How long does juglone last in the landscape after the removal of a walnut?

FAQ
Question: 

How long does juglone last in the landscape after the removal of a black walnut?

Answer: 

Juglone is a chemical that inhibits the growth of certain plant species.  When plants produce a chemical that prevents the growth of other plants, it is called allelopathy.  Black Walnut is a common woodland and landscape tree in the Midwest that produces juglone which causes an allelopathic response (inhibition of growth) in other plants.

Juglone is produced by all parts of Black Walnut (leaves, stems, fruit, etc.), making it a highly effective competitor for space in the landscape.  The name juglone comes from the scientific name for Black Walnut, Juglans nigra.  Other members of the Juglandaceae family (includes Butternut and Pecan) also produce juglone, but Black Walnut produces the most and it does it more effectively than other members of the family.

Juglone breaks down quickly in the environment.  Even though all parts of a walnut produces juglone, a few months after removal most of the juglone is gone.  It is usually safe to replant the next growing season in spaces formerly occupied by walnuts.