How long does weed killer take to work?

What the labels say about the killing time

Two manufacturers put real numbers on it. The Weedol Pathclear label (MAPP 16524) says visible results in 14 days and that weed death may take up to 28 days, adding that in cold weather activity is slower. The professional Roundup ProActive label (MAPP 21225) says symptoms of treatment are generally first seen 7 to 14 days, or longer if growth is slow after spraying, and describes those symptoms as a reddening then yellowing of the foliage, first on grass weeds and later on the broad-leaved ones. So 7 to 14 days for the colour to turn and up to four weeks for the weed to be properly dead is not me hedging. It is what the people who make the stuff print on their own labels.

The reason it is not instant is also the reason it lasts. The treatment has to be drawn in through the leaves and carried down into the root before the weed dies from the inside out. A weed that looks a bit sorry after a few days is a good sign, not a slow one, it means the spray is working its way down. If something wilts within hours, that is usually just heat or damage to the leaf, not the root dying.

Drying time is a different clock

Worth separating, because people run the two together and then wonder why the timings do not match. Dry to the touch is a surface condition and it happens long before the weed is dead. No UK label puts a number of hours on it, they set a condition instead, that children and pets stay off until the spray has dried. Rainfastness is a third thing again, and stricter: at least 6 hours without rain. If that is the clock you are actually asking about, the answer about how long weed killer takes to dry works through the label wording properly, and the one about spraying weeds in the rain covers the shower that lands too soon.

How long it lasts is a different question again

The short of it is residual or not, and the front of the bottle rarely uses the word. Plain glyphosate is not residual: it kills what is green on the day and leaves nothing behind in the ground, so the clock on your next flush of weeds is simply how long the next seed takes to germinate. A path and drive product adds a second ingredient that stays in the surface layer and does hold new weeds back for a few months, and it comes with strings on replanting. The label figures, the half-lives and the wait before you can plant anything are on the answer about how long weed killer stays in the soil.

How long does brush killer take to work?

Longer, and the label tells you how much longer by the interval it allows before a second go. Vitax SBK Brushwood Killer (MAPP 12711) is 48 g/L triclopyr rather than glyphosate, aimed at brambles, nettles, docks and woody saplings, and its instruction is that a repeat treatment may be applied after six weeks, to a maximum of two treatments a season. Six weeks, not four, is the maker's own retreat window, so judging a brushwood killer at the four-week mark you would use for glyphosate on a drive tells you nothing useful. There is more to kill, too. A bramble has a crown and a woody root system that a seedling in a paving joint simply does not.

How to tell whether it actually worked

That four-week window is your test for a normal glyphosate job. If the weeds on a treated area have not visibly died within four weeks of spraying, the treatment never reached the root, whether that is a too-weak product, a bad mixing rate or rain too soon after it went on, and that area needs doing again. Once they are dead you can brush or wash the dead growth away, and the paving stays clear far longer than it would after pulling or burning. Kill the root or it comes back.