Working with the Black Walnut

Black walnut is one of the last trees to leaf out in the spring and one of the first to lose its leaves in the fall.

For this reason and many more, black walnuts, Juglans nigra, seem to be a much maligned tree in landscaping. The mere mention of this tree solicits groans of disgust from the audience and shouts of "cut them down" or "get rid of them!" I understand the consternation around this tree, after all, it is most notorious for releasing a chemical, juglone, through all of its parts which acts as a growth inhibitor to some plants around it and makes it tough to grow certain plants nearby. It does drop large green fruits you don't want to have your car under when they fall and has the problem of Thousand Canker Disease in the southeastern part of the state.

But I am here to shed some light on how to work with Black walnuts if you have them. There are ways to garden with black walnuts, and no need to cut every single one down to send to the lumber mill. According to Michael Williams, in his book, Identifying Trees, an All Season Guide to Eastern North America because of the commercial usefulness of wood and fruit, black walnuts are one of the most planted trees in hardwood plantations. Data on the USDA PLANTS Database show Black Walnuts are native to nearly all the counties in Pennsylvania. This means there are few of us out there that do not have to deal with these in our landscapes. These trees can grow to a majestic 150 feet tall and may grow 8 feet in circumference, though most top out around 90 feet tall and 4 feet around making them a significant addition to any landscape.

Many people, when faced with a black walnut tree feel the only option is to cut it down and get rid of it. While that is certainly an option many people choose, the fact is the chemical will remain active in the soil for a few months to a few years after the tree is removed and many plants grow in association with Black Walnut. Black Walnut is a part of a plant community, meaning it grows in association with a variety of herbaceous and woody plants naturally. The key to landscaping with these trees is to understand what will grow under them and how the chemicals in the soil work.

You can find countless lists online that will tell you what will grow well, and what are particularly sensitive to juglone, the chemical exuded by the tree, try this list from Penn State. Chances are many of the native plants available to us in the trade will do well around the walnuts because they do well in the wild plant community.

As far as working with them and understanding the chemical, it is important to know that juglone is not very water soluable, this means it will not move around a great deal in the soil. It is found in close proximity to the roots, which exude the chemical but it does not move much farther than that in the soil. For this reason raised beds can be used to landscape near black walnuts. In this case, care should be taken to ensure roots will not invade the raised beds and that walnut tree debris is removed.

The amount of juglone affecting plants around the tree can be mitigated by raking up leaves and removing fallen nuts from the area. Some of the highest concentration of juglone can be found in the nuts. Another option to deal with the juglone in the soil is to add organic matter and to aerate around the tree. The increased nitrogen and drainage may enhance populations of soil bacteria that may help break down the juglone in the soil. Studies have shown lower concentrations of juglone in areas of higher organic matter and well-drained soils. The amount of juglone in the soil varies by season and growing conditions and may change from year to year. This may explain why something that didn't struggle being next to a walnut in the past is now experiencing stress.

Black walnuts provide food and shelter for wildlife and even have been used to successfully reclaim surface mined sites because they are so hardy. Black walnuts tolerate a diversity of adverse conditions and are extraordinarily hardy. While you may not consider including them in a planting plan, I recommend considering working with them if you have them.

References

Black Walnut, Penn State Extension Forest Resources. (April 2017)

Black Walnut Plant Guide, USDA NRCS (April 2017)

Dana, Michael N. and B. Rosie Lerner, Black Walnut Toxicity. Department of Horticulture, Purdue University Cooperative Extension Service. (April 2017)

Walnut Wilt. Plant Disease Diagnostic Clinic, Cornell University. (April 2017)

Williams, M. D. (2017). Identifying trees : An all-season guide to eastern North America (2ND ed.). Mechanicsburg: Stackpole Books.