Black walnut toxicity

also called Juglone toxicity, Allelopathy

The classic answer to why nothing grows there: a black walnut nearby, and a chemical in its roots and leaves that poisons tomatoes outright.

Disorder No cure Doesn't spread

What you'll see

Plants wilting, yellowing and dying within a circle around a walnut tree, with the worst nearest the trunk.
Tomatoes stunted, yellow and wilting from the bottom up, then dead — the most sensitive of all.
Sudden collapse of a healthy-looking plant, with no rot, no insect and no lesion to find.
Grass, daffodils and many shrubs perfectly happy in exactly the same ground, which is the confirming detail.
Damage extending well past the visible canopy, since the roots go further than the branches.

Why it happens

Black walnut produces juglone in its roots, leaves, husks and bark. It leaches into the soil and blocks respiration in sensitive plants, killing them outright. The highest concentration is in the root zone, which extends far beyond the canopy — often to twice the branch spread on an old tree — and juglone persists in soil, leaves and sawdust for months after the tree comes down.

Within and beyond the root zone of a black walnut or butternut, worst in poorly drained soil where the compound is not washed away. English walnut on black walnut rootstock produces it too. Sensitivity varies enormously between plants and is essentially all-or-nothing.

Treating it

Move the sensitive plants rather than trying to save them; there is no antidote.
Grow in raised beds with a solid base and imported soil, kept clear of walnut leaves and roots.
Do not compost walnut leaves, husks or sawdust into beds where tomatoes will go; give them a year to break down separately.
Improve drainage in the affected area, which helps the compound leach away.
Give the ground a year after felling a walnut before planting sensitive crops in it — and expect the stump and roots to keep producing for longer.

Keeping it away

Plant the vegetable garden, and tomatoes above all, well outside the root zone — allow twice the branch spread.
Grow the tolerant plants under a walnut instead: daffodils, ferns, hostas, daylilies, astilbe, bee balm, most grasses, black-eyed Susan, and among vegetables beans, beetroot, carrots, parsnips and sweetcorn.
Avoid the sensitive ones there entirely: tomatoes, peppers, aubergines, potatoes, cabbage, asparagus, rhubarb, apples, blueberries, azaleas, rhododendrons, lilac, peonies and birches.
Keep walnut leaf litter off the vegetable beds each autumn.
Site a new walnut with the mature root zone in mind rather than the size of the sapling.

Who gets it

Tomatoes, peppers, potatoes and aubergines are the most sensitive vegetables; apples, blueberries, azaleas, rhododendrons, lilac, peonies and birches among the woody plants.