This lush and low-growing evergreen perennial resembles moss, but it's actually part of the Carnation family (Caryophyllaceae) and has much different growing requirements.
Sometimes referred to as Pearlwort, it's often confused with a similar, but less common species called Scotch Moss (Arenaria verna). While alike, Irish Moss stays a vibrant green throughout the year and Scotch Moss has a more golden shade.
When in bloom, both types have little white flowers. The Irish Moss variety are solitary, whereas those found on Scotch Moss appear in clusters.
|Botanical Name||Sagina subulata|
|Common Name||Irish Moss, Pearlwort|
|Plant Type||Prostrate herbaceous evergreen perennial|
|Mature Size||2 inches in height but spreads widely|
|Sun Exposure||Full Sun/ Partial Shade|
|Soil Type||Variety including sandy, loamy and chalky|
|Soil pH||Not particular|
|Bloom Time||Late spring to early summer|
|Hardiness Zones||4 to 8|
How to Grow Irish Moss
Irish Moss, unlike actual moss species, doesn't do well in full shade and wet conditions.
Conversely, it also doesn't appreciate extreme heat. It likes a sunny or partial shade position, but intense heat isn't good for this ground cover.
In the right conditions, though, this plant grows well and adds a rich and dense spread of color to your garden.
Irish Moss grows best in full sun or partial shade. If it's in an overly shady position, it can result in the plant losing some of its compact quality, and less attractive mounds can develop.
Irish Moss doesn't do well if the soil becomes water-logged. It really needs to be well-drained.
This species prefers a decent quality soil, but it can grow in sandy, loamy and chalky varieties. You may need to add organic matter to help it survive in areas where the summers are hot and dry, or if the soil isn't particularly fertile.
It can be tricky to get the watering levels right with Irish Moss. If you go too far either way, it can result in brown patches developing. It isn't drought-tolerant, and it can't cope with overly soggy conditions. Regular, but not deep watering is best for Irish Moss.
Temperature and Humidity
Irish Moss is fully hardy in USDA Zones 5 to 9. This groundcover is not at its best when planted in areas of intensely hot and dry summers, and this type of weather can result in the dense green shades turning an unattractive burnt brown. Usually, once the weather cools in the fall, the moss reverts to its normal color again.
The hot afternoon sun of the south means a poor placement for Irish Moss..
Using a slow-release fertilizer annually in the springtime can help the plant grow to a tidy uniform one inch height. Under ideal conditions each plant will quickly increase to cover a spread of about nine inches.
However, if Irish Moss receives too much nitrogen as a result of overfertilization, it can result in an irregular growth pattern giving the moss the appearance of undulating rather than having a compact and low-growing form.
Propagating Irish Moss
You can easily lift out an entire clump and divide to replant, or you can cut a strip from an existing section. Just make sure that the sections you replant have decent roots attached.
If you are dividing plants to cover a large area and want to fill up a large space quickly, plant several divisions close together in the same area.
If your Irish Moss is forming less attractive mounds, and stepping them down isn't working, these can be cut to prevent overcrowding. You could always transplant these cuttings to other areas or use them to replace sections that may be overly brown.
Growing From Seeds
Seeds can be grown indoors during the winter season and then transplanted outdoors after the last of the frosts. Alternatively, you can sow them directly into the ground in the spring. Plant them around 12 inches apart to prevent overcrowding.
Make sure you keep the soil moist during the germination period. The seeds also need decent light for germination, so give them only a light covering of soil.
In the right conditions, Irish Moss self-seeds readily. But, if it starts to grow unwanted in particular areas, it is an easy plant to remove.
Irish Moss is relatively pest and disease-free. It can, however, be attractive to slugs.
Irish and Scotch Moss, Sagina subulata and Arenaria verna. University of Wisconsin Horticulture Extension.