If you have pulled weeds out from between paving slabs in a British spring, you already know how fast they come back. Roundup Total Fast Action is one of those products that sits on half the garden shelves in the country, partly because it works and partly because most of us picked it up once, nodded at the dead dandelions a week later, and never switched.

At £6.49 for a 1 litre pre-mixed bottle, it is not the cheapest per litre on the market, but it is almost certainly the most tested. The listing shows 16,721 ratings and a 4.4 star average, which is a remarkable amount of UK garden feedback to draw from. We went through 100 of those reviews in detail to work out what the experience is actually like when you use it, where it falls short, and the one question nobody on the bottle seems to answer: what happens if it rains an hour after you spray?

The Weather Window Nobody Tells You About

Open any review and there is a decent chance someone mentions the rain. One buyer in Wales summed it up with a wry comment: they had been told it was a great product and were just waiting for it to stop raining first. A 4-star reviewer was more precise: it works well when you have a dry spell, and does not work nearly as well if you cannot guarantee a good 24 hours of dry, warm weather afterwards.

The official line on most glyphosate sprays is that rain within 6 hours reduces effectiveness. In a UK spring that is a planning exercise. You need a morning with no forecast showers, ideally mild temperatures, and a day or two of settled weather afterwards so the chemical has time to translocate into the roots. If you spray into a damp afternoon and a storm hits at 4pm, you are basically watering your weeds.

This is the single biggest predictor of whether a buyer ends up in the 5-star camp or the 1-star camp. It is not really a product flaw, but it is the bit most people wish someone had told them before they hit the checkout. Pick your day.

What It Kills And What You Didn't Mean To Kill

This is a non-selective herbicide. In plain English, it does not know the difference between the dandelion and the lawn. One reviewer left a wonderfully blunt 5-star note: it kills anything you spray it on, including grass, and their lawn was ruined. Another 4-star buyer warned about overspray, saying it kills your lawn better than it kills the weeds if you are not careful with the nozzle direction.

That is not a defect, it is the point of the product, but it is easy to forget when you are wandering down a path with a spray bottle on a breezy afternoon. A few tips that come up again and again in the positive reviews: spray on a still day with no wind, aim the wand down at the weed rather than swinging it side to side, and keep the trigger off until the nozzle is directly over what you want dead. One reviewer battling bindweed recommended a clever trick of dropping the bindweed into a plastic bag and spraying inside the bag to protect everything around it.

For patios, driveways, block paving, gravel, and the strip between a shed and a fence, this is exactly what you want. For weeds inside a flower border or scattered through a lawn, this is the wrong tool. Use a selective lawn weed killer for turf and spot-treat borders with a smaller ready-to-use for anything near your prize plants.

The 24 To 48 Hour Promise: Is It Real?

Roundup markets this as a fast action formula with visible results in 1 to 2 days. The positive reviews back that up with specifics. One 5-star buyer said they noticed visible results the very next day with the weeds already wilting, and within a couple of days everything was completely dead and ready to pull out. Another described the process accurately: a few days to notice changes, then the plant slowly dies, turning black before it disappears.

If your expectation is that weeds fall over in 24 hours, some reviewers will feel let down. One 1-star review said they sprayed on Sunday and Tuesday and the weeds were still green, pointing out that the bottle says it should take 24 hours. A handful of these complaints are almost certainly the rain factor above, and a few are probably tough customers like mature bindweed or Japanese anemones, which one reviewer reported needing a repeat treatment to fully clear.

For ordinary patio weeds, dandelions in gravel, and seedlings along a driveway edge, expect 2 to 4 days to visible die-back. Stubborn perennials with deep roots may need a second hit.

The Nozzle: A Real And Repeating Complaint

If there is one consistent product-level gripe in the 1 and 2 star pile, it is the trigger sprayer on the bottle itself. One buyer wrote that the weed killer is fine but the nozzle and spray quality are terrible, with leaks and a lock that does not release properly. Another reported the spray nozzle not spraying contents properly and just dribbling out. A third said the liquid had a funny smell and the package leaked.

This is not everyone's experience. The majority get a working bottle and never think about it again. But it is worth knowing before you open the packaging that the trigger mechanism is the weakest part of the kit. If yours is dribbling rather than misting, it is not a formula problem, it is a hardware one, and Amazon are usually responsive to a quick replacement request.

For anyone covering a larger area, this is also the reason many gardeners buy the 1 litre ready-to-use bottle once, then switch to the 5 litre pump or a concentrate they mix into their own pressure sprayer. The chemistry is the same. The delivery hardware is more reliable when you control it.

The Glyphosate Question

Two of the 1-star reviews raise concerns about the active ingredient itself rather than the performance. One simply states it will give you cancer, another references checking reviews after purchase and regretting it. These concerns are not unique to this product. Glyphosate is the most widely used herbicide in the world and it has been the subject of ongoing regulatory and legal debate for years.

UK domestic use remains permitted, and the product is sold openly in every major garden retailer. The precautionary advice on every bottle is sensible: wear gloves when spraying, keep children and pets off the treated area until it is dry, do not spray in high wind that can cause drift, and do not spray near edible crops unless the product is specifically approved for that use. Several positive reviewers mention wearing gloves as a matter of habit.

If you would rather avoid glyphosate entirely, there are acetic-acid and fatty-acid alternatives on the market that work through contact burn rather than systemic translocation. They are faster on leaves but less effective on roots, so you will reapply more often. That is a real trade-off to weigh up, not a reason to dismiss either option.