Black walnut | Juglans nigra | The Morton Arboretum

Black walnut is a Chicago-area native tree that provides excellent shade for large properties.

Its fruit is a rounded, yellow-green husk containing a nut that is a food source for squirrels. Black walnut also attracts the banded hairstreak butterfly, serving as a caterpillar host.

Black walnut trees need to be sited with care, since the tree produces a chemical that is toxic to some other plants.

This species is native to the Chicago Region according to Wilhelm and Rericha’s Flora of the Chicago Region.

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
Drought tolerance Moderately tolerant
Other tolerances Alkaline soil, clay soil, 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: 3.

Bark Description

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

Leaf Description

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 Description

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

Fruit Description

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, Diseases and Tolerances

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.