Posted: 06/05/2012 at 20:42
Covering horsetail has no effect, I'm afraid - it'll just run along to where there is light. Your best bet is to use a glyphosate-based weedkiller (glyphosate is non-residual, and inactivated completely on contact with the ground), crushing the horsetail first so it absorbs the weedkiller; it's also an idea to add a drop of washing-up liquid to the weedkiller. This won't get rid of it all, but will shift a significant amount. The rest can either be treated again, or you can keep pulling it up when you see it.