Black Walnut Trees

The roots of Black Walnut produce a substance known as juglone.  Plants adversely affected by being grown near black walnut trees exhibit symptoms such as foliar yellowing, wilting and eventually death.
The toxic zone from a mature tree occurs on average in a 50-60 foot radius from the trunk.
 
Not all plants are sensitive to juglone.  Plants grown by Great Garden Plants that have been observed growing under or near Black Walnut include:
  
SHRUBS
Euonymus
Hydrangea
Thuja
 
PERENNIALS
Ajuga
Daisies, Shasta
Dicentra
Ferns: Dryopteris, Osmunda
Geranium
Hellebores
Heuchera
Heucherella
Hosta
Phlox
Pulmonaria
Sedums
Viola