Does bleach kill weeds?

Only the bit you can see. Bleach scorches the leaves within a day or two, but it is not a systemic weedkiller, so it never travels down into the root. On a drive or patio it can also permanently stain block paving, coloured concrete and natural stone, so what you save on a bottle of weedkiller you can lose on the surface underneath it.

Bleach is sodium hypochlorite, and it works as a contact desiccant: it dries out and damages any plant tissue it touches on contact. That is why a weed looks dead within a day, sometimes hours, of being doused. But contact is the whole mechanism. It never gets carried through the plant to the root sitting in the sand between your blocks or down in the sub-base, so an established perennial weed regrows from that untouched root within a week or two, and you are back where you started with a browner-looking drive.

The staining risk is the part most home-remedy advice skips. Bleach can bleach. On block paving, natural stone like Indian sandstone, and coloured or stamped concrete, splashed or pooled bleach can permanently lighten or discolour the surface, and there is no undoing it once it has happened. It also raises the soil pH where it runs off, and it is not sold or labelled as a weedkiller, so using it as one is off-label and outside the manufacturer’s guidance.

Mixing bleach with anything acidic, including vinegar, is also worth knowing about. The two react to release chlorine gas, which is genuinely dangerous to breathe in an open garden, let alone anywhere enclosed. If you have tried bleach and vinegar together on the same patch, that combination is one to stop rather than repeat.

The honest comparison: bleach sits in the same bracket as boiling water, salt and household vinegar. All four can brown off top growth for a week or two on an annual weed, none of them reliably kill the root of an established perennial, and two of the four (bleach and salt) carry a real risk of damaging the ground or the surface around the weed. A proper systemic weedkiller is the only thing on that list built to travel down into the root and finish the job in one go.