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

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

Graham Buck, Nith River Native Plants
Friday, August 26, 2016

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: ClearCreekForest in KentCounty and another black walnut forest near my child hood home along the NithRiver in WaterlooCounty. 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 NithRiver in Waterloo and BrantCounty 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 ThamesRiver. 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 ThamesRiver.

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. 

Juglone-Tolerant Plants

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

Michigan lily   Lilium michiganense

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

Canada wild rye   Elymus Canadensis

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

Semi Shade                               

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 americana

Virginia waterleaf   Hydrophyllum virginianum

Virginia wild rye   Elymus virginicus

Wild coffee   Triosteum aurantiacum

Shade

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

Pennsylvania sedge   Carex pensylvanica

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

Juglone-Intolerant Plants

Smooth serviceberry Amelanchier laevis

Witch-hazel Hamamelis virginiana

Wild plum Prunus americana

Common elderberry Sambucus canadensis

New Jersey tea Ceanothus americana

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.

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