Here's an excerpt from a website:The roots of black walnut trees produce a toxic substance called juglone which adversely affects plants that are sensitive to it. Plants which cannot tolerate juglone will show symptoms such as yellowing and wilting foliage and they will ultimately die from its toxic effects. It is believed that juglone acts as a respiration inhibitor, leaving plants unable to breathe and sapping their energy.

Juglone is produced in the trees’ roots but is present in all parts of black walnut trees, and is strongest in the buds and nut hulls. The leaves and twigs contain smaller amounts of juglone, but black walnut trees have a habit of continually dropping leaves and nuts from late summer through autumn and this debris only adds to the toxicity problem. Because of the accumulation of leaves and nuts beneath the tree, and also because of rain running off the leaves, the entire drip zone beneath a black walnut tree can be a hazardous environment for juglone-sensitive plants. Some plants that are extremely sensitive to juglone won’t grow within fifty feet of the dripzone of a black walnut tree.

Cutting down the offending black walnut tree won’t solve the juglone problem either. The roots will continue to release juglone into the soil and the area can remain toxic for several years after the tree is gone. A gardener I know removed several scruffy black walnut trees from her backyard to make room for a garden, but even six years later she wasn’t able to grow tomatoes in that area.

The good news is that not all plants are sensitive to juglone. Many trees, shrubs, flowers and vegetables will grow in close proximity to a black walnut tree, although even some of the juglone-resistant plants will struggle if they are directly beneath the tree.

Hi PB, I agree with Volunteer, but the affected area is really only about the size of the canopy of the tree. There are a lot of plants that can be used under a walnut. If you copy and paste the link below, you will find a list of sensitive and non sensitive plants re juglone

http://gardening.about.com/gi/o.htm?zi=1/XJ&zTi=1&sdn=gardening&cdn=homegarden&tm=17&gps=322_1413_1255_793&f=20&tt=13&bt=1&bts=1&zu=http%3A//wihort.uwex.edu/landscape/Juglone.htm

Thank you!