I’ve discovered through trial and error what thrives and what dies beneath my black walnut tree (Juglans nigra L.). The black walnut is a species of flowering tree in the hickory family that is native to eastern North America and can be poisonous to plants, to humans and to horses if the wood shavings are used in stalls! It grows from Maine to Michigan and south to Texas and northern Florida. It’s a grand old tree, a specimen of which grew in my back yard as a child, a tree whose nuts stained my hands and clothes brown as I played beneath it where the nuts fell or climbed up the branches to pick nuts. Matter of fact, the hull of the nut does such a good job of staining that early pioneers used it to dye cloth brown.
We knew about the poison way back in Roman times when Pliny wrote about the effect of walnut trees on plants. We now know the specific chemical involved, juglone, a toxin that occurs naturally in the leaves, bark, nut, buds, wood but especially in the roots of the walnut tree. It is not a water-soluble chemical so the poison does not travel too far from the area of the tree and its roots. Plants grown under or fairly close to black walnut trees may exhibit yellowing, wilting, then death.
By monitoring the tree, many plants have been listed as either sensitive or tolerant to black walnuts. Here are five that are said to be intolerant of growing within 50′ according to Ohio State University:
Eastern White Pine, Pinus strobus
Privet, Ligustrum species
Most Rhododendrons and Azaleas
But nobody told this to my white pine, my azaleas or Baptisia that all grow very close by and are perfectly healthy. The spring blooms of the azalea are breathtaking. Baptisa blooms are plump and healthy just beyond the shade of the tree. And privet? If I ignored the area, it would be a jungle of privet beneath the tree. But the most exciting shrub that has been growing well around the tree for the past 4 years is the hydrangea macrophylla. Fast growing in the dappled and moist shade of the white pine and walnut tree, pinks and blues grow side by side filling in an area that was once colorless and bare.
Following the success of the hydrangea, I am experimenting with other plants. The ferns I planted in early spring love growing in the moist, shaded soil beneath black walnut. Viburnum and euonymus have been planted along a pathway beneath the boughs of the tree and are not affected by toxins a bit. Dogwood trees are doing well as is the native magnolia growing nearby.
My only frustration is with the pesky squirrels who, instead of eating the nuts, bury them in the fall in every one of my flower borders and promptly forget them. Each spring, along with the crabgrass and chickweed, I must pull out far too many walnut tree saplings from the borders!
Ann Hohenberger, The Garden Club of Gloucester