How do I kill lawn weeds without killing the grass?

How selective lawn weedkillers work, when to apply them so they actually take, and the cultural changes that mean you need them far less.

Difficulty: beginner
Time: 1 hr
Read: 3 min
Safety: caution

Short answer

Selective lawn weedkillers kill broad-leaved plants and leave grass alone. Apply them to actively growing weeds on a warm, still, non-drought day in late spring or early autumn, and expect to repeat after four to six weeks. Feeding, mowing higher and overseeding does most of the same work without chemicals.

Weeds colonise gaps. A dense, well-fed lawn cut at the right height leaves almost no gaps, which is why the same lawn treated year after year keeps needing treating while the neighbour's never does.

Safety

Lawn weedkillers are pesticides. Read and follow the product label — it is a legal document. Keep children and pets off treated grass until it is dry, do not treat in windy conditions where spray can drift onto beds or a neighbour's garden, do not treat lawns in flower where bees are foraging, and never put treated clippings on a compost heap or use them as mulch.

What you'll need

Selective lawn weedkiller (optional)
Watering can with a dribble bar or a knapsack sprayer (optional)
Hand fork or daisy grubber
Lawn feed and grass seed

Step by step

Identify what you actually have.Daisies, dandelions, plantain and buttercup respond well to selective weedkillers. Moss does not — it is not a broad-leaved weed and needs a different product and a different fix entirely.
Do the free things first.Feed the lawn in spring, raise the mowing height, and overseed thin areas. A thick lawn crowds out most weeds without any chemical at all.
Grub out the big ones by hand.Dandelions and plantain come out with a daisy grubber if the soil is damp. Getting the whole taproot matters — a fragment regrows.
Time the application properly.Weeds must be growing actively. Late spring once growth is strong, or early autumn. Treating in drought or cold does nothing because the weed is not moving the chemical around.
Do not mow for a few days either side.The weedkiller enters through the leaf, so leave leaf on the weed to absorb it, and give it days to travel to the root before cutting.
Apply evenly and at the label rate.A dribble bar on a watering can is far more even than a sprayer for a lawn. Overdosing scorches the grass and underdosing wastes it.
Keep it off everything else.These products kill broad-leaved plants, which includes every border plant, vegetable and shrub. Treat on a still day and shield the edges.
Expect to repeat.Tough weeds like creeping buttercup and speedwell often need a second application four to six weeks later. Speedwell is barely touched by most lawn weedkillers at all.
Overseed the gaps.Where a big weed dies you have bare soil. Overseed at four to six weeks or the next weed takes the space.
Deal with clippings correctly.Box the first two or three cuts after treatment and dispose of them in green waste. Do not compost them or use them as mulch — the residue harms other plants.

Tips

A fed, densely mown lawn is the real weedkiller. Chemicals only clear what is already there.
Speedwell resists almost every lawn weedkiller — hand raking and overseeding works better.
Never mulch or compost clippings from a treated lawn.

Common mistakes

Treating in a hot dry spell — Weeds under drought stress shut down and barely take the chemical in, so it fails and gets blamed on the product. Wait for growing conditions.
Mowing the day before treating — There is almost no leaf left for the weedkiller to enter through, so very little reaches the root and the weed regrows.

If it doesn't work

Weeds unaffected after two weeks

Cause: Applied in drought, cold, or straight after mowing — Fix: Wait for active growth, leave leaf on the weeds, and re-treat.

Yellow scorched streaks in the lawn

Cause: Uneven or over-application — Fix: Water well. Grass usually recovers; overseed anything that dies.

Weeds come straight back

Cause: Bare gaps left after they died — Fix: Overseed every treated area once the weeds have gone.

Moss unaffected

Cause: Selective weedkillers do not touch moss — Fix: Moss needs a moss killer and, more importantly, fixing the shade, drainage or compaction causing it.

Questions people ask

When should I apply lawn weedkiller?

Late spring once growth is strong, or early autumn. The weed must be actively growing and the soil not dry, and the day should be still and warm but not hot.

Is there a way to do this without chemicals?

Largely, yes. Feed in spring and autumn, mow higher, overseed twice a year and grub out the large rosette weeds by hand. It takes two or three seasons but it is durable.

Can I compost the clippings afterwards?

No. Residues persist through composting and damage plants the compost is later used on. Box the first few cuts and send them to green waste collection.