I'm not an expert, so hopefully, someone who is will weigh in, but I've been blessed with an embarassment of riches when it comes to ostrich ferns. Do you know what kind yours are? I've transplanted as soon as I see the curled up fronds appear and the ground is workable. Just shovel up the crown and all the roots that come with it. I've heard that a transplanted fern will sometimes disappear for a season - but if it's an ostrich, it's down there lurking.

Much of my fern patch is in full sun and I've done some mixing in with lilies and phlox. The ferns started out as an underplanting for some ancient spruce, but as the spruce were either limbed up or removed, the ferns simply acclimated. I think they're wonderful and I don't begrudge them popping up in odd places - just dig them out.

I transplant ferns all during the year. The only difference is that if you transplant them after the fronds have emerged, some of the fronds may get damaged in the process and they don't look as good. Therefore, the best time would be when they are dormant or just emerging (in case you need to see the new fiddleheads to figure out where they are).

I second Esh's post. It should be pointed out that Ostrich ferns, mentioned by Duluth, above, are one of only a few ferns that spread so aggressively, but if they are Ostrich ferns, then yes, they will spread!