What Grows Under Walnut Trees? Juglone Explained

Walnut trees release a natural chemical called juglone from their roots, leaves, buds and nut hulls. It stunts or kills sensitive plants – above all tomatoes, potatoes, peppers and aubergines (eggplant), plus apples, blueberries, azaleas and rhododendrons. Many plants are tolerant: grasses, most legumes, onions, beets, carrots, beans, squash, corn, and garden plants such as hostas, daffodils, ferns and astilbe. Black walnut produces the most juglone; the English (Persian) walnut produces less but can still affect sensitive crops. In orchards, keep the ground under the trees covered with grass, mulch or a cover crop rather than cultivating it.

What Is Juglone?

Juglone (5-hydroxy-1,4-naphthoquinone) is a compound walnut trees use to reduce competition from nearby plants – a process called allelopathy. It is found in the highest concentrations in buds, leaves, stems, nut hulls and roots. Its precursor, hydroxyjuglone, is colourless and harmless until it is exposed to air or soil, where it turns into toxic juglone.

• Black walnut (Juglans nigra) and butternut (J. cinerea) are the strongest producers.
• English/Persian walnut (J. regia) and hickories produce less, but still enough to harm very sensitive plants.
• The effect is strongest under the canopy and in the root zone, which can extend well beyond the drip line.
• Juglone dissolves poorly in water and does not move far in soil, but it lingers in poorly drained, wet soils with little organic matter. It can persist for years in the roots after a tree is removed.

Symptoms of Juglone Damage

Sensitive plants turn yellow and wilt, especially in hot, dry weather, and eventually die. Juglone wilting is often confused with drought, but there is a difference: drought wilting develops slowly and recovers with watering, while juglone wilting comes on quickly even in moist soil and later stops responding to water.

Plants Sensitive to Juglone

• Vegetables: tomato, potato, pepper, aubergine (eggplant), and often cabbage family crops and asparagus.
• Fruit: apples, pears, blueberries, blackberries.
• Shrubs: azaleas, rhododendrons, mountain laurel, lilac.
• Flowers: peonies, chrysanthemums (some), petunias, lilies (some).

Lists vary between sources because sensitivity depends on contact, plant health and soil conditions – treat them as guides.

What Grows Well Under Walnut Trees

Gardening Near Walnut Trees

• Site vegetable gardens well away from the canopy and root zone.
• Grow sensitive crops in raised beds lined with landscape fabric and filled with fresh topsoil.
• Improve drainage and add organic matter – healthy, well-aerated soil breaks juglone down faster.
• Rake up walnut leaves, hulls and twigs; do not let them rot near planting areas.
• Do not compost walnut leaves or hulls for sensitive crops unless fully composted for several months, and never use walnut bark or wood chips as mulch around sensitive plants.
• After removing a walnut, wait several years before planting sensitive species in its place.

Managing the Orchard Floor

Walnuts are known for their taproot, but most of the fine feeding roots lie in the top layer of soil under the canopy. Deep ploughing or heavy disking cuts these roots, stresses the tree in summer and leaves wounds that invite crown gall and root rot. Modern orchards therefore keep the ground covered and avoid cultivation unless it is needed – for example to let winter rain soak into compacted soil.

Intercropping Young Orchards

A young walnut orchard earns nothing for several years, and the wide rows can grow a crop in the meantime:

• Keep the intercrop at least 1–1.5 m (3–5 ft) away from each tree row, widening the strip as trees grow.
• Choose low crops that do not shade the trees or compete heavily for water – beans and other legumes, onions, squash or forage legumes work well.
• Avoid tomatoes, potatoes, peppers and aubergines.
• Do not plant apples, pears or quinces in or next to the orchard – they host codling moth.

Cover Crops and Weed Control

Weeds compete with walnuts for water and nutrients and shelter pests. Mowing, mulch and cover crops are the main tools. Legumes such as clover, alfalfa or vetch are the usual cover crops, sometimes mixed with grasses. Benefits include:

• less soil erosion and better water infiltration;
• nitrogen added by legumes;
• more organic matter and healthier soil life;
• fewer weeds and easier harvest on a firm, mown surface.

Keep a weed-free circle around young trees, and never let non-selective herbicide touch young bark or shoots.

Related Walnut Guides

• How to grow walnut trees: complete guide
• When to plant walnut trees
• Walnut tree fertilizer
• Codling moth and husk fly: when to spray

Frequently Asked Questions

What can I plant under a walnut tree?

Juglone-tolerant plants such as grasses, clover, hostas, daffodils, ferns, astilbe, onions, beets, carrots, beans, squash and corn.

What plants are killed by walnut trees?

Tomatoes, potatoes, peppers and aubergines are the most sensitive, along with apples, blueberries, azaleas and rhododendrons.

Do English walnuts produce juglone?

Yes, but less than black walnut. Very sensitive crops can still be affected close to the tree.

How far does juglone spread from a walnut tree?

It is concentrated in the root zone, which can reach well beyond the edge of the canopy; with black walnut, effects are often reported up to 15–25 m (50–80 ft) from large trees.

Can I use walnut leaves in compost?

Juglone breaks down with time and air. Compost walnut leaves thoroughly for several months before using the compost around sensitive plants, or keep them out of the vegetable garden compost altogether.