As mares tail has been around for several million years, don't expect it to be an easy job ! I had it in my allotment fo a few years after taking it over, and have made continuous raids on it, buy digging it out by the root, CAREFULLY, and trying to get EVERY bit of the root our, even the small bits, as they will just regenerate if left, from the tiniest remnant overlooked. Dig, scratch, root out at every chance appearance, and accept that it is an ongoing task. Be vigilant and patient. You will eventuaally reduce it to a manageable level. Goof luck. Oh yes ! If you can get a pig in to rootle it up, then this is even better !