Answer: If you are willing to experiment a bit you might try perennials rated winter hardy into zone 3: these are proven performers through difficult winter conditions and would have the best chance of surviving in your containers. Some of the herbs also winter over fairly well. Among them you might try thyme, sage and fennel. Rather than relying on self sowing, you might have more consistent success by direct sowing quick growing annuals from seed such as nasturtium, sweet alyssum, marigold, zinnia and possibly shorter varieties of sunflower and cosmos as well. For the price of a cell pack you will have an entire packet of seeds. Finally, you might try some of the bulbs that can be saved from year to year such as caladium, canna, and dahlia.
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