Also known as Corsican pearlwort or Scotch moss, Irish moss (Sagina subulata) is a very low-growing, dense groundcover named for its close resemblance to true moss. However, it is more closely related to garden favorites like sweet William and baby's breath. Irish moss will spread indefinitely and given enough time can blanket a yard with an effect similar to a distant view of verdant hills.
About Irish Moss
Native to the British isles, Irish moss is a very cold-tolerant species, able to survive winter lows of -30 degrees Fahrenheit. As such, it also performs better in areas where summers are not extremely hot and has a reputation for being a difficult plant to grow in many gardens. The plant creeps along on the ground, forming dense green mats only 2 inches in height. Patient gardeners are rewarded each year with a show of brilliant white flowers in June and July, scattered across the surface of the growing area like gems on velvet. In many climates, Irish moss is evergreen.
This plant's growing conditions differ drastically depending on the geographic location of the garden. Northern gardeners will find Irish moss performs better in full sun, tolerating only light shade at times throughout the day. Southern gardeners may have considerable difficulty getting Irish moss established due to intense summertime heat, and may have more success if the plant is provided with more shade than it might tolerate in cooler latitudes. Regardless of location, Irish moss prefers to grow in consistently moist but gritty soils; as it tolerates a light amount of foot traffic, it is an excellent candidate for placement between paving stones along a path.
Unlike some plants, such as ornamental grasses that require a full growing season to become established enough to survive a cold winter, Irish moss can be planted during the fall. As it is cold-hardy to USDA zone 4, fall plantings will have enough time to become rooted and established by the time winter arrives in most areas. Careful preparation of the planting site will help Irish moss become established more quickly, including removing all weeds, competing roots and amending the soil with peat moss and composted manure. Extreme winter lows in the Great Plains, Rocky Mountains and northern reaches of New England typically rule out growing this plant in those areas.
Pests and Other Problems
Due to its preference for moist growing conditions, Irish moss may be subject to attacks by slugs, which can be trapped with beer bait, commercial slug bait or killed with salt. Crown rot may also be a problem if soils do not drain properly, indicating the local soil may need a higher sand content. In warm climates, Irish moss may brown out or go dormant during the summer months, but will put on a flourish of new growth once cooler fall temperatures arrive.
Another low-growing, mat-forming groundcover that also goes by the common name of Irish moss is moss sandwort (Arenaria verna). Also native to northern Europe, it is very similar in appearance to true Irish moss and may be sold by nurseries as such. While Arenaria prefers the same moist, gritty conditions as S. subulata, it will tolerate a good deal more shade than its cousin. Plants grow to 3 inches in height and may require shearing to enforce a lower foliage height. Its flowers differ from Irish moss in that they appear in tiny clusters rather than singly. In all other regards, Arenaria can be grown interchangeably with S. subulata, including autumn plantings.