Herbicide damage

What you are looking at

New leaves come out wrong. They are narrowed, sometimes to a thread, with the blade barely developed, so a tomato leaf looks like a fern and a bean leaf looks like a strap. Shoot tips cup, twist or spiral, stems go fluted and ribbed, and the growing point curls into a tight fist.

The older leaves lower down are usually normal, because they were made before the plant met the chemical.

Then look at what is affected and what is not. Tomatoes, potatoes, peas, beans and dahlias show it badly while sweetcorn, onions, leeks and brassicas standing beside them look perfectly well. That split is close to diagnostic, because it follows the chemistry rather than any pattern a disease or a pest would produce.

Aminopyralid, and why it reaches a vegetable plot

The commonest cause of this in a British garden is not a weedkiller anyone applied. It arrives in manure.

Aminopyralid and its relatives are used to control broad-leaved weeds in grassland. They are effective at very low concentrations, they persist on the treated grass, and they pass through an animal that eats the hay or silage essentially unchanged. So they end up in the muck, and in the straw bedding, and then on somebody’s allotment.

Composting does not destroy them. Manure stacked for two years has damaged crops. The residue breaks down through microbial activity in soil rather than in a heap, and that takes time.

The effect is dramatic and it repeats every few years, usually as a wave of ruined allotment crops across a district that traces back to one supplier.

Test before you spread

Four to six weeks in the spring saves a whole season, and the test is straightforward.

Fill three pots with the manure or compost you are thinking of using, and three with ordinary bought compost as a control. Sow two or three broad beans or dwarf French beans in each. Stand them all on a windowsill, water them, and wait.

Beans are exquisitely sensitive to these chemicals. If the plants in the test pots come up with fern-like leaves, cupped tips or twisted stems while the controls look normal, the material is contaminated and should not go anywhere near the vegetable garden.

Do this with every load from a new source. It costs a packet of beans.

If it is already on the ground

The bed is not lost, though this year’s crop probably is.

Dig the material in thoroughly rather than leaving it on the surface, since breakdown happens in the soil. Keep the ground moist and warm and do not let it bake. Then grow the tolerant crops on it for a season or two, which means sweetcorn, brassicas, onions, leeks, garlic, beetroot, spinach and lettuce.

Pull and bin the damaged plants rather than composting them into the same garden.

Most beds recover within a season or two. Test again with beans before putting tomatoes or potatoes back.

The other routes in

Lawn weedkillers contain hormone-type chemicals that damage broad-leaved plants at tiny doses. Clippings from a treated lawn must not be composted or used as a mulch until several cuts have passed.

A sprayer that has held weedkiller keeps enough residue to damage anything sprayed from it afterwards, however well it was rinsed. Keep a separate sprayer, label it clearly, and never borrow one.

Spray drift from a neighbour, from a driveway treatment or from a farm boundary produces damage with a distinct edge along one side of a garden, worst on the plants nearest the source.

What to do about it

Test manure and compost with beans before spreading, every time, from a new source.

Ask the supplier about the forage history of the animals.

Dig contaminated material in and keep the ground warm and moist.

Grow sweetcorn, brassicas, onions and leeks on the bed for a season or two.

Pull and bin the damaged plants and do not eat anything from them.

Keep a separate labelled sprayer for weedkiller.

Keep treated lawn clippings out of the compost heap.

Tell the supplier, because they usually do not know.

What it gets mistaken for

Mosaic viruses distort new growth in a very similar way and mottle it light and dark green as well. They also take individual plants and leave the neighbours alone, where a herbicide hits everything susceptible in an area.

Capsid bugs tear small brown-rimmed holes in the newest leaves as they expand, and the leaf is perforated rather than merely misshapen.

Aphid feeding puckers and curls young growth, with the aphids underneath.

Hormone-like distortion also appears on tomatoes grown near a bonfire of treated timber, or watered from a butt that collected runoff from a treated surface, which is worth checking before blaming the manure.

Genetic sports and fasciation produce flattened or fused stems on an otherwise normal plant, and they affect one shoot rather than every susceptible plant in the bed.