Why So Little Grows Under a Walnut Tree – and What Does
The first nuts are dropping, and everything in the bed below is sulking. Juglone is part of the story, but not all of it.
When the first walnuts start dropping in September, you also notice again how bare the ground beneath the tree looks. There is an old garden saying: nothing grows under a walnut. It is not quite that absolute, but there is real chemistry behind it, and once you understand it, planning the rest of your garden gets a lot easier.
Juglone: the walnut’s chemical elbow room
Walnut trees release a compound called juglone. It sits in the roots, the bark, the leaves, and most concentrated of all in the green husks around the nuts. Rain washes it out of the foliage into the soil, and the roots release it directly into the ground.
In sensitive species, juglone inhibits germination and root growth. Botanists call this allelopathy: the plant chemically claims its space. One thing worth knowing for a home garden — the common European walnut (Juglans regia) is considerably milder than the American black walnut, so advice from US gardening books does not translate one to one.
The neighbours that take it badly
Rootivo’s plant encyclopaedia lists walnut as a poor neighbour for a whole set of species, and that matches what gardeners have observed for generations.
Fruit trees: apple, pear, cherry and plum often stay stunted for good within the root zone of a mature walnut.
Nightshades: tomato and potato are among the most sensitive crops of all. Yellowing, drooping shoots despite decent care are the classic picture.
Soft fruit: red and blackcurrant as well as gooseberry grow weakly and stay small.
For the sensitive ones, no amount of feeding or watering fixes it — only distance does.
Not everything is juglone
This is where honesty pays off, because juglone tends to get blamed for more than it deserves. A mature walnut can reach fifteen metres, casts deep shade and threads the topsoil with a dense mat of roots. In summer it draws enormous amounts of water.
So when lettuce refuses to establish under the tree, the cause is often simply drought plus low light plus root competition. That is good news: those three you can soften with distance, a raised bed or targeted watering — unlike the juglone itself.
What does thrive under a walnut
Some species simply do not mind juglone, usually the ones that are at home in woodland shade anyway. Hostas, ferns, sweet woodruff, comfrey and wild garlic all do well. Spring bulbs like crocus, snowdrop and winter aconite use the window in March and April, before the walnut has even leafed out.
A safety note on wild garlic: it makes an excellent underplanting, but its young leaves resemble lily of the valley and autumn crocus, both of which are seriously poisonous and enjoy the same shady spots. Only harvest leaves that emerge singly on their own stalk and smell clearly of garlic when crushed. When in doubt, leave them be.
What to do with the leaves and husks
Right now in September you get both: the nuts and the green husks around them. Wear gloves when you collect them, as the husks stain hands and clothing a stubborn brown.
For the compost heap: juglone is water-soluble and gets broken down by soil organisms. In a warm, well-aerated heap it disappears within a few months. What you should avoid is spreading fresh walnut leaves or fresh husks straight onto vegetable or strawberry beds as mulch, because that puts the compound exactly where you do not want it.
Planning around it
A rule of thumb: keep sensitive crops out of the drip line, and with old trees add a few extra metres, because the roots reach further than the canopy. Anything in the nightshade, pome fruit or stone fruit family belongs at the other end of the garden.
When you lay out your beds in Rootivo, the encyclopaedia shows you each plant’s poor neighbours, and for walnut that list is unusually long. In the bed planner, the companion-planting lines make clashing combinations visible before you ever pick up a spade.
Im Lexikon: Walnuss