Q: I'm thinking of purchasing a large powder-coated galvanized steel raised bed for planting milkweed in my yard. I'm wondering if the extreme cold temps during our winters will adversely affect perennials with the metal border.
A: Raised beds don’t shelter perennials from winter temps like the earth in a garden bed does. Steel will conduct the cold and also warm up in the winter, which can cause deadly thaw-freeze cycles during sunny days. If you have the option to insulate with bales of straw, bags of leaves, or lots of snow piled all around the bed, the milkweed might survive, but otherwise plants at the sides of the bed will likely die over the winter. But, you can grow them as annuals. Collect seeds to replant next spring.
If you keep the seed indoors over the winter, you’ll need to chill it for a few weeks in the refrigerator before you plant it. Common milkweed and swamp milkweed both require cold to trigger the seed to germinate. Seal the seed in a plastic bag with some damp paper towels and chill for at least three weeks before planting. See monarchlab.org/biology-and-research/monarch-rearing/finding-collecting-and-growing-milkweed.
Written by U of M Extension Master Gardeners in St. Louis County. Send your questions to firstname.lastname@example.org.