Julia they do not have to reseed I deadhead mine a lot so they bloom much longer they are very hardy perennial plants. You will have a much easier time getting them established if you plant a couple of plants it takes them time but after a few years you will have them for years. You do have to be patient getting the plants established mine did not take right off like some plants do but it is worth the effort they are beautiful. I will try to remember to take some pictures this summer when they bloom. I think I have them in about three areas they don't transplant well at least for me they don't so it is best to plant them where you want them to remain for the long term. At one time I did try the pink ones which did not make it for the long term they were very pale in color to me the blue/purple are much nicer.
I have found in my many years of gardening that in my cold climate planting plants of whatever perennial gives me much better results you can always divide a lot of them later if you want more plants.
