Planting Under a Walnut? How to Survive the Juglan Zone!

Question: I have a large, beautiful black walnut tree in my yard.  I’m having trouble growing shade-loving annuals in the ground under my tree.  What can I do?

Answer: Your black walnut (Juglans nigra) is defending its space—that's why you are having trouble getting some plants to grow underneath it.  Using a strategy called allelopathy, the tree produces and releases a chemical called juglone that adversely affects many other (but not all) plants. Juglone can be found in all parts of the black walnut tree. If the roots of susceptible plants come within 12 millimeters (1/2 inch) of the walnut’s roots, they can absorb the juglone, sicken and die. Also, walnut leaf litter and nuts on the ground leach juglone into the soil. The “toxic zone” is not just under the tree canopy, it extends 15-20 metres (50-65 feet) out from the trunk, so there is a large area to consider.

The question of what to plant under or near a black walnut has got to be one of the most commonly asked gardening questions. I suggest taking your inspiration from nature: “Choose the right plants for your site and you'll find them well-adapted, with built-in resilience to changes in temperature and rainfall, and minimum maintenance requirements.” (Joanna Poncavage – The Morning Call, April 26^th, 2006.) In this article I will outline the plants I have encountered in natural spaces thriving alongside black walnuts and how they came together in my black walnut garden.

My interest was not prompted by the frustration of failure, but came from an appreciation of the uniqueness of black walnut plant communities in the wild.  Two come to mind: Clear Creek Forest in Kent County and another black walnut forest near my child hood home along the Nith River in Waterloo County. At Clear Creek, the canopy is composed entirely of black walnut trees. When I inquired about the origins of the trees in this area, I was told it was formerly an agricultural field. After it was abandoned, it naturally succeeded into a forest. Walnuts grow best in openings or along the edge of the forest, where there is adequate sunlight. It typically grows as scattered individuals or in small groups mixed with a wide variety of other hardwoods.  Pure stands of black walnut are rare, but can occur as small groves at the edge of a forest. No universal vegetative indicator of a good walnut site is known. In general, where tulip tree, white ash, red oak, basswood, sugar maple, or red elm grows well, black walnut thrives also.

At Clear Creek the pure walnut stand of walnut started from walnuts from trees along the nearby creek. The nuts were planted by squirrels and quickly grew into trees, creating a black walnut forest. Some of the plants thriving within the grove in the spring and early summer are wild blue phlox, false mermaid-weed, white trillium, red baneberry, northern waterleaf and Virginia waterleaf. Walnut canopies tend to grow far enough apart to allow sunlight to reach the ground throughout the growing season. In the summer, after the spring wildflowers become dormant, a lush layer of silky wild rye and Virginia wild rye takes over. Growing among the grasses are the beautiful tall bellflower and the unusual wild coffee. Sweet cicely is also very common in the summer. The dominance by this single tree species, the spacing of the trees, the lack of a shrubby understorey and the tall grasses led to the nickname walnut savanna.

The Nith River in Waterloo and Brant County hosts a different type of walnut community. Along the floodplain are forests of black walnut, black maple (Acer nigrum), white ash (Fraxinus americana), hackberry (Celtis occidentalis) and bur oak (Quercus macrocarpa). The forest understorey contains many kinds of plants, including wild ginger, toothwort, zig-zag goldenrod, common blue violet, Dutchman’s breeches, running strawberry vine, common wood sedge, Pennsylvania sedge and graceful sedge, to name a few. There are also unusual plants such as Gray’s sedge, green dragon, twinleaf, moonseed, wild coffee and wahoo. Alien species also flourish because high waters wash through the forest which removes the leaf litter, deposits fresh seeds and soil and causes other disturbances to the site. They are namely garlic mustard (Alliaria petiolata), dame’s rocket, moneywort (Lysimachia nummularia), and crack willow (Salix fragilis).   After seeing these captivating examples of natural black walnut plant communities, I wanted to transform the black walnut grove at my nursery in New Hamburg into a walnut garden. Over time, I moved in plants from different gardens on the property. I also found local populations for some of the native plants listed above. After growing them from seed into plants I worked them into a black walnut grove in the spot I thought would best suit them. 

Since the juglone zone at my nursery extends beyond the canopy of trees along an open, south-facing front, I established a walnut floodplain habitat similar to what I have observed along the Thames River. This area of the walnut garden is composed of cord grass, tall sunflower, cup plant, green-headed coneflower, wingstem, oxeye, Michigan lily, tall coreopsis, great St. John’s-wort and golden Alexanders. Nearby, I planted two very rare species: blue ash (Fraxinus quadrangulata) and hairy wood-mint (Blephilia hirsuta), which are very rare in Ontario, and both occur together along a section of the Thames River.

None of my prairie plants showed ill effects from the juglone. A number of the species I choose are aggressive spreaders that can be difficult to control in the small area, such as an urban garden. In a small area or a drier situation, I would suggest a prairie composed of less aggressive species, such as wild bergamot, black-eyed Susan and grasses.

I delight in the challenges of recreating natural-looking plant communities like those associated with black walnut. By using nature as an inspiration to create my garden, I have learned about the environment and landscaping. I have attached an approximate layout of my black walnut garden, in case you wish to create one too.

Graham Buck has been growing native plants for over 10 years. He has recently stared a nursery and landscaping business called Nith River Native Plants. He also works as The Natural Connections Program Coordinator in Brant County and the program coordinator for Tallgrass Ontario. As a botanist he is hired to complete botanical inventories and prepare management plans. He lives in Guelph with his wife Bronwen.