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Miss Floribunda: Walnut tree sabotaging other plants

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Posted on: May 22, 2014

Dear Miss Floribunda,

My husband and I have worked very hard to improve the soil in our yard for several years now, but we still have had no luck with vegetables or flowers  We thought maybe the roots of a walnut tree were taking nutrients away from them, so we tried raised beds. That didn’t work either. Although there is some shade from the tree, which would explain why tomatoes don’t do well, potatoes haven’t survived either. We tried planting azaleas and petunias instead of vegetables, but they shriveled up and died, too. Are we just not cut out for gardening?

Black-thumbed on Oglethorpe Street.

Miss Floribunda
Miss Floribunda

Dear Black-thumbed,

I don’t think your thumbs are really black, but I’m pretty sure your walnut is. My Cousin Moribunda tells me that the black walnut (Juglans nigra) was one of the first plants to be observed to be fatal to plants near it. She says the Greek naturalist Pliny the Elder wrote about it and other allelopathic plants in his Natural History back in the first century AD.

Although the term allelopathic is Greek (meaning to harm another, or each other) it was coined by an Austrian scientist named Hans Molisch in 1937. The way Moribunda describes it, allelopathy is a kind of biochemical warfare waged by certain plants to make sure they are not crowded out by other plants. Black walnuts, for example, exude from their roots a substance called juglone that is highly toxic to many plants, particularly those of the nightshade (Solanaceae)  family: tomatoes, potatoes, eggplant, bell peppers, chili peppers. Even falling twigs, leaves, and the rain dripping through the trees can affect the soil.

You might have better luck with raised beds if you were to make sure that the base of the bed was impermeable and had no contact with the soil. On the other hand, that might cause drainage problems. You might have better luck with plants that are less susceptible, such as beans, beets, carrots, corn, melons, onions, parsnips and squash.  Tolerant flowering plants include begonias, monarda, daylilies, and hostas,  The University of Virginia’s website has lists you may find useful.

Allelopathy has its positive uses. For example, planting larkspur around rose bushes doesn’t harm the deep-rooted rose but keeps a lot of weeds at bay.  There is a lot in knowing which plants help or hinder plants in their vicinity. Many gardeners, for example, plant marigolds with their tomatoes for pest control. Marigolds exude  alpha-terthienyl from their roots that prevents the development of nematodes and their odor repels whiteflies and aphids. On the other hand, neither marigolds nor tomatoes should be planted near bean plants, or indeed any of the vegetables that tolerate the vicinity of your black walnut.

I strongly recommend that you consult a good book on companion planting. The first I read are still highly thought of:  “Roses Love Garlic” and “Carrots Love Tomatoes” by Louise Riotte.

There will be no meeting of the Hyattsville Horticultural Society in May because members will be helping to make flower arrangements to grace the homes of those on Hyattsville Preservation Society’s 35th annual house tour on Sunday, May 18, from 1 to 5 p.m.. You won’t want to miss seeing some of the loveliest homes and gardens in the city.



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Dear Miss Floribunda, My husband and I have worked very hard to improve the soil in our yard for several years now, but we still...