Irish moss (Sagina subulata) is a low-maintenance groundcover that is perfect for use in rock gardens, between pavers, at the edges of a path, and as a low-footfall, limited area lawn substitute. This lush and low-growing evergreen perennial resembles moss, but it is part of the carnation family (Caryophyllaceae) and has much different growing requirements than moss. This is a very hardy but slow-growing plant that can take many months to fill out in the garden.
|Common Name||Irish moss, pearlwort|
|Botanical Name||Sagina subulata|
|Plant Type||Herbaceous, perennial|
|Mature Size||1-2 in. tall, 9-12 in. wide|
|Sun Exposure||Full, partial|
|Soil Type||Sandy, loamy|
|Soil pH||Acidic, neutral, alkaline|
|Bloom Time||Spring, summer|
|Hardiness Zones||4-8 (USDA)|
Irish Moss Care
Irish moss, unlike true 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 groundcover. In the right conditions, though, this plant grows well and adds a rich and dense spread of color to your garden.
Plant Irish moss in spring after your average last frost date. Space plants about 12 inches apart in well-draining soil and water them regularly.
Irish moss grows best in full sun or partial shade. Too much shade results in the plant becoming leggy, losing some of its compact quality, and developing less attractive mounds.
This species prefers good quality soil, but it can adapt to grow in sandy, loamy, and chalky soils. 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. Irish moss doesn't do well if the soil becomes water-logged. The soil must be able to drain well.
It can be tricky to get the watering levels right with Irish moss. If you go too far either way, brown patches will develop. The plant isn't drought-tolerant, and it can't cope with overly soggy conditions. Regular, light watering is best for Irish moss.
Temperature and Humidity
Irish moss is fully hardy in USDA Zones 4 to 8. This groundcover is not at its best when planted in areas of intensely hot and dry summers. Hot weather will result in the dense green shades turning an unattractive burnt brown. If watered consistently, once the weather cools in the fall, the moss usually reverts to its normal color again.
Use a slow-release fertilizer annually in the springtime to help the plant grow to a tidy uniform 1-2 inches in height. For the amount to use, follow the product label instructions. Under ideal conditions, each plant will quickly increase to cover a spread of about 9-12 inches wide. If Irish moss receives too much nitrogen as a result of overfertilization, it can result in an irregular growth pattern. The moss may develop an undulating rather than a compact and low-growing form.
Pruning is not necessary, though you might want to trim the plant back from steps, pavers, and other structures to keep the area looking neat.
Propagating Irish Moss
To propagate Irish moss, divide it in the early spring so it has the entire growing season to get established. Division is also a good way to thin out and rejuvenate an Irish moss patch if some of the mounds have become unsightly over time.
- To divide the plant, lift out the entire clump with a shovel.
- Separate it into smaller sections with the shovel or with pruners if the clump is small. Make sure that each section has decent roots attached.
- Replant the sections in a new location at the same depth as the original plant. You can also use the sections to replace browned spots in an existing Irish moss patch.
- After replanting, trim the wayward stems. Water well and keep the new plants moist until you see new growth.
In the right conditions, Irish moss also self-seeds readily.
How to Grow Irish Moss From Seed
It is possible to sow Irish moss directly in the ground in the spring, however, because the birds like to eat the seeds, it is recommended to start Irish moss in seed flats indoors about one month before your last average frost date.
Lightly press the seeds into the soil but do not cover, as they need light to germinate. Kept the flat moist at all times and at a temperature between 55 and 60 degrees Fahrenheit.
Harden off the seedlings before transplanting them outdoors after the last spring frost.
Potting and Repotting Irish Moss
Although it is mostly grown as a groundcover, Irish moss is also suitable for container growing. Select a pot, preferably terra cotta, that is wide and shallow. Fill it with potting mix and keep the plant well-watered at all times. Fertilize it about once a month with a balanced fertilizer.
Potted Irish moss needs repotting about every three years, or when it outgrows its container.
Irish moss is cold-hardy to USDA zone 4 and can survive the winter without protection but benefits from a thick layer of mulch to insulate it from the cold. The moss will often turn brown during the winter but it will burst back to life with warmer temperatures and spring rains.
Potted Irish moss can remain outdoors during the winter but the containers need some sort of winterization, such as a silo around them, or a protective wrap against cold damage.
Irish moss is relatively pest- and disease-free. It can, however, be attractive to slugs. Moles might wander beneath the moss and lift it as they bore underground. Be sure to pat the moss back down against the soil to prevent it from dying.
What is the difference between Scotch moss and Irish moss?
Sometimes referred to as pearlwort, Irish moss is 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 is solitary, whereas those found on Scotch moss appear in clusters.
Can Irish moss grow indoors?
Though this plant can be grown indoors, it tends to serve as more of a landscaping feature. Some might choose to grow it in wide, flat containers until it is better established, and then transplant it outside.
How long can Irish moss live?
These plants are quite finicky, so don't be surprised if you only get a season out of them. Reseeding Irish moss regularly can help keep the plant looking lush even if you must remove dead growth in nearby areas.
Irish and Scotch Moss, Sagina subulata and Arenaria verna. University of Wisconsin Horticulture Extension.