Is there a spot in your garden where plants just won’t grow? Did your tasty tomatoes all of a sudden up and die?
I might know the reason!
The beautiful big Walnut tree in your back yard may be murdering your plants. That’s right, murder! The Walnut is a native plant with a nasty secret.
Actually, you could consider it self-defense, Walnuts don’t like competition, plants that grow up under and around them take away water, light, nutrients and space, so they take matters into their own hands and get rid of them. The roots of the Black and Butternut Walnut produce a toxic substance known as juglone. This substance is poisonous to most plants and when another plants roots system comes close to or touches the Walnuts roots it unleashes the deadly venom and the competing plant dies. The toxic root zone from a mature tree can extend to a 50 to 60 foot radius from the trunk, on a large old tree it can be up to 80 feet. Young trees two to eight feet high can have a root diameter twice the height of the tree itself.
Walnuts have large seeds which fall directly under its broad canopy and apart from squirrels moving some of them around that’s where they’ll stay. The Walnut helps its young seedlings along by producing juglone knocking back other plants that might compete for water and block sunlight. Once the seedlings are off to a good start they too produce juglone and the cycle continues.
Luckily Juglone does not affect all plants and plants like native Grasses, Eastern Redbud and Forsythia are plant examples that can survive in the presence of the mighty Walnut. It’s actually good for the Walnut to have some plants grow under them to help protect them against erosion, excess heat and water loss.
Although the roots are most toxic the bark, foliage and wood of the walnut also carry the compound with a lower concentration. What implications does this have? Mulching with Walnut wood, leaves and bark is not recommended unless it has been thoroughly decomposed first. The toxic effects of juglone will degrade in four to six weeks with exposure to air, water and bacteria.
As a side note, do not place any potted plants under the canopy of a walnut as any rain water that runs off the leaves to the ground will be mildly toxic. Maybe not enough to kill the plant, but enough to kill some leaves and small branches.
If you plant an English Walnut you may experience the same effects, but only in the ground. This is because most of the time the nurserymen graft onto Black Walnut rootstock for added hardiness. The leaves, bark and wood of the English Walnut are not toxic.