I purchased a potted hosta and a coreopsis mid-summer: both in pots. Apparently, they are both supposed to be planted in the spring. Would it be better to plant them now, in the fall, or somehow overwinter them until spring? I have had this problem before, so a more general question would be - what if you cannot plant a plant when you are supposed to??? I live in central PA, so Zone 5/6.
I think the plants you are talking about do better IF planted spring or early autumn before the soil gets too cold and that's assuming that in your zone, these plants would survive wintering in the garden, I would imagine though that they would be fine and come the mid october I spread a mulch over my peranniel plants a) to protect the underground roots from frost and b) to help reduce the weed seedlings emerging in spring while my real plants are trying to send up new shoots that will give us our summer display.
I agree that somtimes labels are quite confusing regards timing but as a general rule the best time for most plants / shrubs / bulbs etc is spring as the soil begins to warm us and there is still enough moisture about for roots and the other time is end of summer as the weather begins to cool a bit, the soil is still warm enough for new roots to get down or form and the heat of the sun is gone therefore you will have to water but not as mush so the new plants get settled into their possition before the real cold weather arrives.
If it's any help to you I like to add a handful of multi purpose food granuals in early spring when I am getting the beds / borders tidied up for the new season and again end of season while clearing away all the dead or dying plant waist for the compost heap, it's the autumn clear up I also like to lay a few inches of compost / leaf mold or whatever is best for your soil to give winter protection I mentioned earlier.
Good luck and just get your plants in now, Host'a need a bit of shade or part shade and the Coreosis likes more sun, the shade can come from other plants structures or trees / shrubs.