Posted: 18/03/2014 at 09:47
one of the worst weeds you could ever have the misfortune of having. had it in my soon-to-be-previous garden, which incidentally is also a heavy clay, poorly drained and acidic. same conditions under which moss thrives.
so, the answer from these set of circumstances would seem to be to improve the soil structure, as well as cutting any stems ground-level up before spore formation. now, these are very much long term solutions, but in my opinion are the only way. it will take some years to exhaust the root system of the horsetail but frequent cutting and prevention of above ground level foliage appearing will limit any new plants from being produced and will obviously exclude light and thus energy from getting to the plants roots, weakening them. it is not sensible to cut any root material, as not cutting a root cleanly will simply regenerate new plants.
as for soil structure - dig in LOTS of organic matter. horsetail, as with moss, thrives in poor soil. i have read somewhere that the first plant to establish and grow on the burnt earth around the erupted Mt. St. Helens was ... horsetail! that in itself proves that theory. improving this structure over time strategically will help greatly.
good luck ..