what to do about walnut trees
The prior owners of my home had a Black Walnut tree growing next to the garden for many years. It has since been cut down, but nothing grows well at that end of the garden. I'm told Black Walnut leaves contain a natural herbicide that may have poisoned the soil. Is that true? Is there some way to salvage the soil?
I located this article from Minnesota that helps answer your question and a recent Oregonian article (about a week ago) also addressed this same issue.
For starters, please note these Iowa State University recommendations: "Gardeners who have large walnut trees near their vegetable gardens should consider an alternate site. The greatest concentration of juglone in the soil exists within the dripline of the trees. The dripline is the area between the trunk of the tree and the end of the branches. The toxic zone from a mature tree occurs on average in a 50-foot radius from the trunk. Avoid planting your garden in these areas to protect your garden from damage." You may be able to grow plants sensitive to juglone (the black walnut chemical toxic to some plants) successfully near the tree by planting them in a lined (weed fabric) raised bed filled with clean soil. This assumes that the bed will not be under the tree's canopy. Do not create or use compost or mulch that contains leaves or any other parts of the black walnut tree. This web site provides an excellent list of plants known to be sensitive or insensitive to juglone: Black Walnut Toxicity http://hort.uwex.edu/articles/black-walnut-toxicity Learn more here: Toxicity of Black Walnuts Toward Other Plants Best of luck
First off, Thankyou for your quick response, and the usefull information, but it doesn't completely solve my problem. The Black Walnut Tree was cut down already, but my garden still hasn't recovered. Is there anytreatment I can give the soil to recover from the effects of juglone, or do I have to dig out the contaminated soil and replace it with uncontaminated potting soil? Are there any fertilizers that might help? Will composting help?
Further research suggests that the chemical juglone persists after trees are removed and that a raised bed growing container will be most practical, least labor intensive, and eliminate alot of extra effort to rehabilitate the soil by adding organic matter which will take time, have soil tested, have soil removed, etc. If you need any more information I can suggest contacting your local country extension agent in Washington County at 503-821-1150 located at 1815 NW 169th Pl Beaverton 97006
Landscaping and Gardening Around Walnuts and Other Juglone Producing Plants
Walnuts and hickories produce the chemical juglone (5 hydroxy-1,4- napthoquinone), which is exuded from all parts of the plant. The greatest concentration of juglone and hydroxyjuglone (a nontoxic, colorless precursor that is converted into the toxic form juglone by sensitive plants and through oxidation) is found in the vegetative buds, leaves, stems, nut hulls, and roots of the plants. Black walnut (Juglans nigra) and butternut (Juglans cinerea) are the landscape plants most recognized by gardeners as being problems for their other plants. However, English or Persian walnut (J. regia) and hickories (Carya) also produce juglone but to a lesser degree.
The production of juglone is a protective response by the plant to assure its survival. Many plants (e.g., sugar maple, tree of heaven, hackberries, sycamore, cottonwood, black cherry, red oak, black locust, sassafras, fine fescue, and American elm) produce allelochemicals to enhance their survival and reproduction by inhibiting nearby competition. The most common symptoms of juglone sensitivity in landscape and garden plants is the yellowing and wilting of leaves, especially during the hot dry periods during the growing season, ultimately resulting in wilting and death of the plant.
Juglone-induced wilting and wilting due to water stress are often confused. Wilting due to lack of water occurs slowly and can be reversed with watering. Juglone-induced wilting often occurs rapidly even when ample soil moisture is present. Juglone-induced wilting may be partial or may encompass the whole plant. Early wilting symptoms may also be reduced with supplemental water. Later in the season wilting does not respond to additional water, leaves start to brown, and the plant dies. Experimental studies have shown that juglone inhibits plant respiration, depriving sensitive plants of needed energy and cell division as well as water and nutrient uptake.
Sensitive plants located beneath the canopy of a walnut tree are most susceptible to contact with juglone through direct root contact or accumulation of the toxin from leaves and nut hulls in poorly aerated, wet soils with limited microbial activity and organic matter. Juglone is poorly soluble in water and does not move very far in the soil. Well-drained and aerated soils with a healthy population of microbes can accelerate the metabolic decomposition of juglone. Where sensitive plants may survive outside of the canopy of a black walnut, highly sensitive plants may not tolerate small concentrations where decaying roots from a removed tree may still be releasing juglone. Juglone toxicity may persist for years after a tree is removed. So, impatience in replanting an area with juglone-sensitive plants is not advised.
Planting around Walnuts
Understanding the site to be landscaped or developed into a garden is the first step in assuring successful plantings. Identifying your trees and shrubs will help avoid problems with allelopathic toxicity among your future landscape and garden plantings. General tips for planting around black walnuts include:
- Locating gardens well away from black walnuts.
- Creating and plant in raised buds to reduce root contact. This will require lining the bed to reduce root contact using weed fabric and filling the raised bed with new topsoil.
- Improving soil drainage with organic matter additions.
- Preventing leaves, hulls, and stems from decomposing near planting areas.
- Avoiding mulch containing walnut bark, wood, hulls, and leaves