Ask Mr. Smarty Plants

First, we need to explain to you that the Lady Bird Johnson Wildflower Center is dedicated to the planting, protection and propagation of plants native not only to North America but to the area in which the plants are being grown. Ordinarily, to answer this question we would go to our Native Plant Database, and search on Maine, annual, and sun to part shade. When we did this, we really found very little appropriate to your purposes. We recommend native plants, because many non-natives can become invasive, taking over natural habitats, and because they are adapted to the climate, rainfall and soil of the area where they are being grown. None of these seems likely to apply in the artificial habitat you are creating. In order to raise the natives we were able to find, and because of your short growing season and the extreme conditions, you would have to start seeds indoors, and then put them in containers when it was warm enough for them to survive. So, we would recommend that you go to a local nursery and select plants already in bloom or about to bloom, plant them in good potting soil, and you should be able to enjoy the flowers until the first freeze. Few, if any, of those plants are likely to be native to Maine, so giving you a list of plants from our database is kind of pointless. As high as your garden will be, with the ocean winds, and unprotected as the plants will be in pots, you may have to experiment to even find suitable plants. Sorry we couldn't help you.