by Famke Alberts, Sarah Jandricic, Rodger Tschanz and Dr. Al Sullivan
In collaboration with Landscape Ontario, OMAFRA and University of Guelph researchers have conducted a research trial on the attractiveness of different plant species attractive to Ontario pollinators and specifically to determine if there is a role for non-native plants in the pollinator landscape.
Over the course of a three-year study, researchers and technicians have examined 10 exotic plant species typically grown by the greenhouse floriculture industry in Ontario and have directly compared these to 10 species of native plant cultivars or “nativars.”
Earlier studies at the University of Guelph (2016 and 2018) were conducted using individual plant species in test strips, but increased facilities made available at Landscape Ontario in 2020 allowed for the planting of entire beds, with the intent of better replicating a typical homeowner garden.
Preliminary plant selection was based on those varieties that showed promise as a pollinator in earlier studies at the trial gardens located at the Guelph Turfgrass Institute, with factors such as height, seasonality, colour and flower type considered for the final selection. Ornamental species were planted at densities based on the mature size of each plant variety.
Students surveyed the plots for 30 minutes at a time and counted the pollinators that came to visit, making note of the plants that were preferred. Pollinators were counted only if they landed on a flower and stayed to gather nectar or pollen. Site visits were conducted on days with suitably sunny weather conditions. Overall, students logged over 70 hours of observation time.
Data collected in 2020 revealed that garden plots consisting entirely of native plants attracted slightly more pollinators overall, but this difference was not significant. On average, in a garden plot measuring 2m x 4m, a homeowner could expect to observe around 58 pollinators per hour in an all-native plant garden, versus 48 pollinators per hour in an all-exotic garden.
A look at the data reveals the inaccuracy of the commonly accepted premise that native plants are generally the better choice for pollinators. As noted in the table below, both native and exotic plants included species preferred by pollinators. For example, Coreopsis received over 900 pollinator visits in our 2020 trial, but other native plant species such as Phlox and Guara were not very attractive to pollinators (at least those found in Ontario), with fewer than 90 total pollinator visits each.
In both plant categories (native or exotic), plant species with the highest total numbers of pollinator visits tended to attract mostly managed honeybees, native bee species, or both. Although bees tend to be a homeowner’s preferred pollinator, they are not the only contributor to a successful pollinator garden. To play a bigger role in helping all pollinators that have few food sources in urban areas, pollinator gardens should also support as many different groups (or “guilds”) of pollinators as possible. For example, Argyranthemum did not attract many bees in the 2020 trial, however, they did draw higher numbers of both hover flies and pollinating beetles than the other plants tested, making them an important contender as part of a garden that supports both an abundance and diversity of pollinators
Taken together, our data suggests that the best pollinator gardens would likely make use of a mix of species of both native and exotic plants. This list of stand out performers is compiled from the species and cultivars tested in our three years of research. Although far from exhaustive, this list is a good starting reference for anyone wishing to customize their gardens with a mix of pollinator-friendly, exotic and native annuals and perennials.
As a note of caution, the data also shows that plant variety can have an impact on pollinator attractiveness. Changes to the aesthetics of a plant species during breeding can also change plant attributes that are attractive to pollinators, such as volatiles, amount of pollen or nectar, or the colour the insect sees. This information is not new, as previous publications have shown that pollinator attractiveness can vary between varieties by as much as 10 times. This may partially explain why some of our plant choices that should have been successful didn’t perform well (e.g. nativars such as Guara lindheimeri “Variegata Rose”, Phlox paniculata “Purple Improved”, Penstemon digitalis “Dakota Burgundy” and Heuchera villosa “Pinot Noir”).
To overcome these issues, landscapers and garden centres can make specific cultivar recommendations to their clients, such as the ones included here. But another solution is just to recommend that homeowners should plant as many pollinator friendly plants as possible — native and exotic — to hedge their bets.