- A lawn weedkiller kills the weeds, but some of these products stay in the clippings, and the residues can still be there months later in the compost.
- The damage shows up as twisted leaves and distorted fruit on roses, tomatoes, potatoes and vines, never on the grass.
- The fix is a habit: keep treated clippings off the heap and out of the green-waste bin, test suspect compost with broad beans, and pull lawn weeds by hand.
The compost bag you spread this winter could be the problem
Mulching season runs from late autumn to late winter, so this is the month you open bags of compost and empty the heap onto the beds. A mulch is a layer at least 5 cm (2 in) thick. That is a lot of material sitting round the base of your roses.
If any of that material came from a lawn that was sprayed with a selective weedkiller, your plants are about to meet the product the lawn took in. The clippings looked like harmless green waste. They were not.
Here is the fix up front. Keep clippings from a sprayed lawn out of the compost heap, out of the mulch pile and out of the green-waste bin until the label says they are safe. Test any compost you doubt with broad beans before it goes near a bed. Better still, stop spraying the lawn.
The belief: the spray only touches weeds
Most gardeners reason the same way. The weedkiller killed the dandelions and left the grass alone, so whatever is left on the grass must be harmless. Mow, tip the box on the heap, job done.
That reasoning has a gap. A selective lawn weedkiller is selective about what it kills, not about what it contains. The grass takes the product in and survives it. The product does not vanish.
What actually happens in the heap
Products containing clopyralid or triclopyr are hormonal, or growth-regulating, weedkillers. They damage broad-leaved plants and leave grasses unharmed. That is why a lawn can be sprayed and stay green while the broad-leaved weeds suffer.
These products work through the leaves and move down into the roots of the weed. Residues can persist in treated plant material for months.
Clopyralid and a close relative, aminopyralid, take a long time to break down after binding to grass. Composting does not clear them: clopyralid and similar persistent products can survive the green-waste composting process and contaminate compost sold to gardeners.
So the damage has two routes into your garden:
- Your own heap, where you added clippings from a sprayed lawn and then spread the compost or used the clippings straight as mulch.
- Compost or manure you bought, made from green waste that included treated clippings, or manure contaminated with persistent aminopyralid.
Roses, tomatoes, potatoes and vines are the most sensitive plants. They are the ones that suffer when a bed is mulched with contaminated material. Grass is the exception, which comes up again further down.
How to tell if your compost or mulch is contaminated
Look at the plants first. Hormonal weedkiller damage has a recognisable pattern:
- Narrowed or cup-shaped leaves with parallel veins.
- Twisted or distorted leaf stalks.
- Roots growing from places roots do not usually grow, including on some root crops.
- Swollen stems, galls or warts on brassicas.
- Plum-shaped, distorted tomatoes with hollow centres.
- Fern-like growth on a tomato plant growing in contaminated compost.
If a bed you mulched with a lawn heap shows that pattern, the compost is the prime suspect. Sprays that scorch leaves or leave brown spots belong to a different type of weedkiller, the contact kind, which only kills the parts it lands on.
The broad bean test
You can check suspect compost at home with a pot test. It takes three weeks, so run it before you spread anything.
1. Fill two clean pots with fresh, unused suspect compost and label them. Pots of 1 litre (1.8 pt) or smaller are enough.
2. Fill two more clean pots with garden soil or a different brand of compost. These are your controls.
3. Sow four broad bean seeds in each pot and water well with tap water. Broad beans are quick to germinate and sensitive to this type of weedkiller, which is why they make a good test plant.
4. Stand the pots in a greenhouse or on a warm windowsill and keep the compost slightly moist. Make sure the water draining out of the suspect pots cannot run into the controls.
5. After three weeks, check the seedlings. Seeds in the suspect compost that fail to come up, or seedlings that are distorted, slow-growing or fern-like while the controls grow normally, are strong grounds to believe residues are present.
The makers of clopyralid and aminopyralid advise running the same test with radish seed as well. Radishes are unaffected by both products. If the radishes show damage, something else is wrong with the compost, so the beans alone would have sent you after the wrong culprit.
