshy green outer skin which does not split. In the absence of nuts, the surest way to distinguish them is to split a twig down the middle; if the pith is chambered it is walnut, if solid it is hickory. (Wingnuts also have chambered pith, but wingnuts are rare outside arboreta.) Among the Juglans species the black walnut is distinguished from the Persian walnut and the butternut or white walnut (J. cinerea) by the bark, leaves and the nuts. The bark of black walnuts is dark brown and deeply divided into narrow furrows and thin ridges, while that of the Persian walnut is smooth silvery gray with large flat ridges, and the butternut somewhat intermediate with dark gray furrows and light gray ridges much wider than those of black walnut. The leaves of black walnut and butternut are similar with 11 to 20 leaflets, but on black walnuts the leaves are strongly aromatic and the terminal leaflet is often missing, and the butternut has pubescent leaflets and sticky hairs along the petiole. The leaves of Persian walnut have 5 to 9 leaflets, much larger and smoother than the other walnuts. The nuts of butternut are tapered oblong and covered with fine sticky hairs, and the seeds are sweet, oily and edible. The nuts of black walnut are rounded and smooth, exude a strong odor and turn black when bruised. The inner shell (endocarp) is almost black ,deeply sculptured and almost impossible to crack, while the inner seed is very rich and oily. The nuts of Persian walnut are also round but lack the odor and black oxidation, and the endocarp is thinner and smoother and easy to crack.

Walnuts have been in cultivation since ancient times. The name Juglans was the Roman name and is derived from Jovis glans, meaning Jupiter's acorn. The Romans acquired the tree from Persia via Greece. The English name walnut is derived from Old English wealh-hnutu, meaning foreign nut or Gaulish nut (after the Celtic tribe known to Rome as the Volcae). The Persian walnut is the commonest plantation walnut in Europe and places such as California because of the ease of harvesting the nuts. Black walnuts are planted as ornamentals and for timber, and can grow to 75 to 100 feet with an equal spread. Black walnut has an extensive native range, covering most of the midwest and south except for the extreme coastal regions. It extends spottily into the northeast in a few areas of New York, Massachussetts and Connecticut. Most of the trees one is likely to see in New York have been intentionally planted as ornamentals and timber trees, or are offspring of such trees planted by squirrels. Black walnuts like deep soils and are tolerant of both dry and somewhat moist soils

There are a number of problems associated with black walnuts. They have a deep taproot, which make transplanting difficult, and seedling removal frustrating. They harbor a terrific population of squirrels with the result that walnut seedlings will appear everywhere, and walnut shells will find their way into the wall cavities of houses (another explanation of the name?). They are one of the last trees to leaf out in the spring and the first to drop in the fall (almost 8 months of bare branches), and have no appreciable fall color, often dropping green. The catkins produce a prolific amount of pollen in the spring, which can make life miserable for those people (and horses) sensitive to it. After dropping their pollen, the catkins then drop making a mess. Throughout the season, twigs and whole branches often come crashing down with the slightest provocation from wind. Whole trees can break apart in windstorms and icestorms. In the fall, the nuts begin the drop creating a black carpet and making walking hazardous both from falling nuts (hardhats advised) and rolling nuts (like walking on a bed of marbles). They seem to have an inordinate number of latent buds at the base of the tree. Even if one is able to cut down the tree, it will spring back to life from the stump with a jungle of shoots that can reach 8 feet in a single season. They are notorious for poisoning other plants with a toxin, called juglone, exuded from their roots; more on this later.

