Common Whitlowgrass (Erophila verna)
Common whitlowgrass is not a grass at all, as you can see it is a flower. It is a tiny flower at that but one that is worth a closer look under magnification.
The flower head of this plant is so small it is very easy to not see it in the first place! It grows where there is very little soil, often on concrete or tarmac in gutters of roads or car parks. Not only does it grow in harsh conditions it thrives in February and March, long before the majority of other flowers have even started to appear above ground. It can be pollinated by small insects but, flowering so early in the year, the species is basically self-pollinating.
The four deeply lobed petals make this a member of the cruciferae (or cress) family.
Find out more about common whitlowgrass here: