I agree with Rachel. Buy a few pots of erigeron to plant in the gravel and hope it self seeds from there. It's quite hard to get it to intentionally grow from seed in gravel.
I remember Carol Klein saying that erigieron is a bit of a bastard like that. I have had the plants in my front garden with the intention of trying to get them to seed into paving. For years, no a sausage and then - bingo - one seeded. In the trunk of a plane tree. Curmudgeonly and contrary I have found them.
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