An old black walnut in a hardwood forest can be a site to behold with its towering trunk. Their wood can also make wonderful furniture. Black walnut trees can also be majestic shade trees that bring relief in the summer heat. If you have them in your yard and you are trying to grow a garden or landscape, they can also be a pain in the @#$!
You see black walnuts (Juglans nigra) produce the chemical juglone, which is exuded from all parts of the plant. The production of juglone by the tree is used to assure its survival, by killing of other plants that try to invade its turf (bad pun alert!).
You see juglone in the soil causes problems in plants that are sensitive to it. The most common symptoms of juglone sensitivity is yellowing and wilting of leaves. This usually occurs during the hot dry periods during the growing season. It eventually results in the death of the sensitive plant.
Is this all doom and gloom if you have a big walnut in your yard?
Well if you just want grass and your big walnut trees, it’s not a problem as turf seems unaffected. The juglone may actually help to keep some weeds from growing. If you want to grow other plants, then it starts to become tricky. Especially, if any of those plants are on the following list:
Plants MOST sensitive to juglone:
ANNUALS AND VEGETABLES
Cabbage, Peppers (some), Tomatoes, Petunia’s, Eggplant, and Potato.
Colorado Columbine, Wild Columbine, Asparagus, Chrysanthemum (some), Baptisia, Hydrangea, Lilies, Daffodils, Narcissus ‘John Evelyn,’ ‘Unsurpassable’ ‘King Alfred’ and ‘Ice Follies’, Peonies, Rhubarb;
Silver Maple, European Alder, White Birches, Northern Hackberry, Apples and Crabapples, Norway spruce, Pines, Basswood;
Red Chokeberry, Hydrangea, Mountain Laurels, Privet, Amur Honeysuckle, Brush Cinquefoil (Potentilla), Rhododendrons and Azaleas, Blackberry, Lilacs, Yew, Blueberry, Doublefile Viburnum;
It’s not just black walnuts (although mostly)
The greatest concentration of juglone is found in the buds, leaves, stems, nut hulls, and roots of the walnut. The black walnut and the butternut (Juglans cinerea) are the landscape plants most recognized by gardeners as being problems. However, English or Persian walnut (Juglans regia) and hickories (Carya) also produce juglone but to a lesser degree.
Plants okay with Walnuts
Luckily, not all plants are sensitive to juglone. There is a lot of debate as to what plants are not bothered by it. For instance, yew is listed as being very sensitive to it, but I know of a spot where yews have been growing very nicely for that last 12 years right next to a black walnut. Some lists also say crabapples are fine, and some say they are sensitive to it. My advice is to look at several lists and try to find plants that appear on several of them as being tolerant. I have included some links to lists below.
How to grow Juglone sensitive plants
If you can’t live without your tomatoes or some other plant that is sensitive to walnuts. What you need to do is keep their roots away from the walnut roots and leaves.
Option #1 – Grow your plants in pots with potting soil
Option #2 – Grow your plants in a raised bed. This will require lining the bed to reduce root contact using weed fabric and filling the raised bed with new topsoil. This won’t really work for trees and shrubs whose roots will want to grow deeper, but is fine for shallow rooted perennials and vegetables.
Option #3 – Cut the tree down. It will probably take two to three years before you can start growing juglone sensitive plants though.
Landscape Design tolerant of Black Walnuts
If you are keeping the walnut and want to landscape, you will have to pick plants that are walnut tolerant. Next post, I will help a reader who wants a front yard design, but has mature black walnuts in her parkway. Here is a picture of her house:
Her house is in planting zone 5 in dappled shade. Beside the walnuts, she has a large sycamore tree in her front yard. She likes Asian influenced designs but wants to keep it low maintenance. I will show a basic front yard design that hopefully meets her needs.