// SPECIES PROFILE · TREE · NATIVE · PREMIER HARDWOOD

Black Walnut

Juglans nigra

A large, fast-growing, long-lived bottomland hardwood that produces both the most valuable lumber tree in eastern North America and one of its richest edible wild nuts. Native to the deeper soils of NE Oklahoma's stream terraces and bottomland forests, black walnut is also the region's textbook allelopath: its roots and decaying husks release juglone, a quinone that suppresses the growth of many neighboring plants. Plant deliberately, plant for the right place, and you have a tree that will outlive you, feed wildlife generations from now, and (250 years on) yield a $5,000 veneer log to a descendant.

// QUICK FACTS

Family: Juglandaceae (walnut family)
Group: Tree (large deciduous)
Native range: E North America: New England → central FL, west to E Great Plains
USDA hardiness: Zones 4–9 (Tulsa = 7a/7b)
Mature size: 70–100 ft · spread 60–80 ft
Lifespan: 150–250 yrs typical · oldest 350+
Sex: Monoecious (separate male catkins & female flowers same tree)
Sun: Full sun (shade-intolerant)
Soil: Deep, well-drained loam; pH 6.0–7.5; needs depth
Water: Moderate; intolerant of standing water and prolonged drought
Bloom: Pendulous yellow-green catkins, late April–May
Nuts ripen: Sept–Oct, drop in green husks
Wildlife value: Squirrels, turkey, deer; lepidoptera host
Allelopathy: Juglone suppresses many crops & ornamentals
Lepidoptera hosts: Luna moth, regal moth, walnut sphinx, banded hairstreak
Ecological role: Bottomland canopy · nut mast · premier sawtimber

Black Walnut (Juglans nigra) — compound leaves and characteristic furrowed dark bark — *Juglans nigra* — a mature bottomland walnut with characteristic open crown. Photo via Wikipedia / Wikimedia Commons.

Identification

[ field key — bark · leaves · nuts · lookalikes ]

Habit & Bark

Large deciduous tree with a single straight bole and a relatively open, oval-rounded crown — the late leaf-out and early leaf-drop give the tree an unusually short canopy season for its size. Bark on mature trees is dark gray to nearly black with deep, interlacing diamond-shaped furrows separating narrow ridges — visually similar to mature ash bark but darker and rougher. Twigs are stout, with a distinctive chambered (segmented) pith visible when sliced longitudinally — a definitive identification character separating walnuts (and butternuts) from all other native trees.

Leaves

Pinnately compound, alternate, very large — 12–24 in long with 15–23 leaflets. Leaflets are lanceolate, finely serrate, pale green with a slight pubescence beneath; the terminal leaflet is typically smaller than or absent compared to the lateral pairs — a key character separating black walnut from butternut (Juglans cinerea, which has a large terminal leaflet). The whole leaf has a strong, distinctive "walnut" odor when crushed. Fall color is yellow, often dropping early after the first hard frost.

Flowers & Nuts

Monoecious, wind-pollinated: male catkins are pendulous, yellow-green, 2–4 in long; female flowers are inconspicuous spikes of 2–5 at the tips of new growth. Bloom in late April–May after leaf-out. The nut is a spherical green drupe-like fruit 1.5–2.5 in across, ripening Sept–Oct and falling with husk attached. The thick fibrous green husk stains skin and clothing a permanent dark brown (it was the standard black dye source for centuries) and must be removed to access the nut. Inside is a hard, irregularly sculpted shell containing a small but extraordinarily flavorful kernel.

Lookalikes

Most often confused with butternut (Juglans cinerea), which has a large terminal leaflet, a sticky-pubescent young twig, and elongated football-shaped fruit. Butternut is rare to absent in OK; almost any walnut you find here will be black walnut. Confusion with hickories (Carya spp.) is possible at a glance — chambered pith and the late leaf-out are the giveaways. Tree-of-heaven (Ailanthus altissima) has compound leaves that superficially resemble walnut but with a distinctive smooth bark, smelly crushed leaves, and gland-tipped basal teeth on each leaflet.

