GARDENING IN IOWA

Gardening with walnut trees: What is safe to plant?

From the Ground Up | Linn County Master Gardeners

A gray squirrel munches on a walnut high in the branches of a tree in Iowa City. (The Gazette)
A gray squirrel munches on a walnut high in the branches of a tree in Iowa City. (The Gazette)
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Black walnut trees have high levels of the toxin juglone, in all parts of the tree, including roots and leaves. Why keep black walnut trees? They are one only a few nut trees that survive in cold environments, and their branches and nuts provide habitat and food for animals. Some plants are undeterred by high levels of toxins in the soil.

Most maples tolerate juglone, with the exception of silver maple. Oaks are also walnut-tolerant and Iowa natives include red oaks and northern pin oaks. Pin and bur oaks are slow growers. Oaks don’t tolerate shade well, so plant them away from other trees.

Smaller trees and shrubs can also fill out the yard and garden. Dogwoods, both tree and shrub forms, have four-season interest with colorful foliage and branches. The silky dogwood and pagoda dogwood are Iowa natives. Junipers provide green in the garden year-round and two are walnut tolerant. The common juniper is native to Iowa and comes in shrub (up to 5 feet in height) to tree (up to 25 feet) forms. Non-native Chinese junipers vary in size, from shrubs to trees. Junipers don’t tolerate shade, so planted away from the shade of other trees. A more shade-tolerant evergreen is the arborvitae. There are many cultivars of arborvitae that vary in size and shape, but all can thrive in full sun or part-day shade and are walnut tolerant.

Many ornamental perennials can grow in proximity to black walnut trees. Bleeding heart and sweet woodruff (a ground cover) can tolerate full to part-shade conditions. Walnut-tolerant perennials that need more sun include purple coneflowers, daylilies, phlox, asters and vine clematis. All of these comes in a variety of colors, heights and textures.

Can vegetables survive around black walnut trees? Vegetable plants require sun, so they won’t grow well in any shade. But several vegetable plants tolerate juglone in the soil: root vegetables such as carrots, onions, beets and parsnips. Squash, melons, corn, and beans are also walnut tolerant.

Peppers, eggplant and tomatoes, a staple of so many Iowa gardens, are susceptible to walnut poisoning. But these plants grow well in containers. And for some walnut-intolerant food plants, a solution might be a raised bed. Our raised beds have eight-inch high wooden sides. We dug out to about 1 foot in depth to remove any soil contaminated by juglone. Then we put in a combination of compost and commercial garden soil before planting. It was somewhat expensive, but we love asparagus and rhubarb, and it was worth it to have our own crop.

So you see — there are options for gardeners with beautiful black walnut trees in the yard! It may just take a little time and thought to find the right fit for the right spot.

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Call the Master Gardeners with your questions at (319) 447-0647 or send your questions and photos to linncountymastergardener@gmail.com.

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