Help! Mystery plant taking over moss garden.

Liverwort?

Two close relatives of mosses are liverworts and hornworts. I think these are hornworts and the small curved growths are the sporophytes.

A place to start looking is:

It's liverwort all righty or a closely allied equally primitive plant. It can be the bane of greenhouses, & likes both shady & sunny over-watered locations. It can be quite pretty & an acceptable groundcover in polluted highly alkaline sites that won't sustain much else, & it will even break down toxins & absorb heavy metals. Roundup will do wonders for killing the moss & making the area more pleasant for liverwort, which soaks up toxins like a happy sponge.

It does not compete well with taller mosses or with perennial groundcovers, but shorter mosses it can crawl right over the top of them completely barriering the ground like a plastic coat. It is NOT highly adaptable however & only goes to town in conditions that favor it; changing the conditions (in particular, the pH) may be all that is required to keep it from over-competing, though I wouldn't promise that'd work.

Liverworts usually prefers soils that are alkaline & if the area is just slightly acidified, the liverwort should weaken while strengthening the moss, as most decorative mosses prefer acidic soil (there are certainly exceptions but as a generality the really cool mosses live in places with acidic leaflitter or swampmuck or drippings from wet trees, whereas one ideal condition for liverwort is a burned-down forest with gawdawful amounts of potash).

There are alpine houses with very thrilling liverwort collections & it isn't absolutely necessary to hate the stuff. I encouraged some for a while in a fern garden where it thrived for about three years, but the soil was acidic & when I forgot to give it some chalk & clean the leaf litter off of it, it melted away in year three or four despite my intent to let it spread as far as it wanted.

-paghat the ratgirl