Those doggone walnuts. On one hand, they are one of our most valuable hardwood trees, but on the other hand they can be a real nuisance in the home landscape.
Walnuts release a chemical into the soil called juglone. Juglone is present in all parts of the tree, but can be especially high in the roots and walnut hulls. The highest concentration in the soil is directly beneath the canopy of the tree, but it can be present in lesser concentrations anywhere the roots are present (which can be out as far as the tree is tall). The presence of juglone can also be affected by soil type, with sandy well drained soils tending to have less of it present.
Juglone is toxic and even fatal to some plants, while others don’t seen to be affected by it. Affected plants show signs of wilting, yellow leaves, stunted growth, and eventual death of the plant. More than once I’ve been at a customer’s house examining problem plants and sure enough, there was a walnut tree close by.
So, what’s the solution?
1) Susceptible plants can be planted in above ground containers or raised beds. If you build a raised bed, make sure you line it with plastic so the roots won’t grow up in to the soil or you’ll have the same problem all over again. Make sure you rake up any leaves or hulls that end up in the bed.
2) Cutting down the walnut tree can be a good long term solution, but the decaying roots will still release juglone into the soil, and it could be several years before the juglone is out of the soil.
3) Plant susceptible plants as far from the drip line as you can. The farther the better.
4) The most obvious solution is to install plants that are more resistant to juglone. The thing is, there hasn’t been any proven method of determining which plants are truly resistant. Any research I found on plant tolerance was based on simple observation. I compiled the following list of tolerant/intolerant plants using my own observations over the past several years here in Northfield and checking out the university extension sites of Cornell, Purdue, Michigan State, University of Minnesota, Ohio State, and the Morton Arboretum.
Just because a plant is listed as tolerant is no guarantee that it will grow under a walnut tree. You’ll find that many plants labeled as “tolerant”on one list are listed as “non-tolerant” on another list. Therefore, I prefer to list the plants that have been “observed to be more tolerant” or “less tolerant”. This list is compiled of plants that matched my own observations with the above extension sites, plus plants that they had in common with each other.
Plant tolerance to walnut toxicity: Common observations.
More tolerant: Catalpa, Eastern Red Cedar, Eastern Larch, Hemlock, Elm, Locust, Red Maple, Sugar Maple, Redbud, Poplar, White oak, Red oak, , Hawthorn, Beech, Sycamore, Ohio Bucke
Less tolerant: White Birch, Hackberry, Linden, Pine (in general), Black Alder, Magnolia, Norway Spruce, Crabapple, Apple, Silver Maple, Serviceberry, Arborvitae, Basswood (Linden)
More tolerant: Some Viburnums, Juniper. St Johnswort, Euonymus, Forsythia, Honeysuckle, Ninebark, Cotoneater, Potentilla, Sumac, some rugosa roses, Juniper, Mockornge, Forsythia
Less tolerant: Azalea, Rhododendron, Black and Red chokeberry, Hydrangea, Mugo pine, Potentilla, Cotoneaster, Yew, Lilac
More Tolerant: Ajuga, Hollyhock, European wild ginger, Astilbe, Bleeding heart, Foxglove, Sweet woodruff, Geranium, most Grasses, Daylilies, Siberian iris, Bee Balm, Phlox, Jacob’s ladder, Solomon’s seal, Lungwort, Sedum, Tiger Lily, Violet, Trillium, Jack-in-the-Pulpit, Coral Bells, Coral Bells, Hosta, Siberian Iris, most tulips and daffodils.
Less Tolerant: Columbine, Lilies, Paeonies, Buttercup, Rhubarb, Chrysanthemum, Blue Indigo