Irish moss seeds will start outside as easily as they will indoors. The downside is that birds may very well eat them before they get the chance to germinate. Start Sagina subulata seeds in the spring, about eight weeks before the last predicted frost for your area. These plants like lots of sunshine and well-drained soil, and are practically care-free once established.
Cut the lid off of an empty Styrofoam egg carton and set it aside. It will serve well as a tray for your seed starting flat. Flip the carton over and poke a few drainage holes in the bottom of each cell with a toothpick. Combine equal parts Vermiculite and peat moss and fill the cells with the mixture. Set the flat in a shallow container of water until the surface of the starting mix feels moist. Remove it from the water and allow it to drain for two or three hours and set it in the tray.
Sprinkle a few Irish moss seeds onto the starting mix in each cell. Gently firm them into place, but don't cover them with soil. Place the flat in a brightly lit location out of direct sun. A windowsill in a room with a temperature around 60 degrees F is a good choice. Your seeds should germinate in about one to three weeks.
Water the starting mix just enough to keep it evenly moist, but not soggy or wet. Don't allow the seeds to dry out.
Thin the seedlings when they're about ½ inch tall, and continue thinning as they begin to grow and crowd each other until a single healthy seedling remains in each cell. They can be moved outdoors as soon as there's no longer a chance of frost.
Choose a sunny, well-draining location and loosen the top two or three inches of the soil with a gardening fork. Transplant your Irish moss seedlings about six inches apart.
Water just enough to keep the soil evenly moist throughout the growing season.
Feed your Irish moss after it has flowered in its second season. Apply an all-purpose fertilizer according to the packaging instructions.