Sagina lvs
with fine bristle point: select from spp. below
S. procumbens
S. filicaulis
S. apetala
rooting at nodes
flat mature sepals
upright mature sepals
Sagina apetala . . . . Annual Pearlwort
Fine Annual plant with all shoots flowering; sepals acute/pointed, white-edged, erect in fruit; hairy edged lvs.
Sagina filicaulis . . . . Slender Pearlwort
Upright Annual plant with all shoots flowering; sepals red-tinged; sepals less rounded than S. procumbens (which is a more solid plant), spreading in fruit
Sagina procumbens . . . Procumbent Pearlwort
Perennial with some non-flowering shoots and rooting runners; Common widespread sprawling garden weed; perennial with some non-flowering shoots; petals tiny or 0.
Sagina lvs
without bristle tip (or <0.1mm):
Sagina maritima . . . Sea Pearlwort
Round coasts and spreading along motorways; annual without rooting runners, lvs blunt without significant bristle point
See also for 4 petals:
Moenchia erecta