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 26th, 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:
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
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
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.
Black Walnut Nuts
Black walnut is a prolific producer of nuts, which some consider to be another evil trait of the plant. Gourmet cooks, on the other hand, love the rich, smoky flavour (with a hint of wine) that black walnuts lend to cookies, breads, cakes and other baked goods. (Black walnuts have a stronger taste than the more common English walnuts.) Black walnuts can be used in any recipe that calls for nuts, but unless you’re really fond of the strong flavour, use sparingly or combine one part black walnuts with three parts English walnuts.
Preparing black walnuts for baking involves several steps but many people find the time and effort well worth it. Removing the husk can be a difficult and messy job, because the yellow dye will stain anything it comes into contact with. Start by floating the nuts in water and discarding any that float – an insect will have made its home inside one that floats. How you hull the nuts is a matter of personal taste. Driving on them with a vehicle, smacking them with a hammer, or agitating with a mixture of one part water, three parts nuts and a handful of gravel are all common ways that come with there own pros and cons. To enhance the flavour of the nuts, cure them by stacking in shallow layers two or three nuts deep, in a cool, dry location, for two weeks. When you are ready to shell the nuts, soak them in hot water for 24 hours, replace the water and soak in hot water for two more hours.
Full Sun - Moist
Cord grass Spartina pectinata
Cup plant Silphium perfoliatum
Golden Alexanders Zizea aurea
Great St. John’s-wort Hypericum ascyron
Green-headed coneflower Ratibida laciniata
Oxeye Heliopsis helianthoides
Tall coreopsis Coreopsis tripteris
Tall sunflower Helianthus gigantean
Wingstem Verbesina alternifolia
Full Sun –Dry
Sky Blue Aster (Symphyotrichum oolentangiensis)
Smooth Aster (Symphyotrichum laeve)
Big bluestem Andropogon gerardii
Black-eyed Susan Rudbeckia hirta
Goldenrods Solidago spp.
Indian grass Sorghastrum nutans
Ironweed Vernonia spp.
Little bluestem Schizachyrium scoparium
Mountain mint Pycnanthemum spp.
Switch grass Panicum virgatum
Shrubby St. John’s-wort Hypericum prolificum
Wild bergamot Monarda fistulosa
Wild senna Cassia hebecarpa
Bottlebrush grass Elymus hystrix
Blue-stemmed Goldenrod Solidago caesia
False mermaid-weed Floerkea proserpinacoides
Hear-leaved Aster (Symphyotrichum cordifolium)
Northern Waterleaf Hydrophyllum canadense
Baneberry Actaea rubra and A. alba
Silky wild rye Elymus villosus
Sweet cicely Osmorhiza claytonii
Tall bellflower Campanula
Wild coffee Triosteum aurantiacum
Large-leaved Aster (Eurybia macrophyllum)
Alternate-leaved dogwood Cornus alternifolia
Christmas Fern Polystichum acrostichoides
Common blue violet Viola sororia
Common wood sedge Carex blanda
Dutchman’s breeches Dicentra cucullaria
Graceful sedge Carex gracillima
Gray’s sedge Carex grayi
Green dragon Arisaema dracontium
Jack-in-the-pulpit Arisaema triphyllum
Mayapple Podophyllum peltatum
Moonseed Menispermum canadense
Ostrich Fern Matteuccia struthiopteris
Running strawberry vine Euonymus obovatus
Toothwort Cardamine spp.
Trilliums Trillium spp.
Twinleaf Jeffersonia diphylla
Wahoo Euonymus atropurpurea
Wild blue phlox Phlox divaricata
Wild ginger Asarum canadense
Wood Fern Dryopteris marginalis
Zig-zag goldenrod Solidago flexicaulis
Smooth serviceberry Amelanchier laevis
Witch-hazel Hamamelis virginiana
Wild plum Prunus
Common elderberry Sambucus canadensis
Silky dogwood Cornus amomum
Fly honeysuckle Lonicera canadensis
Ninebark Physocarpus opulifolius
Shrubby cinquefoil Potentilla frutiocosa
Choke cherry Prunus virginiana
Purple flowering raspberry Rubus odoratus
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.