Overview:
Black walnut trees are commonly found in the Eastern United States. They are widely used for food and lumber, and make stately shade trees.
Black walnut is an alleopathic tree - it produces juglone, a chemical that is toxic for many plants.
Latin Name:
Juglans nigra
Common Names:
Black walnut, American walnut, Eastern black walnut
USDA Hardiness Zones:
Zones 4-9
Size & Shape:
Black walnut grows to 50-120' tall and 50-75' wide, with a rounded shape.
Exposure:
Plant black walnut in full sun.
Foliage/ Flowers/Fruit:
The leaves of black walnut are pinnately compound - the true leaf is made up of 15-23 leaflets. Each dark green leaflet is 2-5" long. The total true leaf is 12-24" long. The leaves are fragrant.
Black walnuts are monoecious - separate green male and female flowers on the same tree. The male flowers are catkins and the female flowers are small clusters. They appear in spring at different times - more than one tree should be planted to encourage pollination. Black walnut is wind pollinated.
Black walnut trees start producing after 12-15 years. The nuts ripen in September or October.
Additional Facts:
Black walnuts need to be cured after picking. Remove the hull and store in layers for 2-3 weeks, in a place that is cool, dry and well ventilated. Do not let direct sunlight hit them.
Design Tips:
You will need to consider carefully when planting near a black walnut tree. It produces a chemical called juglone that is toxic to many plants.
You can reduce juglone by:
- Cleaning up any fallen leaves, branches or nuts.
- Add lots of organic matter such as compost - this will help soil microorganisms that break down the juglone.
- Avoid using black walnut parts as mulch. It can be used in compost piles if it is given enough time to be broken down by soil microorganisms.
- Choose plants that are tolerant to juglone.
Plant more than one black walnut tree to aid in pollination.
Growing Tips:
Grow black walnut trees in rich, well-drained soil that is deep. The pH should be between 5 and 8.
Black walnut trees are not easy to transplant - they have a deep taproot. Seeds or seedlings should be planted.
Walnuts should be stratified (placed in a cold location) before planting.
Black walnut trees do not do well in competing for nutrients and water, so make sure the land around them is well weeded.
Maintenance/Pruning:
Pruning is needed less if black walnut trees are planted close together.
Per the Virginia Cooperative Extension: "Try to prune trees so that the crown width will be AT LEAST one-half the total height of the tree and maintain at least the upper third of the stem supporting live branches. "
Pruning is best done in early spring before the leaves appear. Make sure that you do not prune while new growth and budding is happening.
Start pruning your black walnut trees when they are 3 years old.
Black walnut trees tend to form several leaders. Prune the tree so that it only has one central leader.
Pests & Diseases:
Pests that may affect black walnut are black walnut caterpillar, tent caterpillars, webworms, scale, mites, bud borers, curculios, ambrosia beetle, and casebearers.
Diseases include walnut anthracnose, blight, root rot, cankers, and leaf spot.
