Stellaria media; the common chickweed is a weedy species. I photographed this species in an enclosure for horses. For those who are willing to look at plants at more detail: have a look at the hairs on the stems. One will see that they are usually arranged in a single line along the stem.
It is a member of the Pink family (Caryophyllaceae). The Icelandic name of this species is Haugarfi.
Next to the common chickweed there are a few other chickweed (Stellaria). One can recognize them as follows:
Stellaria media - common chickweed: the white petals are more or less the same size as the sepals, the other chickweeds have petals much larger than the sepals.
Stellaria crassifolia - Fleshy stitchwort: the sepals are narrow and sharp pointed. It grows on (non-saline) moist grounds
Stellaria humifusa - low stitchwort: much like previous species but leaves more fleshy and more procumbent growth. Sepals not acute shaped. This species grows on salt marshes
Stellaria graminea - lesser stitchwort: much branched slender species with narrow leaves. Grows on disturbed soils.