To conquer mare's tail (equisetum arvense) with its 7ft long roots/underground stems (rhizomes) and extremely invasive nature, first dig out as many roots as you can before planting. Then, as soon as you see the fir tree-like green shoots appear, pull them out. If you never let the horse tail see the light of day, over a number of seasons, the infestation will dramatically reduce. Cutting grass which contains the weed, very short, also keeps it in check. Chemical treatment is not particularly effective as this plant is a strong survivor.