Answer: I empathize with you! I battled horsetail along a slope in my back yard for a couple of years. I'm happy to report that I am the victor and you can be too. Cinnamon fern creeps underground by stolons or rhizomes (as does horsetail). It likes moist soil (as does horsetail). Stoloniferous and rhizomatous plants store lots of energy in their roots and will continue to send up new shoots as long as they have enough stored energy. So here's what you need to do to stop the process. First, dig into the soil with a garden fork. Loosen the soil and gently pull the threads of rhizomes
from the soil. Any root with a node will develop into a new plant. I think you've already completed this step, but if not, now's the time. When a new shoot pops up, immediately cut it to ground level and paint glyphosate (Round Up) on the fresh cut. The roots rely on photosynthesis to collect more energy to store to make new shoots. If you stop the process of photosynthesis, the roots will eventually be depleted of stored energy and will not be able to develop new shoots. The sooner you can cut the new shoots down, the less likely the roots will benefit from sending up a new shoot. This is a labor-intensive process but each time you cut and paint a shoot, that part of the root will die. If you are persistent, you will kill off the cinnamon fern without harming the rest of your garden. It took me 3 years to get my yard completely horsetail free so I expect if you work at it this summer and next, the third year your landscape will be cinnamon fern free.
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