Managing black walnut forests

You will generally find black walnut scattered among other tree species. Pure stands are not common, but do occur. 

Black walnut leaves and roots actively secrete material that’s toxic to some trees, shrubs and herbaceous plants.

Walnut grows best on lower north- and east-facing slopes, stream terraces and floodplains. It is common on limestone soils and grows well on well-drained and fertile soils. 

Poor sites for walnut include steep south- and west-facing slopes, narrow ridgetops and poorly drained sites. Soils with a high amount of sand or sites where bedrock is within two and a half feet of the surface are not suitable.

You can naturally regenerate black walnut from seed, but black walnut trees are normally a minor component of a woodland so you can rarely rely on natural regeneration. Stumps sprout if trees are less than 20 to 30 years old. Plant seedlings to supplement natural regeneration.

Plant seedlings at a spacing of 10 by 10 feet to produce timber, and 15 by 15 feet to produce a combination of timber and nuts. Black walnut won’t tolerate shade. 

To prepare a woodland site, cut or kill all woody vegetation with diameters larger than half an inch with herbicides. Grassy and weedy sites may need herbicide treatments to kill existing vegetation before you plant seedlings.

Plant seedlings in the spring as soon after the ground thaws as possible. Seedlings at least 1/4 inch in diameter at 1 inch above the root collar will compete better with weeds and have a better survival rate than smaller seedlings.

Seeds are easier and less expensive to plant than seedlings, but should be protected from squirrels and other rodents. 

Mechanical barriers (hardware cloth, tin cans, etc.) are the most reliable, but they’re expensive and time-consuming to install. Alternatively, direct seeding a large area can overwhelm seed predators. This approach is becoming more common in Minnesota.

You can sow seed in either the fall or spring. You don’t need to remove the husks for fall planting. Spring planting eliminates overwinter feeding by rodents, but you must stratify seed before planting to break dormancy. Stratification involves subjecting the seed to cold temperatures and regulating moisture for a period of time.

The rotation length for black walnut is between 50 and 80 years.

When establishing a walnut plantation, control weeds for at least three years. This maximizes the sunlight, moisture and minerals available to walnut seedlings and reduces plant cover that encourages rodents. 

Control weeds by mowing or cultivation in open field plantings. But in most situations, herbicides are more cost-effective and will not damage the walnut stems or roots.

Corrective pruning can improve seedling form if there is tip dieback or stem forking. Don’t prune too heavily; young stems have a strong natural tendency to grow upright. Clear-stem pruning is recommended to help produce knot-free wood.

You generally do not need to fertilize on a good black walnut site unless a specific nutrient element is deficient. To find out if there are nutrient deficiencies, analyze the foliage. Weeds are the usual benefactors of fertilizers.

Thin the stand lightly and frequently — perhaps every 10 years — to maintain rapid, uniform growth. After thinning, at least three-fourths of the crown of crop trees should be at least 5 feet away from the crowns of adjacent trees.

Cut or girdle competing trees. To girdle a tree, remove a strip of bark around its trunk. Then, treat with an herbicide to prevent resprouting. 

Dominant and codominant trees will respond best to thinning. Dominant trees extend their crown above the canopy, while the crowns of codominant trees are lower, only extending into the canopy. Both receive full light from above, but dominant trees receive more from the sides.

The major pests of black walnut are walnut caterpillars, bud borers, anthracnose and fusarium canker. Pesticides usually are not economical. 

Here are some guidelines for controlling these pests:

• Caterpillars - You may need insecticides.
• Anthracnose - You may need fungicides to improve nut production. To manage anthracnose, control the weeds that weaken the trees.
• Fusarium canker - Restrict pruning to late winter. 

If you prune incorrectly, it can lead to serious problems, including fusarium canker, bark necrosis and sunscald. Fire is highly damaging to black walnut.