If you live east of the Mississippi River, then you have probably seen a Black Walnut tree at some point in your life. Juglans nigra is the tallest of all the Walnuts, it is not uncommon for a tree to reach 100′ into the sky. Walnut trees have compound leaves that are spaced alternately along the branch. Each leaf is divided into an odd number, from 7 to 23 small yellow-green leaflets. Walnuts are monoecious, male flowers are long, unbranched, drooping catkins and the female flowers are single or short spikes. The fruit is a nut, single or in pairs, and enclosed in a non-splitting shell.
Walnut has an unforgettable scent that is rather difficult to describe, I would describe it as “spicy-citrus”.
Walnut trees have a toxic effect on neighboring plants. Known as allelopathy, the phenomenon involves a plants secretion of biochemical material into the environment to inhibit germination or growth of the surrounding vegetation. Allelopathy enhances tree survival and reproduction. Allelochemicals are metabolic by-products of certain plants that, when introduced into the environment, cause growth inhibition by affecting physiological processes such as respiration, cell division, and water and nutrient uptake. Symptoms include leaf wilting and yellowing, or the death of part or all of the plant.
Black Walnuts are often found growing on landscape sites where they serve primarily as shade trees. When certain other landscape plants are planted close to the tree they will wilt and die. This decline in health occurs because the walnut tree produces a non-toxic, colorless, chemical called hydrojuglone. Hydrojuglone is found in the leaves, stems, fruit, inner bark and roots. When exposed to air or soil, hydrojuglone is oxidized into the allelochemical juglone, which is highly toxic.
The toxic Zone from a mature tree occurs on average in a 50 to 60′ radius from the trunk, but can commonly be up to 80. The area affected extends outward each year as the tree enlarges. Young trees 2 to 8′ high can have a root diameter twice the height of the top of the tree, with susceptible plants dead within the root zone and dying at the margins.
Juglone is exuded from all parts of the walnut tree. Although it has a high water solubility and does not move far in the soil, small amounts may be injurious to sensitive plants. Plant roots can encounter juglone when they grow within a half-inch from a walnut root. Walnut trees have extremely extensive root systems often reaching far beyond the drip line of the tree, affecting susceptible plants far from the tree.
The accumulation and depletion of toxins in the soil is affected by factors such as soil type, drainage, aeration, temperature and microbial action. Soil microorganisms ingest allelochemicals as energy sources, and metabolic decomposition can render the chemicals non-toxic to plants.
Wet, poorly aerated soil, very common in many urban areas, discourages microbial growth. Plants sensitive to walnuts may be at a higher risk when planted in heavy urban soils that lack organic matter. Toxins adhere to organic matter rather than being absorbed by plants, and organic matter also encourages a healthy soil microbial population.
Mycorrhizal fungi are commonly associated with forest tree roots and are considered necessary for normal uptake functions. Allelochemicals can disrupt the uptake process by damaging the root hairs or by inhibiting mycorrhizial populations in the soil. These different soil factors all have an effect on the accumulation or depletion of juglone produced by the black walnut tree.
So, if you’ve read this far… You are probably wondering if anything can grow under a Walnut… There are actually a large number of plants that will grow near allelopathic trees, as well as steps you can take to reduce the allelopathic effects. Regular cleanup and removal of fallen leaves and fruit is the simplest thing you can do, meticulous maintenance is key.
As a side note… I have noticed the real danger even tolerant plants face… Especially anything that is still growing in October… Is the rainfall of 8 ounce nuts falling from 100′ in the air… These falling bastards crush whatever greenery happens to be in their path…
Now to the plants… I have never personally planted a garden under a walnut tree… This article is the result of observation and research… I have always lived near these trees… And ever since I used them as “car grenades” as a child I have been relatively interested in them… When I first learned of allelopathy I became obsessed with the plants that could survive under these trees…
Tolerant Trees – Black Cherry, Crabapple, Dogwood, Elderberry, Hawthorn (my favorite tree), pawpaw, redbud, sassafrass, serviceberry.
Tolerant Shrubs – Black Raspberry
Tolerant Plants – Aster, Astilbe, Bee Balm, Coral Bells, Daffodil, Shasta Daisy, Daylily, Fern, Dutchmans Breeches, Goldenrod, Grape, Great Solomon’s Seal, Hosta, Jack-In-The-Pulpit, Jacobs Ladder, Lambs Ear, Marigold, May Apple, Morning Glory, Sedum, Snowdrop, Sweet Woodruff, Sunflower, Trillium, Tulip, Violet, Yarrow, Zinnia
Tolerant Vegetables – Beet, Squashes, Melons, Beans, Carrots, Corn, Jerusalem Artichoke, Melon, onion, parsnip
Intolerant Vegetables – Asparagus, Cabbage, Peppers, Rhubarb, Tomatoes, Eggplant, Potato
Tolerant Fruit Trees – Peach, Nectarine, Cherry, Plum, Pear
Intolerant Fruit Trees/Shrubs – Apples, Crabapples, Blackberry, Blueberry
Personally, if I had a piece of property with a Walnut Tree on it… I would probably cut that bad boy down and turn it into kitchen cabinets… I’m not a big fan of the flavor of a Walnut, a little bitter for me, plus I find cleaning up the walnuts annoying.
Most walnut varieties are grafted to Black Walnut rootstock, therefore they all contain roughly the same amount of toxins and should be maintained in the same meticulous manner.
plant petunias and question everything – chriscondello
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