Answer
How do I stop weeds growing between patio slabs?
Sound pointing with a quality jointing compound; weeds root where the joints have failed.
In short
Weeds grow where the jointing has failed and seeds can settle, so the lasting fix is sound pointing with a quality jointing compound, not just pulling or spraying. A properly jointed patio on a good base gives weeds nowhere to root. We re-point or re-lay where the joints have gone.
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