Habitat & Range in NE Oklahoma

Black walnut is widespread but selective across NE Oklahoma. It is most common on deep, fertile, well-drained bottomland soils — the river terraces of the Arkansas, Verdigris, Caney, Grand, and Illinois rivers, the alluvial bottoms of their major tributaries, and the rich coves of the Ozark plateau. It is uncommon on the dry sandstone uplands of the Cross Timbers, where soils are too shallow for its deep taproot, and is essentially absent from the very dry mixed-grass prairies west of I-35.

Site requirements are unusually demanding for a tree this widespread: black walnut needs a minimum 3–4 ft of rooting depth, neutral to slightly acidic soil pH, good drainage, and a fairly steady moisture supply through the growing season. It cannot tolerate seasonal flooding the way black tupelo or sycamore can, but it also cannot tolerate the prolonged droughts that scrub oaks shrug off. The classic NE Oklahoma walnut site is a second-terrace forest 50–200 ft above current river level: rich alluvial soil, no flooding, deep loam, full sun after canopy gaps.

Pre-settlement, black walnut was a relatively minor canopy component of mixed mesophytic forests — rarely abundant, almost always present. Its commercial value made it the first tree species systematically removed from much of its range during 19th-century lumbering, and old-growth walnuts are exceedingly rare anywhere in eastern Oklahoma today. The biggest and oldest specimens you are likely to see are in old farmsteads and historic city parks (Tahlequah, Muskogee, Bartlesville, Tulsa), where 100–150-year-old yard trees have escaped the chainsaw.

Ecology & Wildlife Value

[ juglone · mast · lepidoptera · thousand-cankers ]

Juglone & allelopathy

Black walnut roots, leaves, and fruit husks release juglone (5-hydroxy-1,4-naphthoquinone), an allelopathic compound that suppresses germination and growth of many other plants. The active zone extends roughly 50–80 ft from the trunk (deeper around root tips) and persists in soil for years after a tree is removed. Juglone-sensitive species include tomato, potato, eggplant, pepper, blueberry, apple, pine, rhododendron, azalea, hydrangea, and many others; juglone-tolerant species include most native grasses, oaks, hickories, persimmon, elderberry, paw paw, raspberries, and most warm-season legumes.

Hard mast for wildlife

Walnut nuts are a major fall food source for eastern fox squirrel, gray squirrel, white-footed mouse, eastern chipmunk, and white-tailed deer (which gnaw the husks even when they cannot crack the shell). Wild turkeys take fragments dropped by squirrels. Squirrels are the dominant disperser, scatter-hoarding nuts singly — the same caching behavior that produces the classic isolated walnut tree in the middle of an old pasture. Walnut mast is so calorie-dense that fox squirrel populations track walnut crop years measurably.

Lepidoptera larval host

Native Juglans hosts roughly 130 Lepidoptera species in North America. The most spectacular are the giant silkmoths: luna moth (Actias luna), regal moth / hickory horndevil (Citheronia regalis) — whose 6-inch caterpillars feed on walnut and hickory and are among the largest in North America — the walnut sphinx (Amorpha juglandis), and the banded hairstreak (Satyrium calanus). All overwinter as pupae in soil under the canopy — a reason not to over-rake walnut leaf litter.

Diseases & emerging threats

The major emerging threat is thousand-cankers disease, caused by the fungus Geosmithia morbida vectored by the walnut twig beetle (Pityophthorus juglandis). The disease has killed black walnut plantings across the western US and was confirmed in Tennessee, Pennsylvania, Maryland, Virginia, and North Carolina by the 2010s. Oklahoma is currently within the natural range of black walnut but the disease has been detected in the state. Walnut anthracnose (Gnomonia leptostyla) causes leaf spotting and premature defoliation in wet springs, primarily cosmetic. Fall webworm builds large summer tents on walnut foliage; aesthetic only.

Juglone — what NOT to plant near a walnut: Inside the 50–80 ft drip-line zone of a mature black walnut, expect failure from: tomato, pepper, eggplant, potato (Solanaceae generally); apple, blueberry, crabapple, mountain laurel, rhododendron, azalea, hydrangea, pine, fir, spruce, yew, silver maple, white birch, peony, columbine, and a long list of herbaceous garden plants. Even after a walnut is removed, juglone persists in soil for 2–5 years or longer; do not assume the site clears immediately. Confine vegetable gardens to raised beds with imported soil if a walnut is nearby.

