There is a very common myth that says, “nothing grows under walnut trees”, but that is far from being true. If you have walnut trees in your yard or the neighbors yard, you can still have a very lush garden and grow most things.
Walnuts and Juglone
Walnut trees produce a chemical called juglone which is toxic to some plants. All parts of the tree including leaves and roots produce juglone. Rain washes the chemical off leaves and bark onto shorter plants growing below them. When the roots of other plants grow close to the tree roots they also come into contact with juglone. Plants can’t escape this chemical.
Trees related to the walnut, including the English walnut, hickories and pecan also produce juglone, but in smaller amounts.
When juglone contacts other plants, it causes them to grow more slowly. Leaves become yellow and wilt, especially in hot dry conditions. In extreme cases the plant will die either quickly or slowly over a number of years.
Juglone-induced wilting and wilting due to water stress produce similar symptoms. A lack of water results in slow wilting which can be reversed if the wilting is not too sever. Juglone-induced wilting often occurs more quickly, can’t be reversed with watering, at least not in advanced stages, and may only affect part of the plant.
The above description about juglone provides the gardener with the information they need, but it is only part of the story. For a more detailed discussion, including myths about julone – which by the way walnut trees don’t produce – see my other post Walnuts, Juglone and Allelopathy.
Growing most plants under trees is a problem because they want more sun to grow properly. One good thing about walnut trees is that their leaflets are quite small in comparison to something like a maple and these provide the ideal filtered light conditions for many shade and part-shade loving plants.
Walnuts, like most trees have extensive root systems. The active growing roots can be found two to three times as far from the trunk as the drip line and since a walnut tree is a big tree, one tree will have its roots in several normal sized backyards. The root growth is also dense, and they absorb a lot of water, making the soil under the tree very dry. This dry soil is the main reason people have trouble growing things.
The growing conditions under a walnut tree are part shade and dry. As long as you select plants that like these conditions you have a good chance at growing them.
Juglone and Allelopathy
Allelopathy is the technical term used to describe a situation where a plant produces chemicals that prevent other plants from growing. Walnuts get all the blame, but many plants do this. Another common plant that shows allelopathic tendencies is garlic mustard, an invasive weed in North America. It injects chemicals into the soil that prevent other seeds from germinating.
How big of a problem is Juglone? Everybody talks about it, but a recent review of the known literature suggests that it may not be as big a problem as suspected. It is true that some seedlings are easily affected by Juglone, but most mature plants can tolerate it or are not affected at all. It is quite possible that a lot of the stories about walnuts, juglone and plant growth are exaggerated and that poor growth is mostly due to the shady, dry conditions under the tree.
There is no doubt that juglone toxicity can be demonstrated in a lab under controlled conditions, but finding proof in the field has proven to be much more difficult.
Reducing Juglone Levels
As with all chemicals, toxicity is related to dose. High amounts are toxic and low amounts are not. If you reduce the level of Juglone in your soil, you will have less toxicity issues even for sensitive plants. How can you lower the levels – short of removing the tree?
Removing fallen leaves, branches and nuts will help. All of these contain the toxin and by removing them, less chemical enters the soil. You don’t have to throw away this valuable organic matter. Composting it will reduce juglone to safe levels. The compost can then be returned to the garden. The reason this works is that microbes in the compost pile digest juglone into safe chemicals.
Increasing the number of microbes in the soil will reduce the juglone levels. You do this by following standard methods for improving soil. Don’t walk on it, add organic matter, mulch, and grow lots of plants. These will all increase the organic level in the soil, and decrease compaction – all stuff microbes love. Happy microbes means less toxic chemicals.
Use Tolerant Plants
It is quite clear that some plants are sensitive to juglone, but most are not affected at all. Selecting the right plants makes a big difference.
Plants related to tomatoes, including eggplants, chiles, potatoes and petunias, are sensitive. Some sensitive ornamental plants include potentillas, lilacs, columbines, lilies, peonies, mugo pines, and various privet shrubs.
Keep the gardens well watered.
Gardens Under Walnut Trees
If you would like to see a special garden that is grown under walnut trees have a look at Phil Reilly’s garden which was discussed in The Garden Professors Facebook Group. Proof positive that you can have a great garden under walnut trees.
- Black Walnut (Juglans nigra) and Allelopathy; https://www.ces.ncsu.edu/fletcher/staff/rbir/blackwalnutcompanions.html
- Landscaping and Gardening Around Walnuts and Other Juglone Producing Plants; http://extension.psu.edu/plants/gardening/fact-sheets/trees-shrubs/landscaping-and-gardening-around-walnuts-and-other-juglone-producing-plants