Juglone – What is it?

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 Juglandacea family (includes Butternut and Pecan) also produce juglone, but Black Walnut produces the most and more effectively than other members of the family.

Below are a few frequently asked questions about Black walnuts and the chemical juglone.

Are all plants sensitive to juglone?

Not all plants are sensitive to juglone. Members of the tomato family (tomato, potato, eggplant, pepper) are highly sensitive to juglone.  Some shrubs like lilac, hydrangea, rhododendron, viburnum, and yew are also considered somewhat sensitive to juglone.  There are many other plants, however, that are not impacted by juglone.  Shade loving perennials like bugleweed and bleeding heart can coexist nicely underneath walnuts without any injury.  A list of some of the sensitive or tolerant species to juglone are listed below.  It is important to note that there are no definitive lists and very few official studies on the impact of juglone on different plant species.  The plants listed below have been observed to grow well or not grow well when near black walnuts in a landscape or woodland setting.

It is also important to note that many turfgrass species are tolerant of juglone.  They, like many other sun-loving plants, are more likely to perform poorly because of the shade of the walnut tree rather than the juglone.

What are the symptoms of juglone sensitivity?

The degree of symptoms varies with species.  Tomatoes are highly sensitive so they often yellow, grow poorly or look stunted, and eventually die when grown near walnuts.  Plants that are not as sensitive may yellow slightly or grow more slowly, but are rarely killed. And plants that are tolerant to juglone grow well with no signs or symptoms of injury.

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

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.

Can I use woodchips from walnut as mulch in my landscape?

Yes, you can use walnut chips in landscaping – however, you should wait a few months for the chips to compost slightly and break down the juglone chemical before placing the mulch around sensitive plants.