By: Jim Carpenter

Recently, the Elkhart County Master Gardeners hosted their 17th annual spring celebration.

The speaker for the event this year was a nationally known gardener/landscape professional from the Detroit area named Janet Macunovitch. Her first presentation was on edible landscaping, and later she spoke on herbs. Along the way she covered a lot of ground. While it would not be possible to write about everything she discussed in her presentation, there is one thing she mentioned that I have always had a fascination about: allelopathy.

The word allelopathy is derived from two other words. Allelon, which means “each other,” and pathos, which means “to suffer.” Well, the other plants that are susceptible and try to grow near an allelopathic plant certainly do suffer. Plants who are allelopathic secrete a chemical or use other means to discourage competing plant growth in the immediate area. A plant with this ability might have a bigger share of the water and nutrients around it by keeping other plants at bay. This is just another one of the amazing ways that nature can adapt to help a species survive.

The type of allelopathic plant that is probably most important for gardeners to know about is a the black walnut tree. Black walnuts are a beautiful tree that provide a great food supply for many creatures. It also contains a chemical called juglone, which is certain death for many plants that might be growing near it. Juglone is a chemical that is contained in all parts of the tree and is also released through the root system and might even be present beyond the drip line of the tree.

The reason why it is important for gardeners to know about the allelopathic properties of black walnut is because tomatoes are very sensitive to juglone and will not grow properly in its presence. All members of the nightshade family are susceptible to this chemical, including potatoes, peppers and eggplants as well as blackberries. Since these vegetables are popular in many home gardens, it is important to know not to plant them anywhere near a walnut tree.

There are many plants and trees that have allelopathic properties. Another one that comes to mind is the invasive plant called bush honeysuckle. This bush might begin to carve out whole sections of the woods by not allowing any other species to grow near it. While the bush honeysuckle provides some food and cover for wildlife, it does not permit the diversity of plant life that a healthy woods needs in order to sustain itself. Like many invasives, the bush honeysuckle was introduced to the American landscape without realizing the negatives outweighed any positives the plant has.

Another popular tree with allelopathic properties is the Norway maple. This is another plant that is considered invasive. This tree was brought over from Europe many years ago. It has a very shallow root system which spreads out near the surface of the ground and prevents other plant species from growing beneath it. Even weeds and lawn grasses find it hard to survive under a Norway maple.

Back to the walnut tree. Many years ago an older friend gave me a small walnut tree from his property for me to plant. Without realizing the potential effects, I planted it near my garden. After I found out what deleterious effects this tree would have on some of my garden plants I tried to just avoid planting the susceptible ones on that side of the garden. As years went by I realized that the tree and its roots in particular were spreading to an extent that it was going to effect more and more of my garden space.

Enter the chainsaw.

The decision was made and the 12-inch walnut tree was quickly dispatched and turned into firewood. Unfortunately, the juglone toxins remain in the soil with the decaying roots for several years, even after the tree is removed. Granted, the walnut was a beautiful tree that looked nice out there near my garden; in a perfect world it would still be there. However, many times in life, priorities must be given and choices need to be made.

Any walnut trees planted in the future will be in an area far away from my garden.

For more gardening tips and inspirations from Master Gardener Jim Carpenter, subscribe to the Gardening for Life email newsletter.
(Visited 64 times, 1 visits today)