For some gardeners, seed-sowing is solely a spring task. But many seeds germinate more quickly when sown fresh. For proof of this, you need only look around your own garden at all the little volunteers that have sprung up at the foot of their parents. And some seeds need a cold spell to activate growth, so if you wait till spring they will not germinate until after the following winter, unless you acclimatise them in the refrigerator.
A good rule of thumb is that anything that is hardy is better sown fresh: so collect the seed from herbaceous perennials before tidying them up. Use a free-draining compost; old potting compost (provided that the previous inhabitant didn't suffer from any soil-borne pests or diseases) with added grit is used by some gardeners. Proprietary seed compost may also need added grit.
Sow the seed thinly in trays or shallow pots and cover with a layer of grit to hinder moss growth and to prevent seed being washed away by rain. Place the containers in a cold-frame or in a sheltered north-facing area.