Spray it with kitchen vinegar staright out of the jug. You can use an empty pop bottle with a few holes punched into the screw top and drizzle it on the horsetail. With no rain for 24 hrs - they will turn brown and die. Careful, not to get any on or near other good stuff as the acid will burn it out. Then in a week or so leach out the vinegar with water - wait 24 hours - then plant whatever you like in the spot. Worked for me and on any horsetail that did come back (very few) I just used an eye dropper to squirt a bit more vinegar on them. Good luck
