Q: I have a black walnut tree in the back yard and my lilacs aren't doing well. What should I do to fix them?
A: Black walnuts are tricky beasts in the garden, but they are spectacular trees and wonderful to have in your yard. Black walnuts and all plants in the Juglans family, (along with the cacao plant, buckthorn, etc) are called allelopaths. It is a fancy word for plants that are able to secrete chemicals that supress the growth of other plants in proximity to it. Which is why you often can't get other plants to grow near these allelopaths. (Interesting side note: the cacao plant (used to make blow/cocaine) is SO allelopathic that when it is intensively farmed (for drugs which are bad), over time they make the soil surrounding them so toxic that they eventually kill themselves with their own toxicity. Crazy, right?
|Image via Small Things blog, isn't it spectacular?|
Back to black walnuts and their kin. The chemical they secrete is called juglone and it is not toxic to everything, which is the good news. The bad news? The list of plants it is toxic to isn't a short one.
DO NOT PLANT: Tomatos, asparagus, rhubarb, potatoes, cabbage, peppers, eggplant, hydrangea, yew, liacs, blackberry bushes, blueberry bushes, azaleas, mountain laurels, rhodedendrons, columbine, false indigo (Baptisia), chrysanthemum, lilies, peonies, red pines, silver maples, white birches, Norway spruce, Eastern white pine, and apple/crabapple trees. For a more extensive list, check your local extension service which will include local varieties and zone specific plants.
If these plants are planted within the root zone or within 50 feet of the drip line of a black walnut, they will be sickly and possibly die within months. Some plants will limp along, but never reach full potential due to the juglone. Which is exactly what is happening to the lilac bushes in question. The best thing to do with struggling plants (including the lilacs) is to move them to a new location if they are small enough, or cut them down in favor of something less susceptible to the deadly charms of the black walnut. That sounds so dramatic, right? It is, but you won't ever get these susceptible plants to grow how they should, so it is best to cut your losses now and move on.
|Image via the Univeristy of Wisconsin Extension, showing Black Walnut toxicity in grapes|
However, this is the tricky part. You MUST rember that as the black walnut tree grows, so does the toxic root zone. According to the University of Ohio Extension, "The toxic zone from a mature tree occurs on average in a 50 to 60 foot radius from the trunk, but can be up to 80 feet. The area affected extends outward each year as a tree enlarges. Young trees two to eight feet 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." So if you are bent on planting non-compatible plants, keep this in mind lest you lose a lovely plant in a matter of a few years as the black walnut grows.
Instead, try planting these (as appropriate for your growing zone): beans, beets, carrots, corn, mellons, onions, squash, asters, astilbe, trilium, phlox, pansy, ferns, some hosta varieties, calindula, black eyed susans, begonias, bluebell, black locust trees, hickory, oaks, most maples (other than silver maples), dogwood, poplar, arborvitae, clematis, currants, wild roses, willow, wild grapes, and if grass if your flavor, the standby Kentucky Bluegrass does well as do fescues and white clover.
Good luck and remember to check all plants you want to grow near these spectacular trees for toxicity and zone appropriateness. Go forth and plant!
Source: University of Ohio Fact Sheet: Black Walnut Toxicity to Plants, Humans and Horses
DISCLAIMER: All the information/answers posted here are to the best of my knowledge. I have been wrong before and I am sure I will be wrong again (positive, in fact)... but I always try to give the best possible information with the most factual/official sources possible. No one garden, zone, or plant is the same, to always research your particular situation and make sure the suggestions work for you and your problem. If you have specific questions you can always leave a comment or (possibly better yet) call/e-mail your local Master Gardner group or Extension offices.