Afraid that's Marestail aka Horsetail. You won't get rid of it .... it's been around since before the dinosaurs and the roots go right through to Australia ... so just keep hoeing and pulling it to keep it at bay. Afraid that's the best you can do.
'They' do say that it doesn't like lime, so if you're going to lime your brassica beds, that might help.
“I am not lost, for I know where I am. But however, where I am may be lost.” Winnie the Pooh
I saw on TV some time ago that they had Marestail at the RHS garden Hyde Hall. They trampled it to bruise it and sprayed it with Glyphosate, more than once.
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'They' do say that it doesn't like lime, so if you're going to lime your brassica beds, that might help.