EasyBloom Forum

We have a few Black Walnut trees close to where we plant a vegetable garden usually tomatoes, peppers & some herbs. All plants wilt and die soon after being planted. Will a raised bed solve the problem? Or do you have any other suggestions?

Hey Patricia...You might be able to get away with growing veggies around your black walnut trees by using raised beds. Let me tell you how black walnuts cause other plants to wilt and die though. Black walnut is a member of the plant genus Juglans. In this genus you will find a number of plants that produce a compound called juglone. This compound is a known plant growth inhibitor and has been used as a natural herbicide. Tomatoes and peppers are both susceptible to juglone's toxicity. The compound is more or less harmless to people, but some will say that the allergic response to walnut pollen is greater than other plants' pollens.

The reason why I say you might be able to get away with raised beds is how the juglone gets into the soil. Juglone is found in almost all tissues of the plant: leaves, bark, wood, roots. The roots, however, produce more juglone than in other plant tissues. Not only do the roots exude the compound, but the compound is also released into the soil when the roots decompose. A raised bed will work if a couple of things happen: (1) the walnut roots do not invade the raised bed, (2) and you are vigilant with removing any dropped walnut leaves that may find their way into your raised bed. I would recommend laying down a few layers of chicken wire before you fill the raised bed with soil. This will prevent moles and gophers from chewing on any veggie roots you plant in the bed. This may also stymie (for a short period of time) some walnut root development into the raised bed. Normally tree roots grow horizontally and down, but they will also grow towards moisture, which I assume your raised beds would be leaking into the native soil. Speaking of native soil, do not use soil from your yard to fill in the raised beds. You will have to bring in new soil to fill the raised beds because you want to be sure there is no juglone in it. Juglone will break down when exposed to the air or microbial digestion within 1 month or so, but walnut roots that have yet to fully decompose can provide a steady source of juglone for a long time as the root itself breaks down.

Being vigilant with the removal of walnut tree litter from your raised beds is important too. Granted the roots do produce more juglone than any other part of the walnut tree, but there is still some in the leaves. When the leaves decompose, it releases the juglone into the soil. This is also why you shouldn't use walnut leaves for composting.

Now, there are plants (including some veggies) that seem to be immune, or at least relatively unresponsive, to the effects of juglone. These plants include carrots, corn, squashes, beans and melons. I'm sure there are more, but these are the common veggies. You could try growing those plants, but I do think the raised bed issue will give you some success as well. Normally growing plants that are susceptible to the effects of juglone outside of the drip line (the drip line is the area on the ground directly underneath a tree's canopy) will not result in their succumbing to the effects, but walnuts' roots can grow past the perimeter of the drip line as well. I hope this information has been helpful. Let me know if you have any other questions. Cheers...Dr. Robby