Developing a small nursery where you can keep young plants from being destroyed by the usual grazing gangs of muskrats, turtles and ducks will make a diff, where you can start off numbers of young plants, grow them on.
It does not have to be extravagant, made from cheap and cheerful 10' wide 6mil plastic sheet, a safe place to tinker with plants that take your fancy, a space where you can have a good thunk as to what is suitable for your location.
You might determine plantain is way, way too invasive, before you risk letting it loose and regretting it. You may evaluate (quite rightly) Laevigata iris hybrids are total and utter gems, and decide to collect those big time
Many aquatic plants have very fast growth and reproduction, for example Arrow Arum will be a bold clump in a year or two dropping dozens of very viable and reliable seed, when it becomes established
Hardy waterlilies can be rampant strong growers when they get the chance to settle in, multiplying x10 or x20 per year, if Mr.Snapping turtle does not scoff them the first weekend it is planted...
Regards, andy
http://www.flickr.com/photos/21940871@N06/http://i93.photobucket.com/albums/l42/adavisus/
