I used Rosate 36 via watering can to eradicate a large patch of nettles by a new allotment. Strong solution. Will a solution of 30ml to 1ltr water in a 5 litre hozelock sprayer be strong enough to spot kill new nettle growth elsewhere.
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Agree with scroggin. They also make a great compost tea. Just wack them in a dust bin full of water and put the lid on......forget about them for a few weeks and as if by magic you end up with a great high in nitrogen compost tea.