October 2, 2015

Planting Around Black Walnut Trees

Planting a garden and shrubs near a Black Walnut tree is extremely challenging. Finding out some facts about the toxins produce

d by this type of trees is important to ensure proper preparation and success for your garden.

Black Walnut trees along with hickories produce a chemical called Juglone. This chemical is very toxic to plants. Its highest concentration is in the buds, leaves, stems, nut hulls and roots of the tree. The actual concentration in each tree part varies with the season. In spring, Juglone is high in the growing leaves. In the summer, it is high in the roots, and it increases in the hulls of the fruit as they mature.

The toxic zone from a mature tree occurs on average in a 50 to 60 foot radius from the trunk, but can be up to 80 feet. The area affected extends outward each year as a tree grows. 

In most cases, the damage caused by Black Walnuts to other plants is a combination of the presence of Juglone in the soil, and the competition for light, water and nutrients. Cutting the tree down will provide immediate relief for light water and other nutrients, but the Juglone can stay in the soil for a long time. In some cases couple of years.

General tips for planting around black walnuts:
Allan Block Raised Planter (AB Fieldstone)

  • Before anything talk to your local landscape center and ask to have an expert evaluate the soil in your yard to see where the toxin is most concentrated.
  • Locate your garden away from the drip line of the black walnuts.
  • Create and plant in raised beds to reduce root contact. This will require lining the bed to reduce root contact using weed fabric and filling the raised bed with new topsoil.
  • Improve soil drainage with organic matter additions.
  • Prevent leaves, hulls, and stems from decomposing near planting areas.
  • Avoid using mulch containing walnut bark, wood, hulls, and leaves.
  • Consider planting shrubs that are Juglone tolerant around the tree, you can find a whole list online.