Speedwell

According to the Royal Horticultural Society, 

Slender speedwell (Veronica filiformis) was introduced to Britain from Turkey and the Caucasus during the early 19th century. It was much grown as a rock plant until gardeners realised how invasive it could be. It is now widely naturalised in many parts of the country and is a common and troublesome lawn weed.


Fortunately I"m rather fond of it.