From Nursery to Nature: Are native cultivars as valuable to pollinators as native species?
by Annie S. White
The availability of native plant cultivars is widespread and growing in the nursery & landscape industry. Native cultivars are typically marketed for their ecological benefits, but until now, scientific studies neither supported nor refuted these claims. So are native cultivars as valuable in pollinator habitat gardens as the true native species? The answer is complicated; some cultivars are and some cultivars aren’t.
Initiatives to address pollinator decline are widespread and growing in the United States. Pollinator-friendly landscaping and land management practices aim to preserve or restore floral-rich habitat in urban backyards, in agricultural landscapes, and everywhere in between. Enhancing floral resources can improve the abundance and productivity of domestic honey bees, wild bees, and other insect pollinators, even in landscapes with little natural habitat.
Several studies suggest that wild bees prefer to forage—but not necessarily exclusively—on the nectar and pollen from native plants. Native plants are also typically well-adapted to local soil, climate, and other environmental conditions, making them more durable in the landscape. For these reasons, native plants are frequently recommended for pollinator habitat restoration and pollinator garden projects.
The growing demand for native plants in ecological landscaping, including pollinator habitat gardens, has led to the selection and breeding of native cultivars. A native cultivar or “nativar” is a cultivated variety of a native plant, that has been selected by humans (in nature or through repeated selections in a breeding program), cross-bred, and/or hybridized by botanists and plants breeders seeking desirable characteristics that can be maintained through propagation.
The flowers of native cultivars may vary from the native species in size, shape, abundance, color, and bloom time—all attributes known to influence pollinator visitation. In addition to floral traits, native cultivars are sometimes selected for disease resistance, and more predictable sizes and shapes than their wild relatives, making them more desirable landscape plants. But native cultivars can also be less hardy and may prefer different soil moisture and fertility than the species, and most serious of all, may not be as attractive and useful to pollinators.
Some of the traits that humans find attractive in native cultivars, such as a double flowers or an unusual color, may make the flower less attractive to pollinators, and furthermore, may decrease the quantity, quality, and accessibility of the nectar and pollen rewards.
The use of strongly selected cultivars is generally discouraged in ecological restoration projects, but native cultivars are widely available and widely used in the landscape industry. In fact, when gardeners visit their local garden centers, it’s often impossible to find true native species. Gardeners can find echinaceas in every color, size, and shape possible, but will struggle to find a non-cultivated variety of Echinacea purpurea or Echinacea pallida.
With the National Pollinator Garden Network aiming to register one million pollinator gardens; the Pollinator Health Task Force aiming to enhance seven million acres of land for pollinators; and bee habitat on private farms remaining a priority of all conservation programs under the U.S. Farm Bill, it’s important that we understand if native cultivars are comparable substitutions for native species, and if they can perform the same ecological functions in pollinator habitat gardens.
Since 2011, I have been studying this topic at multiple field sites in northern Vermont (zones 4a and 4b) as a PhD student in Plant & Soil Science at the University of Vermont. Under the guidance of Dr. Leonard Perry, I established replicated experimental pollinator gardens to monitor pollinator visitation to 12 native species and 14 native cultivars, to evaluate their garden performance, and to study patterns of nectar production in an additional two native species and four native cultivars.
Some native cultivars attract just as many insect pollinators as the native species. This was seen mostly for open-pollinated seed cultivars such as Asclepias tuberosa ‘Hello Yellow’ and Monarda fistulosa ‘Claire Grace.’ (However, it’s noteworthy that ‘Claire Grace was not as cold hardy in zone 4 given that it’s a selection from a southern ecotype.) One native cultivar selection, Veronicastrum virginicum ‘Lavendelturm’ actually attracted significantly more total pollinators than the native species and had a longer bloom time. This illustrates that there’s potential for pollinator-friendly cultivars (with long bloom periods and high nectar production) to be selected for and marketed.
One clear trend was observed across all species; the more manipulated the cultivars became, the less attractive they became to pollinators. Cultivars such as Achillea millefolium ‘Strawberry Seduction’ and Symphyotrichum novae-angliae ‘Alma Poetschke’, which are the result of repeated selections in breeding programs, attracted significantly fewer pollinators in nearly all pollinator groups. The same held true for hybrid varieties from breeding programs such as Baptisia x varicolor ‘Twilite Prairieblues’ and Tradescantia ‘Red Grape,’ Echinacea ‘Sunrise’ Big Sky and Echinacea ‘Pink Double Delight.’
Although our research doesn’t answer why some pollinators strongly preferred the native species, we hypothesize that color differences and decreased nectar and pollen production in hybridized cultivars are the leading factors.
We also studied patterns of nectar production (in other words, the quality, quantity and distribution of the floral reward) in Lobelia cardinalis (hummingbird pollinated), L. cardinalis ‘Fried Green Tomatoes,’ L. cardinalis ‘Black Truffle,’ L. x speciosa ‘Fan Scarlet,’ L. x speciosa ‘Fan Blue,’ and L. siphilicata (bumble bee pollinated). This study showed slightly decreased nectar volumes in ‘Fried Green Tomatoes,’ comparable nectar in ‘Black Truffle,’ and significantly decreased nectar volume and nectar sugars in the L. x speciosa hybrids, which are crosses between L. cardinalis and L. siphilitca. L. x speciosa ‘Fan Scarlet’ may perform nicely in the garden and may lure hummingbirds to its bright red tubular flowers, but these pollinators are being rewarded with less than 20% of the nectar energy that they would find in the native species, Lobelia cardinalis.
Results may vary in other regions and with other native species and native cultivars, but this research highlights that in some cases, pollinators (bees, in particular) exhibit strong floral preferences for native species. If evaluating native cultivars for use in a pollinator habitat garden, try to limit the use of cultivars to open-pollinated seed-grown “selections” or “sports” of the native species. Cultivars that differ significantly in color and morphology from the native species should be used cautiously and cultivars with hybrid origins should be avoided in the context of pollinator habitat restoration.
This research is just the beginning of what needs to be many more scientific evaluations of native species and all their cultivars.
Three of our research projects related to native cultivars are currently in preparation for publication in 2016. Stay tuned!