Black walnut trees are prized for their rich fruits and lush, scented leaves. Juglans nigra makes an attractive addition to any garden, but planting near black walnut trees offers a challenge when it comes to which flowers will grow underneath them.
The roots, branches and leaves of the black walnut tree produce a toxin known as juglone. High levels of juglone in soil discourage the growth of other plants nearby, reserving more of the area’s natural resources for the tree itself. For many plants, this toxin leads to yellowing leaves, leaf drop, wilting and eventual death. It’s common to see a bare patch around the base of a black walnut tree.
Are All Walnut Trees Toxic to Other Plants?
There are several species of walnut trees. Black walnut is the only variety that produces the juglone toxin in large enough amounts to pose a risk to other plants. But unfortunately, choosing to grow another type of walnut won’t necessarily avoid the toxicity problem.
Most modern walnut trees are grafted onto black walnut rootstock. This helps them grow vigorously and gives them greater tolerance to wet conditions. Because of this, you may come up against the juglone problem when growing any variety of walnut, as the roots will still produce enough of the chemical to be toxic to the walnut’s neighbors.
8 Plants to Grow Under Black Walnut Trees
Thankfully, not all plants are affected by juglone. By choosing tolerant companion species to plant near your walnut tree you can avoid the bare patch of soil that spoils your garden’s look. Here are eight of the most popular perennials, shrubs and smaller trees which can happily coexist with black walnut.
Honeysuckle is a vigorous grower that’s tolerant of juglone. Available in a wide number of varieties, the plant’s greatest asset is the sweet-smelling flowers that fill your summer garden with perfume. The flowers attract butterflies, bees and other beneficial insects. Honeysuckle is a great climber. This makes it useful as a privacy screen if your black walnut tree is at your garden’s boundary with a neighbor or public space.
2. Japanese Maple
Japanese maple is a good walnut tree companion plant and is most notable for its spectacular display of red or purple fall foliage. The plant can be grown as a shrub or a small tree, reaching heights of between 6-15 feet (1.8-4.6 m) depending on how heavily it’s pruned in late fall or winter.
3. Rocky Mountain Columbine
Rocky Mountain columbine provides masses of blooms in blue, pink and white, with flowering starting in late spring and continuing through early summer. The plant has a height of 12-24 inches (30-60 cm), making it an ideal space filler for gardens under the shade of a walnut tree.
4. Sweet woodruff
Sweet woodruff makes a good groundcover under a black walnut tree. Its aromatic foliage produces a fresh, hay-like scent that adds sweet fragrance to an area. Sweet woodruff’s whorls of narrow leaves are especially ornamental in a massed planting. Dainty clusters of white flowers add light to shady spaces from late spring into early summer. Sweet woodruff grows from 12-18” tall (30-46cm).
5. Spice Bush
Spice bush is a tough shrub that tolerates growing near black walnut trees as well as drought, clay soils, deer, and dense shade. Spice bush gets its name from its aromatic foliage (the leaves produce a spicy scent when crushed). Spice bush is a dioecious species, meaning there are male and female plants. Clusters of yellow flowers appear just before the leaves emerge in spring, but it is only the female shrub that produces bright red berries in late summer. Spicebush is a host plant for swallowtail butterfly caterpillars.
Forsythia shrubs provide year-round structure to the landscape and spectacular spring blooms. Forsythia is a good boundary plant, as it can be grown as a hedge and pruned into shape after the flowers have ended. Forsythia prefers some sun so plant just beyond the leaf canopy of black walnut trees, where they can get more light.
7. Coral Bells, also known as Heuchera
Coral bells is a versatile flowering plant with varieties adapted to woodland, mountain and prairie conditions. At a relatively modest size of 6-36 inches (15-90 cm), coral bells make an excellent groundcover plant for the partial shade under walnut trees. The plant produces abundant, bell-like flowers in pink, white and red from spring through fall.
This shade garden favorite grows happily under black walnut trees. Hostas are available in a huge range of sizes and leaf color variations, offering endless design possibilities. Better yet, the trumpet-like flowers of hosta plants attract hummingbirds and pollinating insects!
Looking for more advice on solving landscape planting problems? Read our article on 10 Shrubs That Can Handle Harsh Conditions. It will give you ideas of what to plant in difficult areas where not everything easily thrives.