Q: We are thinking about building a raised bed along one end of our driveway. It will be made from bricks and be three feet high, two feet wide and about 20 feet long and will face south, so will get lots of sun. Do you have any ideas for plants that would look good? I don’t want to do much maintenance so am hoping to plant mainly perennials. I am in zone three. Is there anything I should consider before building it?
A: Make sure you have a way to keep it watered. Because of the heat held and reflected by the bricks and the driveway and the fact that the planter will be raised, you might need to water more than once a day during hot, sunny weather. If you can get a hose to the bed you may want to add a soaker hose or other watering system.
Perennials are tough to overwinter in a raised bed. Hardiness ratings are based on the lowest temperature the plant roots can take. Under normal conditions, a healthy zone three plant may die back to the roots, but should grow again if the temp didn’t get below -40. But that is assuming the plant’s sides and bottom have protection from being planted in the earth and the top is insulated by plenty of snow, so the roots really aren’t exposed to -40. When a plant is in a raised bed, its roots lose that ground warmth. Many sources say you lose a whole plant zone.
If you had a wide bed, this effect would diminish once you get a foot or so from the edge, but with a narrow bed like you are planning the whole bed would be unprotected. Piling snow, straw or some other insulator around the sides and top of your garden would help with winter protection, but that may not be possible if it is in a part of the driveway you plow.
There are some zone two plants, such as creeping phlox, achillia (yarrow) and liatris, but you may want to stick with annuals. You could even plant some vegetables that like warm conditions, such as tomatoes, cucumbers, melons, peppers and even corn.
Adding plenty (10-20%) of compost or other organic material to the planting bed will help retain moisture. Mulching the bed with wood chips or some other organic material (not rocks which will retain and reflect more heat) after planting will help keep the plants cool and reduce water evaporation. It will also keep the weeds down and reduce maintenance.
Written by U of M Extension Master Gardeners in St. Louis County. Send your questions to firstname.lastname@example.org.