Black walnut tree: identification, juglone toxicity, growing and harvest

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At a glance

How to identify a black walnut tree

Black walnut is one of the easiest large native trees to identify once you know four features. NC State Extension, the Arbor Day Foundation and Minnesota DNR describe these hallmarks:

• Leaves: alternate and pinnately compound, 12–24 inches long, with about 15–23 narrow, finely toothed leaflets (Minnesota DNR gives 14–22). The National Park Service notes the tip leaflet is often missing, and that crushed leaves and fruit have a spicy, citrusy smell.
• Bark: dark brown to nearly black, thick, with deep furrows and ridges forming a rough diamond pattern.
• Nuts: round green husks borne singly or in pairs. The husk stays closed when ripe and isn't sticky. Inside is a thick, hard, ridged black shell. The husk heavily stains skin and clothing.
• Twigs (winter ID): slice a twig lengthwise and the pith is divided into chambers by thin, buff-colored partitions. The leaf scar is often likened to a "monkey face."

Look-alikes

Where black walnut grows

Black walnut is native across a broad swath of central and eastern North America and is hardy in zones 4–9. NC State Extension and Minnesota DNR note that it does best on rich bottomlands and moist, fertile lower slopes; NC State adds that survival, growth and quality on less fertile sites are problematic.

It is shade-intolerant. Maryland DNR says young trees can get by with part sun for a couple of years but grow best in full sun, which is why black walnuts are common along fencerows, field edges and forest openings.

Black walnut toxicity: juglone and what not to plant nearby

Black walnut produces juglone. Per University of Wisconsin–Madison Extension, it is released from the roots and also occurs in lower concentrations in the leaves, nut hulls, bark and wood. Sensitive plants wilt, sometimes even in moist soil, then brown and die; tomato leaves yellow, twist and pucker. Because juglone isn't very soluble, its effect is worst at the edge of the crown, but roots reach beyond it, so UW puts the toxic zone of a mature tree within 50–60 feet of the trunk. Cutting the tree down doesn't solve the problem right away: juglone can persist until the roots decompose, which can take five or more years.

Lists are from UW–Madison Extension; NC State Extension also names azaleas, peonies, tomatoes, apples and blueberries as sensitive. Lists differ somewhat between sources, so check a few before committing. The species-by-species list is in plants that tolerate black walnut juglone.

Managing juglone in a yard

• Keep vegetable gardens out of the root zone, especially tomatoes and peppers. Distance is the most reliable fix.
• Rake up husks and leaves and keep them out of beds. UW says walnut leaves are safe only after thorough hot composting, since juglone breaks down with exposure to air, water and bacteria.
• Wood chips are fine: UW notes black walnut wood chips are safe as mulch, because the wood doesn't break down to produce juglone.
• Use root barriers and shallow-rooted plants: UW says barriers may help keep walnut roots out of garden areas, and because walnut roots often don't occupy the surface soil, many shallow-rooted plants grow under the trees without trouble.
• Pull volunteer walnut seedlings while they are small.

How to grow a black walnut tree

From seed

1. Collect nuts after they fall and remove the husks. Iowa State University Extension says to put the nuts in water and discard any that float, which aren't viable.
2. Sow in fall, 1–2 inches deep, where you want the tree, and let winter provide the cold period. Alternatively, Iowa State says to stratify the nuts at 34–41°F for 90–120 days and plant in spring. Direct sowing avoids transplanting: NC State notes the very deep taproot makes black walnut difficult to transplant.
3. Protect the spot from squirrels with a buried hardware-cloth cage or a screen pinned over the soil, removed in spring when shoots appear.
4. Keep weeds and grass away from seedlings for the first few years.

From nursery stock

Plant bare-root seedlings in early spring while dormant, at the depth they grew. Use a planting hole deep enough for the taproot without bending it (see how to plant bare-root trees). Give each tree full sun and plan for a crown 50–70 feet across. For nuts, NC State lists cultivars such as 'Thomas', noted for easy-to-crack nuts with excellent flavor. The Arbor Day Foundation says trees begin bearing in 12–15 years, and NC State says production can start as early as 6 years but usually takes 20 for a substantial crop. Black walnut is self-fertile but wind-pollinated, so Arbor Day suggests planting more than one tree for a better crop.

Care

• Water young trees deeply in dry spells for the first two or three summers. NC State rates established trees as heat- and drought-tolerant.
• Prune in the dormant season to keep a single straight leader, and remove low limbs gradually if you want a clear trunk. Avoid heavy pruning of mature trees.
• Fertilize only on a soil-test basis. Good sites rarely need it.

Harvesting and processing black walnuts

Nuts ripen in fall; Iowa State University Extension says they typically mature in mid- to late September in Iowa, and NC State gives October farther south. The fruit is ready when the hull turns from solid green to yellowish and can be dented with your thumb. Crack a few first to make sure the kernels are filled.

1. Remove the husks right after harvest. Iowa State warns that hulls left on discolor the nutmeat and make it strong-tasting, and the juice stains skin, clothing and concrete, so wear rubber gloves.
2. Wash the hulled nuts and sort them: unfilled nuts float, filled nuts sink.
3. Dry the nuts for two to three weeks in shallow layers, no more than three nuts deep, on a wire screen in a cool, dry, well-ventilated place such as a shed or garage.
4. Crack them with a hammer or heavy-duty nutcracker; soaking for 1–2 hours first reduces shattering. Store kernels in a plastic bag in the freezer, where they keep almost indefinitely, or refrigerate for short periods.