Black walnuts are not all bad. They are easy to grow from seed and fast-growing. I have seen some magnificent specimens, worthy of any arboretum collection or large backyard. They do provide bushels of wildlife food. The nuts are edible, though strong-flavored, and almost worth the effort required to crack them. The wood is highly prized and valuable for use in making furniture and gunstocks. Furniture made from black walnut has a richness of color unrivalled anywhere, but in large pieces can be overwhelming. In my opinion, it is best used in small pieces or as accents. Sawlogs can fetch a high price, as long as they are perfect, at least 18" small diameter, have good heartwood color and very little sapwood. In our area, this condition is rare. To get a perfect log a tree has to be trained to a clear, straight, unbranched log for 20 feet; branches have to be pruned small so that scars are small. Unfortunately, the pruning itself triggers the production of many fast-growing adventitious shoots at the pruning site, and winds and storms often undo any training one can undertake. Moreover, two trees growing in identical conditions next to each other can produce quite different heartwood color and radically different thicknesses of contrasting white sapwood, presumably genetic traits. A really big, old tree with a clear trunk could be sold for veneer, which would be the ultimate prize. Interestingly, prices for black walnut veneer logs are not easy to find in New York, because veneer logs are not generally found here. One has to go to the heartland to find them. New York is at the outer limits of their natural range, which translates as less than optimum conditions. Even if one is fortunate to have a good, clear sawlog of black walnut, many sawmills will be unwilling to take it, unless the tree was growing well away from houses and property lines or old fencelines. This is because of the high potential for harboring hardware, which can make short work of any sawblade. One time we hired a portable sawmill to cut up some logs including black walnut. The saw encountered a complete horseshoe buried inside the walnut log; I can still hear the scream of the blade as the teeth were removed. Of course, there's always firewood for those less-than-optimum logs and branches. Unfortunately, black walnut does not make very good firewood, being relatively less dense and difficult to start. Cutting up a tree for firewood is a lot of work, so better to make it a good wood to begin with. And felling the tree is the easy part; the real work is cleaning up the mess when it's down!

The real problem with black walnuts, and the one that is probably most familiar to gardeners, is the tree's allelopathic qualities. Allelopathy is the ability of a plant to prevent or retard the growth of other plants by chemical means. Many plants have this quality. Sunflowers and potatoes are known to stunt each other. A number of other crop plants are known to be allelopathic, such as wheat, oats, rye, asparagus, corn, cucumbers, and tomatoes. Non-crop plants with this quality include marigolds, wild black mustard, sweet white clover, some salvias, sagebrush, and eucalyptus. There is a great deal of variability in a plants ability to deter competition from others; otherwise there would be too many naturally occurring monocrops. Interestingly, eastern hemlock (Tsuga canadensis) has allelopathic tendencies against its own offspring. Seedlings seldom grow under older hemlocks, unless old needles are removed.

In the case of walnuts, the offending substance is called juglone (5-hydroxy-alphanaphthaquinone). Although butternuts also produce this substance, black walnut appears to produce far more and probably holds the crown for allelopathy. Apparently, Persian walnuts do not produce juglone, at least in sufficient quantity to cause problems, unless they are grafted onto black walnut rootstock. Most juglone is produced in the roots, although some is also found in the leaves, bark and wood. Many plants are stunted or killed by this substance, such as tomato, potato, blackberry, blueberry, rhododendron, mountain laurel, red pine, and apple, sometimes within two months of growing within the root zone of a black walnut. The root zone of a mature tree can reach up to 80 feet from the trunk. Do not use the tree's canopy as a guide to the extent of the root system; it can be up to 3 times as wide. It takes about two years for the roots to lose their toxicity after a tree dies. (Remember that cutting down a black walnut will not kill the tree; it simply resprouts from the stump.) The juglone in leaves will break down in about 4 weeks when exposed to air, water and bacteria in composting, and bark of walnuts can be used as a mulch provided that the material is fully composted for at least 6 months. Some horses are also known to be sensitive to juglone, and the use of walnut shavings and sawdust as bedding may have detrimental effects. Horses may also be allergic to walnut pollen, which is produced in large quantities.

