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The name pearlwort generally refers to the densely clumping herbs that belong to the genus Sagina from the family Caryophyllaceae, or the pink family. There are at least 20 species of these plants, which are mostly native to the temperate regions of the northern hemisphere. They can also be found in east Asia, Europe, and south of the equator in Africa. These flowering herbs have slender and sharp-tipped leaves, are less than 1 foot (0.3 m) tall, and range from evergreen annuals to perennials.
Depending on the species, the leaves of pearlwort plants can be succulent or non-succulent and are usually green. A cultivar of the S. subulata species called aurea has golden leaves, however. These plants usually have dense, moss-like foliage, though cultivated varieties are denser compared to species found in the wild. Their simple or branched stems bear small purple or white flowers that bloom in the spring and summer. The fruits of these plants look like small capsules and contain light tan to red-brown seeds.
Most species in this genus are adaptable to different climates. For example, the species called arctic pearlwort, or S. saginoides, can grow in the northern Greenland region down to the southern region of California in the United States. There are species, however, that are on the threatened and endangered lists of several regions of the United States, namely S. decumbens, or trailing pearlwort, and S. nodosa, or knotted pearlwort.
Generally, pearlwort plants are cultivated as rock garden or ground cover plants. S. subulata, which is more popularly known as Irish moss or Scotch moss, seems to be the most popular species to cultivate. This perennial pearlwort measures 2 to 4 inches (5 to 10 cm) tall and has a spread of up to 1 foot (0.3 m), making it ideal ground cover or filler in the crevices between stepping stones. This species favors moist and well-drained acidic to neutral soils and can grow whether placed under full sun or partial shade. Capable of spreading very quickly, it can be invasive and therefore should be controlled.
Some of these plants are reported to have medicinal uses, particularly the annual pearlworts abundant in China, Japan, and Korea. The leaves of S. japonica are sometimes used for the treatment of ailments that vary from fevers to skin diseases. A decoction from the leaves of another species, S. maxima, is typically used to treat boils, sores, and snakebites. This plant has also been used to stimulate blood circulation.