Black walnut | The Morton Arboretum

The black walnut is a Chicago-area native tree that provides excellent shade for large properties. It needs to be sited with care, since the tree produces a chemical that is toxic to some other plants. The fruit is a rounded, yellow-green husk containing a nut that is a food source for squirrels. The black walnut also attracts the banded hairstreak butterfly, serving as a caterpillar host.

This species is native to the Chicago region according to Swink and Wilhelm’s Plants of the Chicago Region, with updates made according to current research.

Family (English) Walnut
Family (botanic) Juglandaceae
Planting site City parkway, Residential and parks, Wide median
Tree or plant type Tree
Foliage Deciduous (seasonally loses leaves)
Native locale Chicago area, Illinois, North America
Size range Large tree (more than 40 feet)
Mature height 50-75 feet
Mature width 30-50 feet
Light exposure Full sun (6 hrs direct light daily)
Hardiness zones Zone 4, Zone 5 (Northern Illinois), Zone 6 (City of Chicago), Zone 7, Zone 8, Zone 9
Soil preference Alkaline soil, Moist, well-drained soil
Tolerances Alkaline soil, Clay soil, Occasional drought, Road salt
Season of interest early fall
Flower color and fragrance Inconspicuous
Shape or form Round
Growth rate Moderate
Transplants well Yes
Planting considerations May be difficult to find in nurseries, Messy fruit/plant parts
Wildlife Cavity-nesting birds, Game mammals, Songbirds
Has cultivars Yes

Native geographic location and habitat:

Black walnut is native to much of the Eastern United States. C-Value: 5.

Bark color and texture:

The bark is medium brown and has thick, interfacing ridges.

Leaf or needle arrangement, size, shape, texture, and color:

Alternate, pinnately compound leaves with the terminal leaflet often missing. Leaves are 1 to 2 feet long. The leaflets are toothed and aromatic when crushed. Leaf color is green in summer, changing to yellow in fall.

Flower arrangement, shape, and size:

The flowers are inconspicuous. The male flowers are in drooping clusters and the female flowers in terminal spikes.

Fruit, cone, nut, and seed descriptions:

The fruit is a yellow-green, rounded husk, up to 2 inches in diameter, and contains an edible nut.

Plant care:

Falling black walnut fruit is a potential safety hazard and can cause yard litter as well as staining on sidewalks. All parts of the tree produce a chemical, juglone, which is toxic to many plants. The trees are difficult to transplant due to a deep taproot.

Do not prune in spring as black walnut is a ‘bleeder’ (sap will run from wounds). Black walnut should be pruned in the dormant season.

List of pests and diseases:

Black walnut is targeted by walnut and yellow leaf caterpillars. It is susceptible to anthracnose, which may lead to late summer defoliation. Thousand canker disease is a serious problem occurring in some states (not yet reported in Illinois). It is tolerant of high pH soil and shows some tolerance of salt.