Irish moss (Sagina subulata) is a spreading ground cover that forms a neat, dense mound. Not a moss at all but a perennial, Irish moss is perfect for rock gardens and between flagstones, according to the University of Illinois. Sometimes also called Scotch moss, the plant has finely textured foliage and grows to only an inch tall. In the spring, Irish moss blooms with tiny white flowers.
Irish moss is native to Ireland and Scotland. It thrives in cool, moist, temperate climates. Sagina subulata is cold-hardy to USDA Zone 5, according to the University of Illinois. The plant will not grow well in hot, dry areas such as tropical and subtropical climates.
Irish moss does best in partial shade or even full shade if you live in a climate that has hot summer weather. Exposure to a full day's worth of sunlight will cause this plant to become scorched and turn brown.
Soil and Water
Irish moss loves damp, cool soil. Water just enough to keep the soil barely moist, but not soggy. While this plant is very attractive growing between stepping stones, Irish moss may develop root rot if water tends to collect in depressions between the stones. For that reason, this plant grows best in well-draining soil. Irish moss thrives in soil that is rich in organic matter, so work a handful or two of compost into the soil before planting.
Fertilize in the spring with a slow-release, water-soluble fertilizer for flowering perennials. Apply according to the instructions on the label for the size and age of your Irish moss.
Growth and Problems
Irish moss can be somewhat difficult to grow, according to Washington State University. Small plants tend to get lost in the soil, stepped on, or drown in standing water. In addition, the growth rate of this plant is slow. You will have more success buying the largest plant you can afford rather than growing Irish Moss from seed.