Irish moss (Sagina subulata) is also known by the name of Heath Pearlwort and Corsican Pearlwort. It is not a true moss, but is instead a perennially growing, evergreen, mossy-type ground cover. As a ground cover, Irish moss can be planted in borders or in between stepping stones to form a dense mat of spreading greenery. Irish moss is extremely cold hardy, withstanding temperatures of --1.11 degrees C, according to Wisconsin State University. Irish moss seeds are very small; for best results, plan on planting them in seed-starting flats.
Fill up seed-starting flats with a soilless seed raising mix until they are well filled. Plan on planting Irish moss seeds in March through April for planting outside in July. Pack down the seed-raising mix using the bottom of a glass, or the base of a 3- or 4-inch pot.
Put the seed-starting flat in a watering tray, or other object that can hold approximately 1 inch of water. Pour water slowly into the tray letting the seed raising mix soak up the water until it is visibly dampened. Allow any excess water to drain away before proceeding.
Scatter the Irish moss seeds thinly across the surface of the seed raising mix. If you are experiencing problems handling the tiny seeds, measure out an equivalent amount of Irish moss seed in fine sand into a bowl. Pour the Irish moss seeds into the bowl and mix the seed with the sand.
Spread a layer of cling film over the tray. You can also use a sheet of glass or place the tray inside of a clear plastic bag.
Put the tray in a warm location out of direct sunshine. Provide a temperature range between 18.3 and 21.1 degrees C. Plenty of light is required for the seeds to germination; ideally 8 to 10 hours of light should be provided each day.
Water often as needed to keep the seed-raising mix moist. Pour water 1 to 2 cups at a time into the watering tray.
Remove the sheet of glass, clear cling film or clear plastic bag once the Irish moss seeds begin germination, usually within 5 to 10 days. Move the tray to a cooler location, approximately 15.5 to 18.3 degrees C.
Transplant the Irish moss once the seedlings are established, usually within 2 to 3 months. Use a knife and cut out 2-inch wide by 2-inch wide squares in the seed-raising tray. Plant Irish moss in the shade in hotter southern climates; in cooler northern climates, you can plant them in full sun to partial shade.
Fertilise Irish moss 3 to 4 weeks after transplanting. Use a 10-10-10 or similar liquid fertiliser. Read the instructions the manufacturer provides on the products label so you will know how many tablespoons, or teaspoons, to use per gallon of water.