Whenever I leave plants in containers over the winter, I bury them in the garden up to the top of the planter, then cover with about a foot of straw. Sometimes the containers will crack, but the plants survive. You'd be taking a chance to leave it out in the open, because of the freeze/thaw cycle during winter. But a thick layer of straw might work. As far as flowering perennials...there's just so many of them, depending on your favorite color and how tall you like them. Here's a website that has flowering perennials: http://www.ext.nodak.edu/county/cass/horticulture/perennials.htm

Thanks for the link, Kim. I figured it would be difficult. My 2 boxes are wooden(formerly ammo boxes) and they are in the open, but next to the apartment building, on a bed of drain rock. I wouldn't be able to pile anything on them, as that would be 'defacing ' the building. I keep trying different annuals and so far petunias and marigolds do best, but I'm getting tired of paying for plants and I don't have much room to start things inside. The morning shade makes it difficult to plant sun lovers and the afternoon sun fries shade lovers. I tried WSing some annuals this year and it didn't go well(dwarf cosmos which are still quite tall and haven't bloomed yet). I thought if I could find the perfect perennial for this situation, I could still stick in an annual here and there if I felt like it, but have something I could otherwise depend on. Thanks again for the feedback.

try Sempervivums and some dwarf sedums...as long as the boxes are not held high, but can be covered by the snow in winter they should do fine. But use a VERY well drained soil if you do.

Dave
Milwaukee

I do like Sedums...and yes, the soil in the boxes drains very well. Thank you for the suggestion.