I have had a lot of experience with decorative water lilies and lotus in my 3 acre pond. The vast majority of hybridized hardy water lilies will not take over your pond, in fact, they may need to be coddled to get them started. I usually start mine in large pots with garden soil and transfer out of them after the first year. Plastic chemical drums sawed in half work great and are very cheap. Suppliers often don't have a clue about which species are invasive as most people are planting in pots in cement lined backyard water gardens. Lilypons and Slocum's are 2 mail order companies I have used and both have some knowledgeable personnel to give advice. I have personally had excellent results with "Pink Sensation" (dark pink, fragrant),"James Brydon" (deep red), Sunrise(yellow). Tropical water lilies will definitely not overwinter in zone 7(same zone as me in Norman, OK). Muskrats also love the roots of tropical water lilies and I had $250 of plants destroyed overnight once before I learned this. Once tropicals are established the muskrats tend to leave them alone but I used to put up chicken wire cages around my tropicals for several weeks. Two years I grew the Amazonian tropical "Victoria" which has the leaves 4-6 feet in diameter. It is thorny beyond belief but has enormous visual drama. I have not grown these for a few years now. If you decide to pursue this next spring, post again and I will answer what questions I can.
