Two years ago I planted a clematis..It's purple..I forgot the name. Anyway I was surprized when it grew again the next year. What should I fertilize it with..and when?
Generally any type of slow release fertilizer that is balanced should be fine and it generally applied in early spring when the buds start swelling. Many people use different things including compost, rose fertilizer, tomato fertilizer, alfalfa pellets, composted cow manure, etc. You can use synthetics if you wish but if you do I would hold off in spring until you are sure your last frost is past since the synthetics will be readily available for use by the plant and can cause it to start growing before it should and the tender growth could get zapped by any last hard frosts you might experience (if you tend to have them in your area).
If you are growing type IIIs that you cut back after flowering, you could always hit them with additional fertilizer after you cut them back to stimulate more growth and a second flush of blooms.
I use tomato fertilizer as I saw that suggested somewhere. Once per year or when I get around to it. Most of my clems seem to do fine without it yet even where I didn't amend the soil and forgot to plant one small one 2 inches deeper, suppose it depends on the size of the plant when you get it, like you're supposed to, don't know why.