Minnesota’s black walnut forest type

April 20, 2007 at 5:53 pm Leave a comment

Products and Uses

Wood products from black walnut include sawlogs, veneer logs, gun stocks, and smaller novelty pieces. Frequent nut crops make it an excellent tree to plant for wildlife, especially squirrels. In some areas nuts are collected and sold for human consumption.

Growing Conditions

Black walnut bark, Mankato MN. Click for a better view.Black walnut generally is found scattered among other tree species. Pure stands are not common, but do occur. Walnut grows best on lower north-and east-facing slopes, stream terraces, and floodplains. It is common on limestone soils and grows well on deep loams, loess soils, and alluvial deposits that are fertile and moist but well drained. Poor sites for walnut include steep south- and west-facing slopes, narrow ridgetops, and poorly drained sites. Soils with acid clayey subsoils, coarse sand or gravel layers, or bedrock within 2-1/2 feet of the surface are not suitable for walnut. Black walnut leaves and roots actively secrete material toxic to some trees, shrubs, and herbaceous plants.

Regeneration

The rotation length for black walnut is from 50 to 80 years. Black walnut can be regenerated naturally from seed and stumps sprout well if trees are less than 20 to 30 years old. Since black walnut trees normally are a minor component of a woodland, natural regeneration is seldom reliable. Seedlings should be planted to supplement natural regeneration.

Black walnut plantation, Mankato MN. Click for a better view.Seedlings often are planted at a spacing of 10 x 10 feet for timber production purposes, and 15 x 15 feet for producing a combination of timber and nuts. Black walnut will not tolerate shade. To prepare a woodland site, cut and/or kill with herbicides all woody vegetation larger than 1/2-inch in diameter. Grassy and weedy sites may require herbicide treatments to kill existing vegetation prior to planting seedlings.

Plant seedlings in the spring as soon after the ground thaws as possible. Seedlings at least 1/4inch 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 most reliable, but they are expensive and time consuming to install. Alternatively, direct seeding a large area can overwhelm seed predators. This approach is becoming more common in Minnesota (read more about direct seeding in Minnesota).

Seed can be sown in either the fall or spring. You do not need to remove the husks for fall planting. Spring planting eliminates overwinter feeding by rodents, but requires that the seed be stratified before planting to break dormancy. Stratification involves subjecting seed to cold temperatures and regulating moisture for a period of time.

Intermediate Treatments

Control weeds for at least three years when establishing a walnut plantation to maximize the sunlight, moisture, and minerals available to walnut seedlings and to reduce plant cover that encourages rodents. You can 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.

map showing range of black walnut
Range of black walnut

Corrective pruning can improve seedling form if tip dieback or stem forking has occurred. Do not prune too heavily; young stems have a strong natural tendency to grow upright. Clear-stem pruning is recommended to help produce knot-free wood. Read more about how to prune trees here.

Fertilization generally is not needed on a good black walnut site unless a specific nutrient element is deficient. Foliage analysis will reveal a nutrient deficiency. Weeds are the usual benefactors of fertilizers.

Thin the stand lightly and frequently-perhaps every 10 years-to maintain rapid, uniform growth. Following thinning at least three-fourths of the crown of crop trees should be 5 feet or more from the crowns of adjacent trees. Competing trees should be cut or girdled and treated with a herbicide to prevent resprouting. Dominant and codominant trees will respond best to thinning. Read more about thinning on our Thinnings & TSI page.

Pests
The major pests of black walnut are walnut caterpillars, bud borers, anthracnose, and fusarium canker. Pesticides usually are not economical. Fungicides may be necessary to control anthracnose for the purpose of improving nut production, and insecticides may be necessary to control caterpillars. Anthracnose can be managed by controlling weeds that weaken the trees. Fusarium canker can be controlled by restricting pruning to the late winter. Fire is highly damaging to black walnut.

Incorrect pruning can lead to serious problems, including fusarium canker, bark necrosis, and sunscald.


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News and information of interest to Minnesota woodland owners. Sister site to MyMinnesotaWoods.org.

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