Botany on the Byways: Black Walnut

National Walnut Day (May 17th) is important in Iowa, as black walnut is the most valuable tree in the state. Iowa continues to rank as one of the leading producers of walnut lumber and veneer. The rich brown color, attractive grain of the wood, and the woodworking quality of the wood makes it the most valuable timber tree in Iowa. Although not as commonly eaten as an English walnut, what you would find in a grocery store, black walnuts can also be eaten if harvested, processed, and stored correctly.

Black walnut (Juglans nigra) is native to Iowa and found in all 99 counties. It occurs on a variety of sites, but is most common on bottomlands, stream terraces, and moist sheltered coves and slopes. It grows best on deep, rich, well drained soils. It usually has a full, well-formed trunk with no branches for quite some distance from the ground.  The crown is oval to rounded and somewhat open. A typical black walnut reaches 50 to 75 feet, but it may reach maximum heights of 125 to 150 feet. The leaves are alternately arranged on the twig, compound with 1 to 23 leaflets, and ranging in size from 1 to 2 feet.

Common associates of black walnut vary as much as the sites but include oaks, basswood, maples, ashes, and Kentucky coffeetree. Walnut is considered a good invader species and often is one of the first species to become established in abandoned fields and pastures. Squirrels are especially fond of the nuts and aid in the establishment of walnut on these sites. 

Black walnut roots produce chemicals called juglones which are very toxic to certain other plants such as azaleas, rhododendrons, blueberries, peonies, and solanaceous crops (tomatoes, peppers, potatoes). Most of the toxicity is limited to within the drip line of the tree, but the area of toxicity typically increases outward as the tree matures.

sources: Iowa State University Extension and Outreach, Dickinson County Conservation

Black Walnut Fruit - Photo by Paul Wray, Iowa State University

Black Walnut Leaves - Photo by Paul Wray, Iowa State University

Black Walnut Fruit - Photo by Paul Wray, Iowa State University