Killing grape hyacinths
 

Does anyone know a way of killing grape hyacinths?

Obviously I can dig up the ones I can get to, but over the years they have propogated underneath my patio over a very large area and come up through the cracks every year.

I have tried weed killer without much effect, but I guess timing would be critical because they can just store up their energy in the bulb.

Any advice would be gratefully received.

dch222