Hi Nadine, your lovely purple flower is a Clemetis, these are climbing plants and like to have their roots in shade and the flowers/stems in sunshine, so what I normaly do is after planting, lay a large stone over the planting hole and train the branches/stems up a trellis or other support, or give a good thick mulch, when planting, it is a good idea to plant them about 3'4 inch deeper than they were in the pot when you bought it, this saves you loosing the plant to Clemetis stem rot, some are prone to this and no one knows why, but it means should the plant get this, it will send up new growth the next year, it looks like one we call Clemetis X Jackmanii but you guy's may know it as different name, you would prune this about March time but only if you need to contain the size and room it takes up, but deadhead the flowers as they fade unless you want to get seeds from it, I dont let mine seed as I want strong growth, and would take cutting instead, it is a lovely plant and some folks grow theirs through a large shrub or tree for support instead of up a trellis. good luck, hope this helps you. Weenel.
Thank you so much ecrane3 and WeeNel! You've solved the mystery. Funny thing is, I could swear I looked up that very flower on the internet but the photos did not look like these! Must be a different type. Thanks again for your help!