The suggestion to seal in a bag would be for indoor, not outdoor sowing.

But I don't WS astrantia, I Fall sow it, giving it a warm, cold, cool sequence. I'd purchased seed and not had germination from it more than once, bought one plant and had more self sown seedlings than I felt like weeding out. It was giving itself warm moist, cold moist, cool periods and coming up like lettuce :)