Charlie Pridham writes
The best way to establish them in a garden is to buy the corms in late
summer, do them up into pots ( lots in each)and place under the
greenhouse bench, protect from mice and keep just moist, in spring when
the shoots appear check for roots and as soon as you see roots plant
the out where you want them (they actually quite like a heavy soil) the
problem is that wild ones have their roots already at the time you are
able to buy the corms so they can cope with wet while the bare corms
can not and they need re hydrating slowly

Thanks Charlie - that explains why I've had so much more success buying
them in the spring potted up and ready to flower. They've established
easily that way and spread into clumps, whereas corms planted into the
garden in late summer often disappear without trace.
--
Kay