New Studies Weigh Ecological Value of Native Cultivars

Luckily this tiger swallowtail didn't mind that we planted phlox 'Jeana' instead of the straight species in our garden.

I recently wrote about the key differences between native cultivars and straight species and why we should care. My conclusion was that while it wasn't necessary to completely eschew cultivars of native species (sometimes referred to as nativars), if you wanted your garden to provide crucial ecosystem services, like supporting pollinators, you should be sure to include a significant number of straight species plants, ideally from local ecotypes, or geographic origins, if possible.

It makes intuitive sense that if you want to support wildlife and natural processes (and you should!), you'd want the plants you select to reflect those processes, as opposed to choosing genetically indistinct clones that may exhibit unusual characteristics. But is there a scientific basis for this?

Luckily this is a question that a few intrepid scientists are beginning to address. Doug Tallamy, entomologist and celebrated author of Bringing Nature Home, has been studying caterpillar interactions with native plants and their cultivars alongside graduate student Emily Baisden at Delaware's Mt. Cuba Center. And Annie White recently completed doctoral research at the University of Vermont which tracked pollinator visits to straight species and cultivars in the context of pollinator restoration projects. Hopefully this will be the start of even more robust research, but early results, which should be of great interest to land managers, the ecological restoration community, and even home gardeners, are highlighting the value of straight species while not dismissing cultivars out of hand.

Tallamy's findings are preliminary, but they show that different characteristics of cultivars can have different effects on how they fit into the ecosystem. In terms of cultivars with distinct plant size or habits, caterpillars had no preference between them and the straight species. The same went for specimens selected for disease resistance, which is good news for the folks working to restore elm and chestnut populations. Leaf color, however, was an area where differences started to come through. Cultivars with purple leaves, for instance, were not nearly as attractive as their green straight species counterparts. This could be the result of visual cues, or as Tallamy suggests, the unpalatibility of the specific chemical compounds that lead to those unusual colors. Variegation in leaves (streaks of color) was less conclusive. Some caterpillar species actually preferred the variegated cultivars, though most avoided them (and since they contain less chlorophyll, they're probably less nutritious).

This coneflower cultivar, ‘Double Delight,’ has a double flowered structure. White found that this major morphological difference from the straight species plant made it difficult for pollinators to physically access the nectar. Image: Ltshears, Wikimedia Commons

White's research in Vermont focused not on caterpillars and leaves, but on adult insect visits to flowers during the pollination process. "Because cultivars have been selected primarily based on ornamental and cultural traits," she writes, "it is not clear whether or not they perform the same ecological roles as the species, which evolved naturally in the landscape." To test this performance, she painstakingly counted thousands of visits by bees, moths, butterflies and flies to the flower varieties she was studying. Her findings were for a general preference for the straight species plants among pollinators, but, she writes, "not always, and not exclusively." Among the species and their cultivars that she studied, all commonly found in restoration projects and home landscapes, she observed "seven native species to be visited significantly more frequently by all insect pollinators (combined) than their cultivars, four were visited equally, and one native cultivar was visited more frequently than the native species. Bees (both native and non-native) and moths/butterflies exhibited similar preferences, whereas flies showed no preference between the native species and the native cultivar." The differences could be profound in some cases. During her observation periods, she recorded 1,414 pollinator visits to Achillea millefolium (common yarrow), but only 119 to the popular 'Strawberry Selection' cultivar. She cautions that further research is required to account for the wide array of species and cultivars as well as regional differences, but her data do certainly support a more widespread use of native straight species plants.

And the insects' preferences notwithstanding, both Tallamy and White warn about the lack of genetic diversity that comes from widespread plantings of cultivars. “It is a bad idea to load the landscape with plants that have no genetic variability,” says Tallamy. “I’m not a hardliner on this issue, but gardeners ought to have access to straight species. We have to convince the nursery industry that native plants are about more than just looks.”

In my own experience this can be a problem. While straight species plants are generally available to the restoration community, home gardeners will often have a hard time finding them compared to cultivars. In order to find many of the plants for our own garden, we've had to seek them out through wholesale growers that typically don't do much retail business, along with the one-day native plant sales that many nonprofits offer. And there is certainly a growing desire among home gardeners to have a positive impact on wildlife, especially pollinators, so the overabundance of cultivars in the market may mitigate the potential impact they can have.

Like Tallamy, I see no need to be a hardliner on this issue. If you want an accent that lends a splash of unexpected color, if you're gardening in a small space and need a more compact shrub, or if you've had issues with certain pests or diseases, there's no reason not to make use of native cultivars in your garden that have those desired characteristics. But if knitting your garden into the wider ecosystem is important to you, making heavy use of straight species should be a key part of that strategy.