Answer: Scotch and Irish moss are common names for Sagina subulata. This broadleaf perennial is quite susceptible to the chemicals in Preen and Green Pre-Emergent herbicides because the products target both grassy and broadleaf weeds. There is nothing registered for use in a bed of Sagina. One option to help control weeds is to cover the bare earth with organic mulch. A mulch will shade the soil and stop weed seed germination, which will give your mosses an opportunity to grow thick enough to crowd out weeds on their own. Aside from completely digging an area, setting aside the Sagina and discarding the weeds, then replanting the moss and covering the bare earth with mulch, I can't think of an alternative. Once your plants grow thick and lush and completely cover the area, they will help crowd out all but the most tenacious weeds.
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