If you mean bulbs, can do it any time up until late autumn and they'll do fine in my experience. Unless you plant hundreds and hundreds of bulbs, you need to wait a few years for them to naturalise and bulk up to really make an impact, each one is so small
I put a diary reminder in for this weekend to "sow snakes head in pots for next year".
Anyone know why?
They're bulbs in my bog garden patch. I think I planted them late so they never really did much.