If the test is positive, report it to the compost maker and to the Health and Safety Executive. For bought compost, tell the maker where and when you bought it.
What to do with the clippings and compost you have
Read the label on the lawn weedkiller. A selective product states when you can collect the clippings and how long they need to compost before the residues break down. Follow the label, not a guess. If a lawn-care company treats your grass, ask them directly whether weedkiller has been applied and which product it was. Lawn-care companies often offer a weed and feed treatment as routine maintenance, so ask rather than assume.
Keep treated clippings out of the green-waste bin. Persistent products survive the large-scale composting process, so a bin of treated clippings can turn into contaminated compost on someone else’s garden shelf.
For bought compost, choose a peat-free product that clearly states it contains no composted green waste, or make your own mix. That claim on the bag is the signal to look for.
Compost that fails the bean test is not waste. Compost suspected of holding these residues can be used as a topdressing for lawns, as grass is unharmed. Spread it on the lawn and keep it off the beds.
If you have already damaged plants, the news is better than the symptoms suggest:
- Prune out abnormal, yellow, brown or dead growth. New normal shoots can take a while to appear.
- Water in dry spells, mulch around permanent plantings with clean material, and feed if growth is slow.
- Plants that showed symptoms early in the growing season can grow out of the problem as the residues break down.
- Replant the bed only after enough time has passed for the residues to break down. The weedkiller label gives the timing.
Two habits cut the risk further. Weedkiller travels on footwear and tools, so do not walk the sprayed lawn and then work a rose bed in the same boots. And triple-rinse any sprayer or watering can that held weedkiller, using the rinse water only on areas the label lists. Ready-to-use products skip that step.
How to stop it coming back
The simplest answer is the Royal Horticultural Society’s: you do not need a lawn weedkiller. Non-chemical methods work, and no weedkiller means no residues in the clippings. Selective lawn weedkillers can damage other plants when contaminated clippings are added to compost and used as a mulch.
That removes the whole problem. You can skip the spray and keep your clippings, your heap and your mulch.
Here is how to deal with the weeds themselves:
- Spot weeds early, while they are small.
- Dig out single weeds by hand with a hand fork or a dandelion weeder, and repair any bare patch by reseeding or laying turf in spring or autumn.
- Cut out and lift a badly weedy patch with a sharp spade, remove the weed roots, fork over the soil, rake it level, tread it down and lay fresh turf. This works well on creeping weeds such as clover that run through the grass.
- Rake creeping weeds before mowing, so the mower picks up their low stems.
- Keep the mower blades higher. Cutting very short, especially in summer, weakens the grass and leaves bare patches for weeds to take over.
- Cut off the flowers of the weeds you tolerate before they set seed, with secateurs or by regular mowing.
If you want worked examples, getting rid of chickweed comes down to the same hand tools. It needs no spray bottle.
Bare patches invite weeds, and pink-flowered weeds in bare patches shows how to close them without spraying.
For a tall weed that keeps returning, the leafy weed with rusty seed spikes has its own hand-removal fix.
A thick, healthy lawn is the long-term defence. A programme of spring and summer care plus autumn care keeps the grass dense, which makes it harder for weeds to establish.
You also get something back for doing less. Weed and feed products are sold together as routine maintenance, but most lawns do not need regular feeding to stay green and healthy, and the weedkiller half reduces lawn biodiversity. Lawn weedkillers are designed to eliminate every plant that is not a grass. Dandelions, for example, are an excellent early source of pollen for flies, bumblebees and solitary bees, and they flower even in the winter months. Stop spraying and the lawn does not turn brown or untidy overnight.
When to give up on a heap
If you cannot say where the clippings in a heap came from, or a lawn-care company has treated the grass and will not say with what, treat the heap as suspect. Run the bean test before you spread it on a bed. If the beans fail, tip it on the lawn as a topdressing and start a new heap from clean material. It costs you a few weeks and one packet of seed, against a bed of ruined roses or tomatoes.
