At least when you see this plant by the side of the road, the answer to the question: “What’s that doing there?” is clear. This is soapwort, Saponaria officinalis, growing by the roadside here in Pennsylvania and since earliest times it’s been used to make soap.
Found wild in Southern Europe, and growing over much of Britain but perhaps native only in the south west (the botanists seem undecided), this pale form is the one seen in here in PA though in some areas a darker rose pink form is seen, and a pale semi-double form (‘Rosea Plena’) also sometimes establishes itself in the wild.
It was brought to North America by colonists specifically for its use as soap and for use in brewing, and long ago escaped from homesteads. It can be made into a very gentle soap - it’s said to have been used to clean the Bayeux Tapestry - and there’s little in the North American native flora that will do the job, especially in colder areas. Soapwort is hardy down to zone 3 (-40C/-40F). Boil it in rainwater to create a mild soapy lather. It’s been used to help create a head on beer, and is also used to make halva.
To be honest, the plant is a bit of a thug, even in poor soil it grows vigorously as long as it has plenty of sun, and it takes the salty run-off from roads in its stride. In fact it’s so adaptable that it’s naturalized in every American state except Alaska and Hawaii. Here in Pennsylvania, it often grows along roadsides with double-flowered daylilies – about which more next time.