Hi, I am an intern Master Gardener myself but have a problem I'd like to discuss with someone more experienced. I suddenly have Equisetum coming up all over a 12' x 55' area that was disturbed a couple of years ago. It's an area along our back fence that had a raised bed on a slope that backed up to a fence and underneath was some very heavy-duty geotextile material (possibly placed there many years ago when the school behind us put in a driveway that is upslope). We removed the railroad ties and geotextile cloth, and knocked down the soil so that we could put in a driveway and have a more natural gentle sloped area for planting native trees and shrubs. After doing research using MG principles, I fear that we have unleashed a monster! Prior to this spring I had not noticed more than just a couple of plants, and they were in the gravel driveway about 8-10' from this area. As I said, suddenly this spring I have them popping up all through the area (see photos)! Last spring (2016) we knocked down the grass and weeds growing there with a weed whacker and put a layer of cardboard and about 3" of chips from an arborist on the area. This spring they are coming up right through the cardboard and running through the mulch like they love it! I've researched and found that it is extremely difficult to kill via mechanical means due to the rhizomes running up to 5 feet deep. The geotextile cloth that we removed may have been what was controlling it, although it took 2 years after we removed this cloth for it to appear. I have been pulling every frond I can find, but it sounds like that is somewhat futile. I looked up chemical controls and found that OSU recommends MCPA, but not for home use as it requires a certified applicator. WSU recommends Caseron/dichlobenil, but that is a pre-emergent and it is too late for that. Both are also listed as "Highest Hazard" herbicides on the "Grow Smart Grow Safe" web site. I'm trying to get my yard certified as a Backyard Wildlife Habitat and these herbicides are highly discouraged, however I would be open to using something that could be spot applied on a limited basis this summer. I had hoped to plant native trees, shrubs, and ground covers in this area in the fall. Now I am trying to figure out what I should do and would appreciate any suggestions.
Clackamas County Oregon horsetail
Thanks for your question about equisetum, and welcome to the Master Gardener program! Control of horsetail falls into the "good luck" category, since this plant was alive in the time of the dinosaurs. Here is a link to an excellent UC Davis article on this weed, along with IPM recommendations. Since it grows from rhizomes, you will only slow it down with mechanical and/or cultural means. I made the mistake of planting my first vegetable garden over some (previously unknown) rhizomes, and the addition of frequent fertilizer and water only encouraged it. I suspect you will have like results. If you neither fertilize nor water, you may find it 'goes into remission,' but there's no 'cure.'
Sorry. Good luck!