I think this very invasive plant came in with a load of pine or cedar mulch I bought when I was putting my garden in the summer/fall before last. My friend, Patty McKnight suggested I ask you. We are in Southampton, zone 5b (I think) with very sandy soil. Plant is about 3″ tall. Picture uses my toe as a size guide. If I can attach a second you’ll see how it has spread. Thanks for the help. Bill Bowden
From your picture I believe you have a form of horse tail. It is a native to Canada and well loved by grizzly bears but can be poisonous to horses.
Though a native it can be very invasive. It spreads with rhizomes that go deep underground as well as by spores it releases in the spring. Another way for it to spread is through pieces of the plant being relocated to a new site. Your suspicion that it came from the mulch is a pretty good guess.
Eradicating Horsetail can be very difficult. There is some mention of herbicides in the literature but the most effective method seems to be persistence and depleting the plant of light. If you dig out as much of the underground roots as you can then cut off any shoots that show up you will set the plant back a few steps. Do not throw the discarded plants into compost as it will encourage the plant to spread in new locations. Continue watching the site and cut off or mow any plants that surface. If you continue depriving the plants of a source of light the plant will weaken and eventually die. Planting other plants near by which increases the shade on the horsetail which will also help.
I have included some links below for more information
Good Luck, I hope this helps.