It is NOT too late to plant! As long as the ground is not frozen it's best to get them in the ground if you have their long term spot identified. The only way I'd pot them is if you'll have to move them within the first year or so. Peonies growth is very focused—cool soil cues root growth so getting them in the ground, even if you only have a few weeks before it freezes, is best for the growth. They'll go dormant during the cold winter then spring will wake them to a bit more root growth before it warms enough to see above ground growth. Best of luck with your new plants, peonies are the greatest!
