Black Walnut

Black Walnut Juglans nigraBlack walnut (Juglans nigra) is the most valuable tree in Iowa. The leaves are alternately arranged on the twig, compound with 1 to 23 leaflets, and ranging in size from 1 to 2 feet. The terminal leaflet is either absent or much smaller than the leaflets near the middle of the leaf. The leaflets are 2 1/2 to 4 inches long, finely toothed and slightly hairy below the surface.

black walnut tree showing bark
Black Walnut Tree - Photo by Paul Wray, Iowa State University

Habitat: Found in moist woodlands, especially bottomlands. Grows in most of the state.

Hardiness: Zones 4 through 9

Growth Rate: Moderate to Fast

Mature Shape: Usually has a full, well-formed trunk with no branches for quite some distance from the ground.  The crown is oval to rounded and somewhat open. 

Height: 50-75 feet (may reach maximum height of 125 to 150 feet)

Width: 50-70 feet 

Site Requirements: Rich, well-drained soils of bottomlands, terraces and lower slopes.  It can tolerate drier soils, but will grow much more slowly in dry conditions.

Leaves: Alternate compound with more than 11 leaflets

Flowering Dates: April - June

Seed Dispersal Dates: October - November

Seed Bearing Age: 12 years

Seed Bearing Frequency: Yearly 

Seed Stratification: Prechill for 3-4 months at 34°F to 40°F

black walnut twig and cross section of twig showing chambered pith
Black Walnut Twigs - Photo by Paul Wray, Iowa State University

Twigs are moderately stout to very stout, gray-green or light brown in color and have a chambered pith which is tan to light brown in color. The fruit is a round nut, 1 1/2 to 2 inches in diameter composed of a single seed enclosed by a hard, corrugated shell and a green, semi-fleshy husk that eventually turns black. The bark is dark gray or black, deeply furrowed, with interconnecting ridges that become thick and blocky on larger slow growing trees. 

Black walnut is native throughout the state. It occurs on a variety of sites, but is most common on bottomlands, stream terraces, and moist sheltered coves and slopes. It grows best on deep, rich, well drained soils. 

Common associates of black walnut vary as much as the sites but include oaks, basswood, maples, ashes, and Kentucky coffeetree. Walnut is considered a good invader species and often is one of the first species to become established in abandoned fields and pastures. Squirrels are especially fond of the nuts and aid in the establishment of walnut on these sites. 

Walnut is a moderately fast to fast growing (2-3 feet in height per year) species especially on the best sites. It commonly attains heights of 90 feet and diameters of 2 to 3 feet. 

Although black walnut has been used a lot as a shade tree, it is considered to be a poor choice because its relatively open crown provides less shade than other species, its leaves tend to drop early and the nuts tend to be messy. However it is prized for its nuts which proved a distinctive taste for baked products. The Iowa Nut Growers Association continue and have made several selections of black walnut for the outstanding size and quality of their nuts. 

Iowa continues to rank as one of the leading producers of walnut lumber and veneer. The rich brown color, attractive grain of the wood, and the woodworking quality of the wood makes it the most valuable timber tree in Iowa.

Diseases that Can Affect Black Walnut

Insects that Can Affect Black Walnut


black walnut fruit
Black Walnut Fruit - Photo by Paul Wray, Iowa State University

green black walnut fruit
Black Walnut Fruit - Photo by Paul Wray, Iowa State University

black walnut leaves
Black Walnut Leaves - Photo by Paul Wray, Iowa State University