September brings the first official day of fall and noticeably shorter days for gardening. Our native plant garden tasks include trimming tall plants along paths, harvesting seed, updating species lists for each garden area, and weeding. Plants die back (a process accelerated by dry conditions in August) and summer blooming species produce seed. Fall migration is underway for some birds and monarch butterflies. Bumble bee gynes (future queens) are mating and digging their overwintering chambers. Seasonal change is in the air.
Our gardens vary in size, location, exposure, soils, and available light. For those with limited space (or unlimited curiosity), this column deals with a frequently asked question: how to grow native plants in container gardens. Because I have very little experience with container gardening with native plants, I’ve drawn on the experience of two gardeners with successful balcony plantings here in Madison. The two examples demonstrate what types of containers and soil work well.
A west-facing second-floor balcony garden has cedar boxes (2 x 5 x 1.5 foot boxes along the railing and 2 x 2x 2 foot boxes along the wall of the building) with plywood liners painted with waterproof paint. Small drainage holes are drilled in the bottom of the boxes. The boxes are filled with 2 to 3 inches of limestone gravel, covered with porous fabric and a soil mix of 50 percent sand, 25 percent gravel, and 25 percent loam. This soil mix is intended for drought tolerant plants, which are also of relatively short stature. These plants are not watered if it rains about an inch per week, but they are hand watered up to twice a week if temperatures are above 85 degrees F, and/or there are drying winds.
The plants, a mix of short prairie grasses and forbs, were planted five years ago, spaced a foot apart. A few species were added in 2019. Several species have survived and bloomed but not increased: for example, prairie phlox (Phlox pilosa), harebells (Campanula rotundifolia), and spiderwort (Tradescantia ohiensis). Several species have increased: mountain mint (Pycnanthemum virginianum), shooting-star (Primula meadia), Canada rye (Elymus canadensis), and prairie dropseed (Sporobolus heterolepis). Several have increased abundantly by seeding: purple prairie clover (Dalea purpurea), little bluestem (Schizachyrium scoparium), smooth blue aster (Symphyotrichum leave), and prairie blazing-star (Liatris pycnostachya). Two species did not survive to the second year: butterfly-weed (Asclepias tuberosa) and prairie dock (Silphium terebinthinaceum). Weeds are not an issue; only a couple of Canada goldenrod (Solidago canadensis) have seeded into these boxes.
A north-facing second floor balcony garden has 20-gallon and 10-gallon fabric pots as well as plastic pots and window boxes, all placed along the railing. (VivoSun fabric pots work well for this gardener. Another option is DIY grow bags). These containers receive several hours of full sun in the middle of the day. Fabric pots are economical and durable but dry out faster than plastic or clay pots. These containers are filled with basic topsoil supplemented with compost and coconut fiber. The pots are not watered if there is a good rain shower once a week. Otherwise, the fabric pots are watered two to three times a week if there is no rain.
In this garden, shorter plants are selected, especially species that are valuable to pollinators. Plants that have done well include lavender hyssop (Agastache foeniculum), sky-blue aster (Symphyotrichum oolentangiense), Michigan lily (Lilium michiganense), harebells, and wild columbine (Aquilegia canadense). The hyssop has seeded well into other pots and may outcompete other species. The aster is also seeding prolifically. Columbine can dominate a pot if it gets a good start: it does not establish well in pots that are already occupied. Monarda and prairie phlox have not been successful in this setting.
Both gardeners recommend an experimental approach to developing your container garden. Taking photos or keeping written records will help you remember what you tried and if it was successful. Midwestern native plant nurseries, a few vendors at the Dane County Farmers’ Market, and the Friends of the Arboretum Native Plant Sale provide excellent plants and information about selecting plants for your site.
Thanks to gardeners Darrel M. and Sherry H. for sharing their years of experience container gardening with native plants. I welcome input from other gardeners as well and will include additional success stories and tips in a future column. Enjoy fall in your garden, no matter its size.
—Susan Carpenter, native plant gardener