Thousand-cankers disease — do not move firewood: The most actionable thing any NE Oklahoma resident can do for black walnut conservation is buy firewood locally (within ~50 miles) and refuse to transport walnut logs across state lines. Federal and state quarantines restrict movement of walnut wood from infested areas. If your walnut tree dies suddenly with progressive crown dieback, contact OSU Extension Plant Disease & Insect Diagnostic Lab in Stillwater for confirmation.

Horticulture & Care

[ siting · direct seed vs container · care · cultivars ]

When to plant intentionally

Black walnut is the right choice for: large rural properties with deep soil and room to spare, nut orchards for serious harvest, shade and windbreak trees in farmsteads, and multi-generational hardwood investments on land that will stay in family or trust ownership for 80+ years. It is wrong for: small urban lots, anywhere within 80 ft of a vegetable garden, fruit orchard or sensitive landscape plants, or compacted shallow-soil sites.

Establishment

Direct seeding (preferred for forestry/nut plantings): collect fresh hulled nuts in October, plant 2–3 nuts 2 in deep where the tree is to grow permanently, cage to prevent rodent excavation. Walnut taproot is 2–3 ft on the seedling and rarely transplants well.
Container stock: use only when small (1 in caliper or less) and plant in fall or late winter. B&B trees often die from root disturbance.
Sun: full sun — walnut is highly shade-intolerant.
Soil: deep (3+ ft), well-drained loam or sandy loam; pH 6.0–7.5; tolerates higher pH than most eastern hardwoods.
Spacing: 30–40 ft for shade or specimen; 12–15 ft on center for forestry plantings (plan to thin); 30–40 ft for nut orchards.
Mulch: 3–4 in wood chip mulch out to dripline; do not pile against trunk.
Water: deep weekly soakings during the first three growing seasons. Established trees handle moderate drought but suffer in extreme heat.

Pruning & care

For lumber-quality trees, plan a structural pruning regime in the first 10–15 years to develop a single straight central leader and a clean butt log. Remove competing leaders early and prune low lateral branches gradually to a clear stem of 16–20 ft. For nut or shade-tree applications, only minor corrective pruning is needed. Always prune in dormant season — black walnut "bleeds" from cuts in spring (not generally harmful but unsightly). Avoid mechanical bark damage; walnut compartmentalizes wounds slowly.

Pests & diseases

Thousand-cankers disease — the major emerging threat (above).
Walnut anthracnose — leaf spotting in wet springs; cosmetic.
Fall webworm — large communal tents on terminal branches; aesthetic only.
Walnut caterpillar (Datana integerrima) — periodic outbreaks defoliate trees; recovery typical.
Walnut husk fly — lays eggs in the green husk; maggots tunnel and discolor the husk but rarely affect kernel quality.
Carpenter worm & ambrosia beetles on stressed trees.

Notable cultivars (for nut production)

Cultivar	Origin	Distinguishing feature	Notes for Tulsa
'Thomas'	PA, ~1881	Large nut, thin shell, ~25% kernel	Most widely planted nut cultivar; reliable in Zone 7.
'Sparrow'	IA	Heavy producer, medium nut, easy crack-out	Useful for serious home nut harvests.
'Emma K'	IL	Very large nut, excellent kernel percentage	Compact tree habit; suitable for smaller orchard rows.
'Surprise'	MO	Thin shell, late ripening	Pollinates well with Thomas.
'Football' / 'Football-II'	TN	Large oblong nut, easy crack	Widely available in mid-South.
Wild local seedlings	OK river bottoms	Locally adapted, wild flavor	Use for landscape and ecological plantings; flavor often surpasses cultivars even with smaller kernel.

Cultural & Material Uses

Black walnut may be the most economically productive native hardwood tree in North America, with three independent revenue streams (lumber, nuts, dye/husks) and a 200-year cultural footprint of furniture, gunstocks, and culinary craftsmanship.

