None of them 'last' these days. That's the whole point of enviromentally friendly chemicals.
Best is to spray Roundup or SBK. They are the most effective around really.
Not cheap though .. none of the good ones are.
I use Glyphosate. £21.99 for a drum of 18 sachets from a garden centre. I use it twice a year (end of Feb, end of June) on my gravel drive, and it works very well. You drop the sachets in a watering can, allow a couple of minutes for the sachets to dissolve and release the chemical into the water, and away you go.
Dennis66: re your query "salt"? just a pinch of salt in the middle of a dandelion will kill the whole root within a couple of days for good. I periodically go all over my lawn looking for a weed to dare show itself and zap it with salt
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