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Black Walnut Growing Profile

Juglans nigra

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Black walnut tree Photo © Wikimedia user Mongo
Black walnut leaves and fruitPhoto © Wikimedia user Mongo

Overview of Black Walnut:

Black walnuts are deciduous trees are commonly found in the Eastern United States. They are widely used for food and lumber, and make stately shade trees.

This is an alleopathic tree as it produces juglone, a chemical that is toxic for many plants.

Latin Name:

This walnut tree is Juglans nigra and it is found in the Juglandaceae family.

Common Names:

You may see this listed as black walnut, American walnut or eastern black walnut.

USDA Hardiness Zones:

Zones 4-9 are preferred for this plant.

Size & Shape:

It grows 50-150' tall and 40-75' wide, with a rounded or oval shape.

Exposure:

Plant this tree in full sun.

Foliage/ Flowers/Fruit:

The leaves are pinnately compound and 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. They are fragrant.

These trees are monoecious. The male flowers are catkins and the female flowers are small clusters. They appear in spring at different times, so more than one tree should be planted to encourage pollination. The flowers are wind pollinated.

Trees start producing after 12-15 years. The nuts ripen in September or October.

Design Tips:

You will need to consider carefully when planting near a black walnut tree as it is a prime example of allelopathy. 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 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.

These trees are not easy to transplant since they have a deep taproot. Seeds or seedlings should be planted.

Walnuts should be stratified (placed in a cold location) before planting.

This plant does 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.

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. UC Davis did a study and found that more nuts were produced if trees were pruned every other year.

Start pruning your trees when they are three 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 ambrosia beetle, black walnut caterpillar, bud borers, casebearers, curculios, mites, scale, tent caterpillars and webworms.

Diseases include blight, cankers, leaf spot powdery mildew, root rots and walnut anthracnose.

Additional Facts:

These nuts 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.

It is related to the pecan and hickory.

Walnut Recipes to Try:

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