- Name also: Morison’s Spurry (USA)
- Family: Pink Family – Caryophyllaceae
- Growing form: Annual herb.
- Height: 8–25 cm (3.2–10 in.). Often with many stems. Stem ascending–erect, glabrous–sparsely straight-haired, without glandular hairs. Internode under inflorescence 2–3 times as long as lower ones.
- Flower: Corolla regular (actinomorphic), white, under 1 cm broad; petals five, 2.5–4 mm (0.1–0.16 in.) long, same length as sepals or slightly shorter. Sepals 5, membranous margins, often blue–violet. Stamens usually 5. Gynoecium syncarpous, with 5 styles. Inflorescence a cyme.
- Leaves: Opposite, but apparently whorled, stalkless, stipulate. Blade linear–thread-like, with entire margins, not grooved, glossy, shiny, herbaceous–bluish green. Lowest leaves clearly shorter than internodes. Stipules membranous, thin, dropping early.
- Fruit: Egg-shaped, yellowish, 5-valved, 4.5–6.5 mm (0.18–0.26 in.) long capsule. Seed dark, with membranous margins, membranous margin broad and grooved.
- Habitat: Lichen rocks, sandy moors, sandy areas, gravel pits, roadsides.
- Flowering time: May–June.
Granite rocks are not usually a happy hunting ground for plants: its important minerals are tightly bound with silicate minerals where the rootstock cannot get access to them. The monotony of plants that can grow here is enriched by a few species that favour infertile habitats, such as pearlwort spurrey, which is an annual. The autumn weather affects how many seeds germinate – a certain amount do not germinate as insurance against disaster. Sometimes the plant can stay out of sight for years in known habitats. A long, warm and rainy autumn promotes germination and strong leaf rosettes are able to cope with the challenges that winter brings. Conditions vary greatly from year to year, especially in the extreme conditions on rocky outcrops, and good conditions for germination may be followed by a bad spring, which can exhaust the plants before they flower. On the other hand, pearlwort spurrey needs dry years because they destroy perennial dwarf shrubs and other large plants that like the same rocks and which would otherwise colonise the habitat. The luckiest shoots survive the winter cold and the summer drought, grow their branched inflorescence in spring, and finally amass their seed store. Pearlwort spurrey flowers as long as possible. In a hot, dry spring its flowering time is cut short and the plants wither already in the summer. A warm and rainy spring, however, ensures an abundant inflorescence throughout the summer and the last pearlwort spurrey plants only wither at the end of the summer.
Pearlwort spurrey grows most abundantly in south-west Finland on dry lichen-dominated rocky outcrops. It is very like corn spurrey (S. arvensis), which grows almost all over Finland in places that have been affected by human activity. Corn spurrey is usually much hairier, however, and often also sticky with glandular hairs. The internode under pearlwort spurrey’s inflorescence is 2–3 times as long as the internode below, while corn spurrey’s internodes are mainly the same size. Additionally, pearlwort spurrey’s leaves are always shorter than its internodes, while corn spurrey’s are around the same length as its internodes.