I don't recall it having the white outer margin, but it was the first thing that came to mind when I saw the leaf shape. It was growing in a cultivated location alongside a clump of Iris, planted in partial shade. It was also not spreading aggressively as I understood was the case with Galium oderatum (though this clump was contained by a footpath and a large expanse of rock behind it). Hoping for some guidance from the experts! Will post a follow-up with a closer look at the leaf. Can't say for certain about the flowering period, saw no signs of seeds, buds or flowers present. Thanks!