Not all plants are sensitive to juglone. In fact, the growth of Kentucky bluegrass and other grasses as pasture can be enhanced by black walnuts, provided sufficient sunlight and water is available. Curiously, black raspberry grows well under walnuts, while blackberry is killed outright. From my own experiences and observation, I can say that the following plants can be grown successfully under walnuts:-

• Acer palmatum Japanese maple
• Ajuga reptans Carpet bugleweed
• Astilbe spp. Astilbe
• Carya spp. Hickory
• Catalpa speciosa Catalpa
• Cercidiphyllum japonicum Katsuratree
• Cercis canadensis Redbud
• Chasmanthium latifolium Northern sea oats
• Chionodoxa sp. Glory-of-the-snow
• Cladrastis kentukea Yellowwood
• Cornus alternifolia Pagoda dogwood
• Cornus florida Flowering dogwood
• Cornus mas Cornelian cherry
• Crocus sp. crocus
• Daphne mezereum February daphne
• Dicentra spp. Bleeding heart
• Euphorbia dulcis spurge
• Galanthus nivalis snowdrop
• Geranium spp. cranesbill
• Helleborus spp. hellebore
• Hemerocallis spp. daylily
• Heuchera spp. Coral bells
• Hosta spp. hosta
• Impatiens balsamina balsam
• Lilium spp. lily
• Lindera benzoin spicebush
• Lysimachia punctata yellow loosestrife
• Morus spp. mulberry
• Muscari sp. Grape hyacinth
• Narcissus sp. daffodil
• Oenothera spp. Evening primrose
• Panicum virgatum switch grass
• Parthenocissus quinquefolia Virginia creeper
• Phlox paniculata garden phlox
• Physostegia spp. Obedient plant
• Podophyllum peltatum Mayapple
• Polygonatum biflorum Solomon seal
• Prunus avium wild sweet cherry
• Prunus virginiana chokecherry
• Quercus rubra red oak
• Rubus occidentalis black raspberry
• Salix spp. willow
• Sambucus spp. elderberry
• Sassafras albidum sassafras
• Scilla sp. squill
• Sedum acre gold-moss sedum
• Stachys byzantina lamb's ear
• Tulipa sp. tulip
• Viburnum burkwoodii Burkwood viburnum
• Viburnum lantanoides Wayfaringtree viburnum
• Viburnum trilobum Highbush cranberry
• Vinca minor myrtle

I have not tried to produce a list of plants that will not grow under walnuts, because it is rarely easy to determine why a plant does not survive. I have had three Stewartia die in a particular spot close to walnuts, and finally decided to blame the juglone; similarly, with two Erie viburnums. I have lost five Nanking cherries (Prunus tomentosa) which were planted within thirty feet of a large black walnut. Anecdotal evidence suggests that blackberries were once very abundant on the farm, while black raspberries were relatively scarce. Now the reverse is true. Is this the result of the spread of black walnuts? I do know that today blackberries are found where walnuts are not, but this does not prove cause and effect.

There are undoubtedly many more plants which will grow under walnuts. It is possible that other species of the genera listed above will work, although not guaranteed (cf. blackberry and black raspberry). A review of internet resources will produce more lists. Have your search engine of choice search out "black walnut" and go from there. The problem is that a comparison of any two lists is likely to contain contradictions. For example, I've seen a list of walnut tolerant plants suggested by Doc and Katy Abraham, which included Norway Spruce (Picea abies), lilac (Syringa sp.) and blackberry (Rubus allegheniensis), all three of which are on the proscribed list of Ohio State University. Most lists are compiled from field observations and not from any scientific trials, and it is possible that plants can grow under walnuts and not come into contact with walnut roots and juglone, which is not readily soluble in water and therefore does not travel far in soil. In other words, it's potluck. If a plant survives under a walnut tree, it is either tolerant or not in contact with juglone. If it dies, it's probably not tolerant. So look on the bright side; go ahead and plant that favorite, and see what happens. If it dies, you can always blame the walnut. As for planting a black walnut legacy, I would think twice; you may get more than you bargained for.