A slender, low-growing annual or biennial, an often overlooked plant due to its diminutive size, which grows in open ground and gardens scattered over all of Scotland, but less frequent in the North and Isles. It has sprawling untidy, multi-branched stems and tends to colonise bare ground, though it likes a relatively nutritious soil, so tends to grow where humans have cultivated.
Stems are fragile, turning upwards at the ends, and the stems and leaves are rough and hairy all over.
The leaves are small and sessile, as the name implies, look like thyme leaves, being oval and pointed, and growing to a mere 8mm at most. The edges are unbroken. Leaves grow in opposite pairs along the stems and are greyish-green in colour.
The flowers are equally small, growing to between 5 and 8mm across. Invariably white, with 5 equal oval petals that grow above 5 spear-shaped pointed green sepals, the sepals growing slightly longer than the petals. Inside the flower are 10 or fewer yellow anthered short stamens clustered around 3 stigmas. Flowers appear on a branched cluster of upward pointing stems.
The fruit is a pear-shaped capsule which nestles within the sepals after petals have fallen.