Answer: Equisetum (horsetail/scouring rush) have far-reaching, tuberous roots with what seems like tons of stored energy. While you may be able to kill the tops down with glyphosate (Round-Up), the roots have plenty of energy left to send up new shoots. Equisetum thrives in moist places; if you can dry the soil out, you'll discourage the plant. I've found the only reliable and permanent way to get rid of horsetail is to cut it down as soon as it sends up shoots. Don't wait - as soon as the new shoots appear, cut them off at ground level. If you do this consistently, the roots will eventually run out of stored energy and they'll die. If you allow the shoots to grow 2-3 inches tall, photosynthesis will recharge the roots, so keep everything cut down. It took me one full year of whacking off new shoots to rid a hillside of horsetail, but once it went away, it never came back. Best wishes with your project!
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