Sagina procumbens Procumbent Pearlwort
by Cameron Diekonigin
A check on the distribution of this plant and one thing is very clear, it occurs throughout the UK including almost all of the islands yet it is one that is hardly noticed. It is walked past and walked upon without any consideration. It grows on disturbed ground, pavements, roadways, waste soil, in potted plants; basically anywhere there is freedom from competition. In fact, Blamey and her co-authors say that it occurs on ‘all types of bare ground from the Duke of York’s steps in London to Ben Lawers in Scotland’.
Carpeting the compost of a pot of Laura nobilis
A perennial, Sagina procumbens forms a rosette of leaves with shoots that grow out, rooting at the nodes to create a dense matt of bright green. The plant may never grow more than a couple of centimetres high under ideal conditions and if growing from within cracks on a footpath may be only a few millimetres tall. With a superficial glance, it can be mistaken for a moss.
Despite being regularly walked upon, it will produce flowers. Each has four small white petals which are insignificant and may even drop, leaving a ‘green flower’ created from the remaining four sepals.
Close up of the flowers and seed head of Sagina procumbens
The specific name of procumbens reflects its prostrate, creeping growth whereas the generic name of Sagina is derived from the Latin to feed/fatten.
The above description gives no real indication of which family the plant belongs to; the Carophyllaceae. This family is much better known for the showy, decorative and sought after plants such as the carnations, pinks, and sweet Williams of horticulture and the Dianthus, Silene and Lychnis known to botanists.
Some features of the Carophyllaceae are readily visible in Sagina – leaves on the stems are in opposite pairs with each pair at right angles to the pair below, and the point where the leaves join the stem (the node) is swollen. The form of the plant in the close up image highlights some of these features.
There are other Sagina species in the UK but only one may cause confusion. Sagina apetala is also widely distributed with flowers producing insignificant white petals to leave a green flower created from the four sepals. It however is an annual, does not form a central green rosette, and its growth is much more lax.
Reference
Marjorie Blamey, Richard Fitter and Alastair Fitter 2013 Wild Flowers of Britain and Ireland. Domino Guide, Bloomsbury, London.
Botany in Scotland 
This is also found in the cracks of my driveway here in California, one of the few species native to both here and Scotland!
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