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.
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 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.
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 consistently, 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 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
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. 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.
Overwintering
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.
Common Pests
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.
