What actually kills weeds for good

Short version: “strongest” is the wrong thing to shop for. What kills weeds for good is a systemic weedkiller that reaches the root, not the one with the scariest label. Glyphosate is the one that does it, and how well it works comes down to how it’s put on far more than the number on the bottle.

There is a real gap in what’s in the bottle, mind. The ready-to-use trigger sprays off the shop shelf often carry only about 7 to 9 grams of glyphosate a litre. The concentrates the trade uses run 360 to 490 grams a litre, so the strongest professional concentrate is roughly fifty times stronger than a shop trigger-spray before it’s even diluted. That’s a lot of watering-down between what you buy and what actually goes down a weed.

But strength you can see in a day is usually the tell that it went nowhere. A lot of the “extra strong” and “fast-acting” shop products are contact killers, so they scorch the top off by teatime and look brilliant, while the root sits there untouched. Systemic takes its time on purpose. If it browned off overnight, it didn’t reach the part that matters.

It’s no secret what councils and groundskeepers use, either: professional, or amenity, glyphosate, put on at the right rate. Same active ingredient councils have long used on pavements and kerbs. The difference between that and a shop bottle isn’t some stronger mystery chemical, it’s the concentration, mixing it to the label rate, and being certificated to spray it. Chasing the strongest bottle on the shelf tends to cost you twice: once for the bottle, again when it grows back.

Boiling water, vinegar, salt and bleach all get passed around online. They share a problem (they only hit the top, so perennials regrow) and, on a hard surface, most of them bring a second problem: they damage the thing you’re trying to keep nice.

• Vinegar is a contact burn. Household vinegar is too weak to do much, and even the strong stuff scorches the leaf without reaching the root. It can also corrode any metal it runs onto.
• Salt does knock things back, but it sterilises the ground. Washed down into the sub-base it stops anything growing there for a long time and can’t be flushed out easily, which is the last thing you want next to borders or a lawn.
• Bleach stains. It can permanently mark block paving, coloured concrete and natural stone like Indian sandstone, and it still doesn’t kill the root. Never mix it with vinegar either, that gives off chlorine gas.
• Boiling water is the safe one for your paving, but it only scalds the top. The root down in the cool sub-base usually shrugs it off.

The straight verdict: for a stray weed in a crack, boiling water is harmless enough if you enjoy repeating it. For a whole drive or a patio full of weeds, salt and bleach risk your surface and the ground around it without ever settling the weeds. A proper systemic spray is the only one on that list that reaches the root without wrecking the paving.

Here’s the honest bit no product label will tell you: nothing stops new weeds forever. Seed blows in on the wind and arrives in bird droppings all year round, so a month or two on you might see fresh green in the joints. That is brand-new seed germinating, not the weeds you treated coming back to life.

It matters because the two are different jobs. The weeds that were there and got sprayed should be dead and stay dead, and that’s the test of the treatment. New seed settling in later is just what happens to any outdoor surface, and it’s managed with a bit of aftercare rather than another full treatment.

There’s one real exception worth knowing about. A few deep-rooted weeds, horsetail and mare’s tail chief among them, genuinely do keep pushing back through repeated treatments, because their roots run a couple of metres down. That isn’t the spray failing, it’s just how that plant works, and it needs its own approach over several seasons. I’ve set that out in how to get rid of horsetail and mare’s tail.

The weedkiller I use is professional-grade glyphosate, the same systemic, non-selective active councils use on pavements, and I’m qualified to spray it. It’s taken in through the leaves and carried, or translocated, down into the roots, so the plant kills itself from the inside rather than just browning off on top. It needs a dry spell after it goes on, at least six hours and preferably twenty four, which is what the labels themselves demand. Once it reaches the soil it stops working (it’s non-residual), so it won’t linger in the ground or creep across to your borders.

If the whole drive is overgrown, the answer isn’t to strip it first, it’s to treat the lot and wait it out. Everything green takes the spray up, then dies back together over the next few weeks, and it gets cleared and re-sanded once it’s dead. It’s not instant, and anyone promising overnight results is guessing, but it’s the version that’s still clear months later, on block paving, gravel and paths alike.

If you are still choosing what to buy, the honest verdict on which product type actually suits which job goes into that in more depth than a shop shelf ever will.