I too find seedlings of bonariensis everywhere..in sw Cornwall too. Homestead purple and rigida are easy. I take layered roots and cuttings..do that in August.....keep,in greenhouse and plant out in spring. However, I find it does best in good soil even though I mostly grow it in a dry wall. I have a yellow osteospermum next to it and elymus magellanicus behind and they have flowered together all summer, still going strong. Took cutting of osteospermum in august too.