Lumber & veneer: the most valuable native hardwood in North America. Heartwood is dark chocolate-brown with purple highlights, straight-grained, dimensionally stable, and works beautifully — the standard for fine furniture, gunstocks, musical instruments (acoustic guitar backs and sides, grand piano cabinets), interior millwork, and high-end veneers. Veneer-quality logs from a clear-stemmed 20–30 in DBH tree can sell for $5,000–$20,000+ each. Sustainable harvest rotations are 80–120 years.
Edible nut: the strongest-flavored of all commercial nuts — rich, slightly bitter, perfumed. The Hammons Products Company (Stockton, MO) processes the bulk of US wild-harvested black walnuts and pays hullers per pound at fall buying stations across MO, AR, KS, and OK. Nuts go into premium ice cream (Black Walnut is the classic Missouri-Ozark flavor), baked goods, and divinity candy. Cracking is slow work; commercial mechanical crackers are now available for serious home harvests.
Husk dye & ink: the green-to-black husks are the source of walnut ink — a permanent, lightfast dark-brown ink used by Old Master draftsmen including Rembrandt and Da Vinci, and still produced today for archival drawing and calligraphy. Husks also dye wool a permanent rich brown (no mordant required) and were the standard dark hair dye through the 19th century.
Husks as natural cleaner: ground walnut shell is widely used as an industrial abrasive blast media (softer than sand, harder than baking soda), a cosmetic exfoliant, and a polishing medium for jewelry and gun barrels.
Indigenous uses: documented across many Eastern Woodlands and southern Plains nations including the Cherokee, Iroquois, Meskwaki, and Comanche. Uses include nut foods (raw, roasted, ground into oil and meal), dye for baskets and skins, husks as a fish-stunning toxin in small impoundments (a practice now illegal), and bark/leaf decoctions for various medicinal purposes. Black walnut has long been a tribal trade good across North America.
Folk medicine & modern phytochemistry: walnut hull tinctures were widely used in 19th- and early-20th-century American herbalism for intestinal parasites and skin complaints. Juglone shows in vitro antimicrobial and anti-fungal activity but is also moderately toxic; do not consume green husks or hull tinctures without informed guidance.

Sources & Further Reading

USDA NRCS PLANTS Database — Juglans nigra: plants.usda.gov/plant-profile/JUNI
USDA Forest Service Silvics of North America, Vol. 2 (Hardwoods) — Williams, R.D., chapter on Juglans nigra: srs.fs.usda.gov — juglans/nigra
USDA Forest Service Fire Effects Information System (FEIS) — Juglans nigra: fs.usda.gov/database/feis — jugnig
Lady Bird Johnson Wildflower Center — Juglans nigra: wildflower.org — JUNI
Missouri Botanical Garden Plant Finder — Juglans nigra with juglone-tolerance lists.
OSU Extension — Black Walnut Toxicity (Juglone) and Plant Sensitivity, fact sheet HLA-6452.
OSU Plant Disease & Insect Diagnostic Lab — Thousand Cankers Disease of Walnut, EPP-7331.
Hammons Products Company (Stockton, MO) — commercial wild-harvest pricing and buying station network across the Ozarks and southern Great Plains.
Williams, R.D., USDA NA-FR-04-99 — Black Walnut: An American Wood.
Tisserat, N. et al. (2009), "Thousand cankers disease of black walnut," Plant Health Progress, on the discovery and spread of TCD.
Wikipedia — Juglans nigra: en.wikipedia.org/wiki/Juglans_nigra (CC BY-SA 4.0; portions of the description summarize Wikipedia content).
Native Tree Society & Eastern Native Tree Society archives — documentation of largest known black walnut specimens.

Black Walnut

Identification

Habit & Bark

Leaves

Flowers & Nuts

Lookalikes

Habitat & Range in NE Oklahoma

Ecology & Wildlife Value

Juglone & allelopathy

Hard mast for wildlife

Lepidoptera larval host

Diseases & emerging threats

Horticulture & Care

When to plant intentionally

Establishment

Pruning & care

Pests & diseases

Notable cultivars (for nut production)

Cultural & Material Uses

Sources & Further Reading

Companion Planting