Whether to pot up or not depends on the size of the rootball and how the plants look. More than likely potting them up into one gallon sized pots and growing them out over the course of the summer will be the best way to handle them. Then in the fall, you will have good sized plants to put in your garden and get settled in before winter arrives.

Just like any plant, clematis will need some sort of fertilizer and water over the course of the growing season to remain healthy and to grow. What type of fertilizer is up to you. Some people swear by rose or tomato fertilizer since it tends to be lower nitrogen and higher in phosphorus to encourage root growth and blooms. I use whatever is handy and readily available when I need to fertilize as long as it isn't high nitrogen and is slow release. As to when to plant, I prefer the fall since our springs turn to summer very fast and plants seem to struggle to get established in the heat. In the fall, the plant has cooler temps since the daytime temps are gradually falling, natural rain is more plentiful, and the cooler soil temps allow the plants to grow more roots. Mulching the area alos keeps the soil temps. moderated and allow the plant's roots to continue growing well into early winter. You know how your weather tends to do in your zone, so plant accordingly.