How to Grow Irish Moss

Perfect for rock gardens, between pavers and as a lawn substitute

Irish moss ground cover with thin moss-like leaves and tiny white flowers

The Spruce / Jayme Burrows

Irish Moss (Sagina subulata) is a low-maintenance ground cover 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.

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 is 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
Flower Color White
Hardiness Zones 4 to 8
Native Area Europe

How to Care for Irish Moss

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 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 ground cover square on wooden surface

The Spruce / Jayme Burrows

Irish moss ground cover with thin vine-like leaves and small white flower at tree base

The Spruce / Jayme Burrows

Irish moss ground cover with bright green leaves and small white flowers with slug on top

The Spruce / Jayme Burrows

Light

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.

Soil

This species prefers a 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. One thing that is a must, Irish Moss doesn't do well if the soil becomes water-logged. The soil must be able to drain well.

Water

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 consistantly, once the weather cools in the fall, the moss usually reverts to its normal color again.

Fertilizer

Use a slow-release fertilizer annually in the springtime to help the plant grow to a tidy uniform one to two-inch height. Under ideal conditions, each plant will quickly increase to cover a spread of about nine inches wide. 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 foot traffic isn't keeping them in check, trim the plants to prevent overcrowding. Thin the plants by dividing the clumps and then trim the wayward stems. You could always transplant the plants to other areas or use them to replace sections that may be overly brown.

Growing From Seed

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.

Common Pests

Irish Moss is relatively pest- and disease-free. It can, however, be attractive to slugs.

Article Sources
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  1. Irish and Scotch Moss, Sagina subulata and Arenaria verna. University of Wisconsin Horticulture Extension.