Hello Angela! Thanks for reaching out to Ask Extension.

Horsetail (Equisetum spp.) is a very tricky plant to fully eradicate, as it sounds like you’ve found with your digging efforts. It thrives particularly well in disturbed soils and spreads via both spores and underground rhizomes (which can reach up to 6 ft in depth!). It can also store large amounts of food/nutrient reserves in the rhizomes, which can take a while to deplete. It’s a very resilient plant.

There’s no simple solution to horsetail removal, but I can suggest a couple of additional tips:

1. Since horsetail can spread via spores, any removal method will be most effective early in the season, before the shoots have time to produce fruiting heads. Disturbing them after they fruit might just make them spread further. One option is to remove the above-ground shoots when they come up in the spring and then continue to mow or clip them down as more green shoots come up. However, depending on how extensive the rhizome system is, you will likely need to do this for at least two years and possibly more before the horsetail stops coming back. You’re essentially starving out the root system.

2. Horsetail does particularly well in poorly drained, acidic soil low in organic matter. One approach to getting rid of horsetail is to mix compost into the soil where you’re getting the horsetail to increase organic matter and drainage, and add lime to increase the pH. These conditions should make the horsetail struggle, especially if there are other plants around that can outcompete it in more well-drained, organically rich, neutral soils.

A couple of resources you can take a look at for info on horsetail and horsetail removal include this Michigan State University Extension factsheet - https://www.canr.msu.edu/news/scouting_for_weeds_field_horsetail - and this article from the Brooklyn Botanic Garden - https://www.bbg.org/news/weed_of_the_month_field_horsetail.

Best of luck,