Yes, sounds like they could use a good transplant. Do not put compost three or four inches deep on top of the soil however as that would be about two inches too much. Instead remove them from their pots, then trim an inch or so off the root ball on the sides and on the bottom. Try to tease the roots apart if they are going in a spiral. They sell root saws for this job but a very sharp serrated bread knife also works. Now put some fresh potting soil mixed with compost (about one third compost, two thirds potting soil) into the bottom of the whiskey barrel. Set the maple tree in and fill in around the sides with the rest of the potting soil and compost mix. Water well and make sure the drainage holes are not blocked. I like to raise large pots off the ground by sliding plastic bottle caps from water bottles under the edges of the pot. This helps them drain. A one inch layer of compost on top is okay. Fertilize all container gardens in the spring and for trees an shrubs I use a slow release plant food like Osmocote. It comes in different forms and I don't have to worry about over fertilizing and burning the leaves. Japanese maples are sensitive to fertilizer burn. You can perform this transplant operation any time the temp is above freezing but be sure to push the fluids after surgery. Keep growing, Marianne