With leaf-raking season upon us or nearly so, gardeners might need a refresher about juglone, the natural chemical compound produced by some nut tree species.

For the most part, juglone is harmless and we can ignore it. However, a few of our favorite plants can be harmed and even killed by the juglone from neighboring trees.

Juglone is a toxin to some plants. And since leaves falling from juglone trees do contain the compound, we need to decide if, where and when those leaves can be used as plant mulch or added to compost piles.

Juglone is produced by a few tree species, especially walnut trees. As a matter of fact, it is the nut tree group scientifically known as the Juglandaceae family that harbors juglone producers. I do not know which was named first, the tree family or the toxin produced by it. In this area, the top juglone-producing species by far is the black walnut.

The walnut relatives hickory and pecan also generate some juglone, but not nearly to the extent that walnut trees do. Juglone is produced in all parts of the trees.

The most common transfer of the toxin to susceptible plants is from roots through soil. There is actually a tomato malady known as Walnut Disease. It has occurred in the past when fields of tomatoes were planted too close to walnut trees. Based on time and weather, the tomato plants begin to turn yellow and eventually just wither and die because of soil-borne juglone.

Vegetable plants sensitive to juglone poisoning include the Nightshade family of tomato, potato, pepper and eggplant. Asparagus is also a possible target. Beans, peas, corn, cucumbers and melons are not bothered by juglone. Fruit plants harmed by contact with juglone are blueberry and apple while sensitive ornamentals include hydrangeas and daffodils.

Dealing with fall leaves, the good news is the highest level of the juglone compound is in spring in new leaves and drops during summer and fall. Also, pecan and hickory leaves don’t rate up there with black walnut for the amount of juglone.

It’s the black walnut leaves that might, but rarely, be problematic.

So what do we do to avoid juglone toxin hurting garden and landscape plants? For starters, I suggest not using walnut leaves as a mulch around blueberry plants anytime soon. Pile them up and forget about them for six months or so and the juglone chemical will fade away.

As for adding walnut leaves to compost piles, leaves that are mixed in with lots of other plant stuff are going to be diluted and the composting action of decomposition will do away with juglone in a few months, sooner in warm weather.

As for any logical questions about us handling walnuts or eating roasted walnuts, the answer is simple. The human species body is not sensitive at all to juglone, roasted or raw.

 

Terry Rector writes for the Warren County Soil and Water Conservation District.