Why is my weed killer not working?

The weed was too stressed to take it in

This is the one that catches most people, and it sits in the Restrictions section of the label rather than buried in the small print. Roundup ProActive (MAPP 21225, 360 g/L glyphosate) puts it plainly: do not spray onto weeds which are naturally senescent, or where growth is impaired by drought, high temperatures, a covering of dust, flooding or frost at, or immediately after application, otherwise poor control may result.

A systemic weedkiller is not a poison you paint on. It has to be drawn in through the leaf and carried down inside the plant on the same flow that moves sugars to the roots. A weed baking in a July drought has shut that down to survive, so the dose sits on a leaf that is barely drinking. Research published in Weed Science found severe drought stress cut glyphosate efficacy hard, suppressing mortality by up to 63 percent. Nebraska extension gives the plain-English version of the same finding: wait for some rain, then spray once the weeds are healthy and actively growing again.

The dust clause is the one to read twice if this is a drive, a path or a gravel edge. A weed growing in block paving joints beside a road, or under a dry hedge, is often carrying a film of dust on its leaves, and the label counts that on its own as a reason for reduced control. Let a shower rinse it first, or start with the weeds that are clean.

Rain landed inside the rainfast window, or the leaves were already wet

Six hours is the floor, not the target. The Roundup ProActive label requires a period without rain of at least 6 hours and preferably 24 hours after application, and Barclay Gallup Hi-Aktiv Amenity (MAPP 17681, 490 g/L glyphosate) asks for the same thing in nearly the same words. If a shower came through inside that window, a good part of the dose went down the drain before the plant had taken any of it in, and the weed carries on as though you had never been out there.

The same label adds a line that almost nobody sees: a covering of dew may reduce efficacy where run-off occurs. Spraying first thing on a dewy morning is not free, then. You are diluting the mix on the leaf and letting it run off the tip into the ground. Mid-morning, once the dew has gone and with a dry afternoon ahead of you, is the window that costs you nothing.

The mix was too weak, or you drowned it

Two opposite mistakes, and both end up in the same place. Too weak is usually a rate lifted from a different product. Glyphosate concentrates on the UK shelf run from 90 g/L up to 490 g/L, so a millilitres-per-litre figure copied off a forum, a video or the last bottle you owned can have you spraying a fraction of what your label asks for. Read the rate off the pack in your hand, every single time.

Drowning it is the surprising one, because it feels like being thorough. The Roundup ProActive label tells you to avoid high water volumes, above 300 litres per hectare, because they lead to run-off from the treated vegetation and reduced control. Spraying a weed until it drips does not give it a bigger dose. It gives it a wet leaf, and it gives the ground the rest. An even coat that stays put on the leaf is doing more work than a soaking.

There is a third possibility hardly anyone checks: the water itself. A 1995 paper in Weed Science on the hard-water antagonism of glyphosate set out the mechanism, which is that calcium and magnesium ions in the spray water lock onto the glyphosate in the tank before it ever reaches the plant. Much of England is hard-water country and the knapsack gets filled from the outside tap. I would not reach for this as the first explanation, but on a mix that was already marginal it is one more thing quietly working against you.

You used a contact product on a perennial

This is the failure that looks like a success for about a week. A lot of what is sold for paths and drives is not glyphosate at all. Pelargonic acid and acetic acid products are contact herbicides: they destroy the plant tissue they physically wet and are carried nowhere else afterwards. On a seedling that is enough, because at that size the leaf more or less is the whole plant. On a dandelion, bindweed or couch grass rooted in the joints, you scorch the top off and the root underneath never registers it, so the same weed is back through the same crack a fortnight later.

The labels do not hide this, they just phrase it as a routine. Roundup Weedkiller Total Optima, 43.1 g/L pelargonic acid, tells you to reapply every 7 to 21 days, up to four applications a year. Read that for what it is. A product that finished the weed would not need a diary. If you are on your third bottle of the season, it is the chemistry that is wrong rather than your technique.

The weed was at the wrong growth stage

The Roundup ProActive label is direct about this too: it is important that all weeds are at the correct growth stage when treated, otherwise some re-growth may occur and this will need re-treatment. It then names the stages. Perennial broad-leaved weeds are most susceptible around the flowering stage. Annual grasses and broad-leaved weeds want at least 5 cm of leaf, or two expanded true leaves. Perennial grasses need full emergence of healthy green leaf.

The thread running through all of that is leaf area, because the leaf is the only door in. A weed you have just strimmed, mowed or scraped out of a joint has almost nothing left to take a dose through, so spraying it is close to spraying the ground. If you have already cut it back, let it come into proper leaf again and treat the regrowth instead.

It is working and you are looking too early

Glyphosate is slow by design and week one tells you very little. The Roundup ProActive label says symptoms of treatment are generally first seen 7 to 14 days, or longer if growth is slow, and that they show up as a reddening then a yellowing of the foliage. It also asks you to leave the weeds undisturbed, with no further treatment or cultivation, for 7 days, then to allow 2 to 3 weeks for symptoms to develop before spot-treating anything still unaffected.

So the honest answer to day three is that there is nothing to see yet. The commonest self-inflicted failure I come across is somebody deciding on day four that it has not worked and then jet washing, scraping or re-spraying, which interrupts the exact process they were waiting on. Write the date down and leave it alone.

Sometimes it is the weed, not you

There is one case where you did everything right and the product still cannot finish the job in a pass, and it is worth knowing because it stops you buying something stronger for no reason. Two separate 360 g/L professional glyphosates admit it on their own labels. Roundup ProActive states the product will not give an acceptable level of control of horsetails, Equisetum arvense, and that repeat treatment will be necessary. Barclay Gallup XL (MAPP 17663) states in as many words that it does not give acceptable control of the same plant.

If what you are fighting is horsetail, or mare's tail as most people call it, no amount of correcting your technique changes that outcome. It is a repeat-treatment campaign run across seasons, and I have written it up properly on its own page. It is also an unusually straight thing for a manufacturer to print about their own product, and it is worth more than any review you will read.