By Laura Gardner, Master Gardener in Training
Last year I posted about Doug Tallamy’s most recent book and talked about how Quercus (Oaks) are the number one “keystone plant species.” A keystone plant is one that supports the entire life cycle of many different wildlife species—all critical to the food web. The list of keystone plants is actually quite short as only 14% of native plants support 90% of butterfly and moth species.[i] Some of these, like Danaus plexipplus (Monarch Butterfly), are specialists in that they require host plants from the genus Asclepias (Milkweed) to complete their lifecycles. Recently I learned that while most native bees are generalists and they seek out a range of plants for pollen, there are certain specialist native bees that are restricted to either a single plant genus or to a few genera. Horticulturist Jarrod Fowler determined that of native bees in the Northeastern United States, only 15% restricted pollen foraging to 33 plant genera and only 201 native host plants.[ii]
As examples of specialist bees, authors Lorraine Johnson and Lorraine Johnston mention in their book A Flower Patch for the Rusty-Patched Bumblebee: Creating Habitat Gardens for Native Pollinators in the Greater Toronto Area, two whose sole pollen source plants include Oenothera (Evening Primrose) and Monarda (Bee Balm): Lasioglossum oenotherae (Evening Primrose Sweat Bee) and Dufourea monardae (Bee Balm Sweat Bee). The former is considered vulnerable and the latter is imperiled in Ontario. A recent online presentation by biologist Heather Holm indicated that there are also specific plants that are the sole providers of pollen to Bombus (Bumblebees). For example, Monarda (Bee Balm) provide nectar to them but they do not provide pollen. Pollen is a necessary protein source as is also nectar as a carbohydrate source. Other plants are required for their pollen sources. This list can help as a guide to some of these.
When I first started gardening, I planted different Milkweed and it was all for the endangered Monarch Butterfly. I think I was influenced more by aesthetics and an influential marketing campaign than anything else. While I will continue to have these plants in my garden and continue to support Monarchs, I have become more thoughtful in my choices—especially since the percentage of native plants that are supportive is so small. What can we do to improve our native garden palettes? A good approach is to choose a wide range of geographically appropriate native plants from the top keystone genera that have flowers of different shapes, colours, season-wide blooming periods, and provide nectar and pollen. Plants that historically or genetically evolved in our region will be the most supportive of the native wildlife in our region. Consider also adding some individual species that support specific specialists. As with all plants, you still need to consider whether your planting site is suitable [e.g., light level, soil type (loam, clay, sandy), pH (acidic, alkaline), moisture, drainage, etc.].
To help, here are two lists provided by the National Wildlife Federation that can apply to gardeners of the Peterborough area—one for northern gardens in the Northern Forests ecoregion and one for southern gardens in the Eastern Temperate Forests ecoregion. There are also two other related lists for Eastern Temperate Forests and Northern Forests—these also provide examples of ferns, vines, and grasses that are host plants and/or provide nectar and pollen. Heather Holm has also put together a wonderful list of native trees and shrubs for pollinators with their flowering periods. There are a few plants on a couple of the lists that are not found in nature in Peterborough County, however, the majority are.
You may be surprised to see Solidago spp. (Goldenrod) listed as the top flowering keystone plant genus. There are 25 species native to Ontario and some of them are easily managed and do not spread like the ubiquitous S. canadensis (Canada Goldenrod) that you tend to see along roadsides and in fields. Last year I added S. caesia (Blue-Stem Goldenrod) to my garden and this year I am looking forward to adding S. rigida (Stiff Leaf Goldenrod) and S. flexacaulis (Zig-Zag Goldenrod). What will you be planting this year?
For Expanded Learning
Johnson, Lorraine and Ryan Godfrey. Get to Know Goldenrod. Online: https://www.toronto.ca/wp-content/uploads/2021/09/97fc-DS-21-0224-GoldenRodFactsheetDigital.pdf
Holm, Heather. Pollinators of Native Plants: Attract, Observe and Identify Pollinators and Beneficial Insects with Native Plants (Pollination Press, 2014).
Ohio State University’s online learning program: Tending Nature: Native Plants and Every Gardener’s Role in Fostering Biodiversity
Pollinator Partnership Canada. Selecting Plants for Pollinators: a Guide for Gardeners, Farmers, and Land Managers in the Manitoulin-Lake Simcoe Ecoregion. Online: https://pollinatorpartnership.ca/assets/generalFiles/Manitoulin.LakeSimcoe.2017.pdf