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Do Black Walnut Trees Kill Other Trees?

The allelopathic (harmful to other plants) nature of Juglans nigra, Black Walnut, is widely thought to pose challenges for nearby plant life due to the secretion of juglone, a substance harmful to many other plants.
 

This effect, stronger in wet soils low in organic matter like sand or clay, appears to inhibit the growth of neighbouring flora.
 

Research indicates that juglone concentrations diminish significantly shortly after the tree's removal, suggesting soil may recuperate in less than a year.
 

However, the details of this phenomenon are murky:

"The common statement “nothing grows under walnut trees” is not true. “Walnuts produce juglone”, is not entirely true either. “You need to compost walnut wood chips before using them in the garden”, is false. “The allelopathic properties of walnuts are well understood” – definitely not true."

- Robert Pavlis, from his in-depth article Walnuts, Juglone and Allelopathy 


While we may not understand exactly how Black Walnut trees prevent many plants from growing close to them, it's an observable fact that they do.

How Long Before I Can Use Black Walnut Woodchips? 

The juglone content in woodchips declines rapidly, especially when the tree is felled during the less active winter months. 
Two weeks is enough time for Black Walnut Woodchips to be safe, but it's still recommended not to use it around seeds and seedlings.

There is certainly no need to compost the woodchips for a year, as some old sources suggest.

List of Plants Resistant to Black Walnut Poisoning 

Numerous plant species exhibit tolerance to juglone, and should be fine growing next to Black Walnut.

However, black walnut casts dense shade and has an aggressive root system that dominates the soil underneath it, drying it out, and those factors not well tolerated by many plants if you try to plant them directly underneath.