Does glyphosate kill grass?

Why grass has no way out of it

The mechanism is the same on every green thing you point it at. Glyphosate blocks an enzyme called EPSP synthase, which sits in the shikimate pathway plants use to build three of the amino acids they need to grow. Grasses run that pathway exactly as broad-leaved plants do. There is nothing in a blade of ryegrass that gives it a way round, which is why the label wording is so flat about it: Roundup Ultra describes itself as a non-selective, translocated weedkiller that moves through the whole plant, above and below ground, to control annual and perennial weeds including grasses. So the question is never whether glyphosate can kill your lawn. It is only whether any of it lands there.

Grass usually goes first, too. Two professional UK labels say so in almost the same sentence. Roundup ProActive (MAPP 17380) describes the leaf symptoms as a reddening then yellowing of the foliage, first seen on the grass weeds but taking longer to appear on broad-leaved weeds, generally starting 7 to 14 days after spraying or longer if growth is slow. Barclay Gallup Biograde 360 (MAPP 17612) repeats it nearly word for word. If you have caught a lawn edge with the spray, the yellow stripe will show up before the docks in the border have registered anything at all.

Is grass harder or easier to kill than weeds?

Easier, mostly, and the rate table on the bottle is the proof. Roundup Ultra asks for 10ml per litre on annual weeds, which is the band annual meadow grass sits in, 15ml for perennial grasses including couch, and 15ml for the tough broad-leaved perennials like bindweed, docks and thistles. Couch, the grass with the worst reputation of the lot, is on the same rate as the weeds everyone already thinks of as difficult, not above it. Only killing a whole lawn outright goes higher, at 20ml.

Where couch genuinely does catch people out is timing rather than strength. The Roundup ProActive label wants perennial grasses to have full emergence of healthy green leaf, and states that common couch becomes susceptible at the onset of tillering and new rhizome growth, usually when the plant has 4 to 5 leaves each carrying 10 to 15cm of new growth. Spray a rhizomatous grass before it has that much leaf area and you are asking a small solar panel to power a long journey underground. Gallup Biograde 360 adds the other half: couch and everything else is less susceptible when growth is restricted by drought, waterlogging, frost, very high temperatures or natural dieback.

Spraying a drive that runs alongside turf

Two different things put glyphosate on grass you wanted to keep, and only one of them is spraying in the wrong place. The first is drift. Roundup Ultra tells you to avoid spraying in windy conditions because spray drift to lawns and other wanted plants will damage or destroy them, and Gallup Home and Garden says to take extreme care to avoid drift and not to spray in windy weather or near desirable plants. The second is transfer, and it catches far more people. The Fast Action Roundup ready-to-use label is approved for edging lawns and, in the same panel, tells you not to walk into treated areas until the product has dried, precisely to avoid transfer to lawns and other desired vegetation. Wet spray on the sole of a boot or on a dog's paws is a delivery system, and it walks straight onto the grass.

On wind there is a UK reference standard, although it was written for professionals with field sprayers rather than for you and a trigger bottle, so treat it as the benchmark rather than the rule. The Code of Practice for Using Plant Protection Products, published by Defra and the Health and Safety Commission in 2006, puts the safest conditions at a steady wind of 2 to 4 miles an hour blowing away from anything sensitive. Its wind table then singles out herbicides by name in the band above that: at 4 to 6 miles an hour it says there is an increased risk of spray drift and to avoid spraying herbicides, and at 6 to 9 miles an hour it says do not spray. It also makes a point that matters more on a drive than in a field. The usual assumption is that a crop slows the air near the ground, so where there is no crop, and it names spraying hard surfaces in amenity areas as the example, the wind at nozzle height is faster than that assumption allows. A bare, open drive is about the windiest place on your property to be spraying anything.

Which gives you the practical version. Work on a still day rather than a bright breezy one, keep the nozzle low and the pressure down, use a coarse spray rather than a fine mist (Gallup Home and Garden asks for a medium to coarse spray, and the professional Gallup label caps nozzle pressure at 2.5 bar), work with what breeze there is pushing away from the turf, and stay off the treated strip until it has dried. If you do clip a wanted plant, Roundup Ultra says to wash the weedkiller off immediately and thoroughly with plenty of water.

It will not creep across to your lawn through the soil

This is the part people worry about, and it is the one thing glyphosate does not do. It is inactivated the moment it touches soil. The Roundup ProActive label states that it is rapidly adsorbed onto particulate matter in soils and quickly degraded by the micro-organisms present, and that until it is degraded it is practically immobile in soil. Gallup Home and Garden says it is inactivated on contact with the soil and broken down by soil organisms to leave no lasting residues. So the grass a foot from where you sprayed is at risk from a droplet landing on its leaves, and not from anything travelling underground to its roots. That is also why nobody has to worry about the roots of a tree whose canopy you sprayed under.

The same fact settles the follow-up question, which is whether the ground is now poisoned for new grass. It is not. Plain glyphosate has no residual action at all: the Gallup Biograde 360 label states outright that there is no residual activity and that weeds germinating after application will not be controlled. That is a nuisance when you are trying to keep a drive clear, because the next seed that blows into a joint germinates as happily as ever. It is exactly what you want when you are trying to get grass back.

Putting the grass back after you have killed it

The repair sequence is on the Roundup Ultra label itself, in the lawn destruction section, which makes it about as authoritative an answer as this question has. Treat actively growing lawn before the onset of dormancy, usually between May and September. Allow at least 7 to 10 days for the product to be translocated to the roots. Remove the dying and dead lawn. Then re-seed or re-turf to the seed or turf supplier's instructions. The wait is about letting the chemical finish its journey downwards, not about waiting for a residue to clear, which is why the same label allows replanting and sowing the day after treatment where the cover was only light annual weeds, and asks for at least 7 days where it was dense or perennial. Gallup Home and Garden gives the same 7 days, with the sensible caveat that in cool weather you should wait for the leaves to yellow or die back first.

None of this makes glyphosate the wrong thing to reach for next to a lawn. It makes it a product whose only real weakness is aim. Everything that decides whether your grass lives is under your control on the day: the wind, the spray quality, how close you work to the edge, and staying off the treated ground until it has dried. Get those right and you get the thing nothing else on the shelf gives you, which is a weed killed at the root rather than burned off at the top. Kill the root or it comes back.