Blackened, slimy husks are often a sign of walnut husk fly. Harvesting and hulling promptly limits the staining.

Black walnut wood and what a tree is worth

Black walnut is a hardwood prized for furniture, cabinetry, gunstocks and veneer. Minnesota DNR describes the heartwood as of superior quality and value, and Iowa State University Extension calls it one of the most valuable timber trees in Iowa. Whether a particular tree is worth much depends on factors that are easy to check:

• Trunk diameter and length: buyers want a large-diameter, straight, branch-free log. Minnesota DNR notes that small trees contain mainly light-colored sapwood, which is not durable.
• Defects: rot, large knots, splits and embedded metal from fences, nails or clotheslines reduce value. Yard trees often contain metal.
• Access: trees near houses, wires or septic systems cost more to remove safely, which lowers what a buyer will offer.

Before selling, ask your state forestry agency or a consulting forester to appraise the tree. Get more than one bid, and don't accept a door-to-door offer on the spot.

Pests and diseases

• Thousand cankers disease: the most serious threat. Per the Tennessee Division of Forestry, the walnut twig beetle carries the fungus Geosmithia morbida, and repeated attacks create many small cankers under the bark. The first symptom is usually flagging (leaves wilting and yellowing in midsummer), followed by a thinning crown and branch dieback; once symptoms show, the tree declines rapidly and can die within a few years. Some states have quarantined walnut logs, lumber, firewood and mulch (Tennessee did until 2021). Don't move walnut wood, and report suspected cases to your state forestry or agriculture department. See also how to identify tree cankers.
• Walnut anthracnose and leaf spot: NC State Extension lists anthracnose, bacterial blight, leaf spot and canker among potential diseases. Rake up and remove fallen leaves.
• Caterpillars: NC State lists fall webworm and a variety of leaf-chewing caterpillars; usually cosmetic on large trees.
• Walnut husk fly: blackened, slimy husks. Harvest promptly.
• Squirrels: NC State notes squirrels eat the nuts, and they bury them widely. Pull volunteer seedlings while small.

Common mistakes

Frequently asked

How do you identify a black walnut tree?

Look for long compound leaves with about 15–23 narrow leaflets (the tip leaflet is often missing), dark bark with diamond-patterned furrows, and round green-husked nuts borne singly or in pairs. In winter, a twig cut lengthwise shows chambered, buff-colored pith. Butternut has lighter bark and elongated nuts.

What should you not plant near a black walnut tree?

University of Wisconsin–Madison Extension lists tomatoes, potatoes, peppers, eggplant, apples, pears, blueberries, azaleas, rhododendrons and lilacs among sensitive plants. Beans, squash, onions, corn, some hostas, ferns, daylilies, redbud and oaks are more tolerant. UW puts the toxic zone of a mature tree within 50–60 feet of the trunk.

Is black walnut toxic?

To many plants, yes: the roots, and to a lesser extent the leaves, husks and bark, contain juglone, which damages sensitive species growing nearby. The nut kernels are edible. NC State Extension lists black walnut as a problem for dogs and horses, so ask a veterinarian about any animal exposure.

How fast do black walnut trees grow?

Fast on good sites: NC State Extension rates its growth as rapid, and Minnesota DNR says it grows rapidly in good soil. On thin, dry or shaded sites it struggles. The Arbor Day Foundation says trees begin bearing in 12–15 years, and a substantial crop can take about 20.

How do you grow black walnut from seed?

Remove the husks, discard nuts that float in water, and sow the rest 1–2 inches deep in fall right where you want the tree, covered with hardware cloth against squirrels. Or stratify them at 34–41°F for 90–120 days and plant in spring, per Iowa State University Extension. Direct sowing beats transplanting because of the deep taproot.

How much is a black walnut tree worth?

It depends on trunk diameter, straightness, length of clear log, defects such as rot or embedded metal, and how easy the tree is to remove. Small trees are mostly sapwood and worth little. Ask your state forestry agency or a consulting forester for an appraisal and compare several bids.

Is black walnut a hardwood?

Yes. Black walnut is a deciduous hardwood whose heavy, hard, chocolate-brown heartwood is prized for furniture, cabinetry, gunstocks and veneer.

Sources

1. Juglans nigra (black walnut), NC State Extension Gardener Plant Toolbox
2. Learn About Black Walnut, Arbor Day Foundation
3. Black walnut (Juglans nigra), Minnesota Department of Natural Resources
4. Black Walnut (Juglans nigra), University of Minnesota Urban Forestry Outreach, Research & Extension
5. Landscaping In Spite of Black Walnut, University of Wisconsin–Madison Extension
6. Native Plant Profile: Black Walnut (Juglans nigra), Maryland Department of Natural Resources
7. Black Walnut Trees, National Park Service
8. How to Harvest, Crack, and Store Black Walnuts, Iowa State University Extension and Outreach
9. Germination of Tree Seed, Iowa State University Extension and Outreach
10. Thousand Cankers Disease, Tennessee Department of Agriculture, Division of Forestry

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