Native Species or Cultivars of Native Plants–Does it Matter?
There is a lot of excitement about planting native plants, and the reasons for choosing them are well-researched. However, when home gardeners look for native plants in nurseries, they often find cultivars of natives rather than straight species. Straight species native plants have grown in a particular area or ecoregion for hundreds or even thousands of years, are open-pollinated, and grow true to seed. Many plants marketed as “natives” in garden centers have never grown naturally in the wild. The word cultivar means a cultivated variety; thus, a cultivar is selected and cultivated by humans. Although some cultivars can occur in nature as plant mutations, most cultivars are developed by plant breeders and are called hybrids. The plant label will give the species and genera, followed by another descriptive name in single quotes. This last name indicates that the plant is a cultivar. Examples are Echinacia purpurea ‘Pink Double Delight’, and Cercis canandensis ‘Appalachian Red’. The first is a purple coneflower with double blooms; the second is an eastern redbud tree with bright pink flowers.
Cultivars are developed for attractive characteristics such as striking flower colors; shorter, bushier forms; colored or variegated leaves; winter hardiness; and improved disease resistance. These qualities make them easier to incorporate into a home garden design, particularly when space is limited. There have been many field trials on native cultivars that assess these “improvements” relative to the straight species and to different cultivars. Gardeners can look up the findings on specific plants to see which cultivars have performed best. This article, however, focuses on a different aspect: “Do native cultivars provide the same benefits to our home garden ecosystems as straight species native plants?”
To answer this question, we’ll focus on current research. At this point, there are two main areas of study. The first looks at woody plants, and the effects of cultivar characteristics on leaf-eating insects, caterpillars in particular. The second area focuses on herbaceous plants, and the effects of cultivar characteristics on pollinators.
Readers who would like to refresh their familiarity with terms such as straight species, open pollination, variety, cultivar, and hybrid, can refer to a past article from The Garden Shed on plant nomenclature.
WOODY NATIVE CULTIVARS AND NATIVE LEAF-EATING INSECTS
Many of us are familiar with Douglas W. Tallamy, noted entomologist from the University of Delaware and author of Bringing Nature Home (2007) and Nature’s Best Hope (2019). Although his research has been ongoing for many years, he more recently has been conducting research trials at the Mt. Cuba Center with doctoral student Emily Baisden. The studies focus on whether cultivars of native woody plants are as productive as straight species, productive in the sense of supporting native insects, caterpillars in particular. Gardening for wildlife, such as butterflies, birds, and bees, requires plants that can support juvenile stages of insects, not just the nectar-sipping adults.
The field study included 16 species of woody plants, both trees and shrubs, with cultivars of each species, for a total of 160 plants. The species were planted in the middle of a ring surrounded by the cultivars matched to each species. Tallamy chose cultivars that varied from their straight species counterparts in one of four ways:
- plant habit
- leaf color variation
- increased berry size
His team studied three insect behaviors: how lepidoptera caterpillars react to the changed characteristics found in cultivars; how and whether hatching bagworms recognize plant differences; and the overall insect impact on the plant during a season. Researching insect response takes years to reach conclusions. To appreciate the challenges of setting up a trial study and collecting data, as well as a listing of plants in the study, and a discussion of study results, see this video of Kim Eierman of EcoBeneficial interviewing Tallamy. Or, read a transcript of Eierman interviewing Tallamy.
Do his study results vote yay or nay on using cultivars? The only trait out of the four that consistently deterred insect eating was changing green leaves to red or purple or blue.
As Tallamy explains, “Red leaves remove chlorophyll from the leaf and load it with anthocyanins, which just happen to be feeding deterrents.”
Regarding the other cultivar traits, slight variegation did not affect insect feeding; however, as the percentage of variegation increased, the leaves became less attractive. Changing plant habit, such as making a shrub more compact, had no effect on insect feeding. On the other hand, the larger-berried highbush blueberry cultivars supported more insect species than their straight species counterparts.
As Tallamy has noted, “What we’re looking at is different types of genetic changes, and then we can extrapolate because there’s no way we’re going to look at all tens of thousands of cultivars. Fortunately, there are only a few types of genetic changes that create a cultivar. We are looking for patterns that emerge from these few changes.”
Cultivar research offers opportunities for important work on plant diseases. Preliminary results of Tallamy’s study of American elm (Ulmus americana) indicate that the ‘Princeton’ cultivar, which is resistant to Dutch elm disease, is no less attractive to insects. A very good sign, according to Tallamy who holds great hope for the introduction of “back-crossed” American chestnuts (Castanea dentata) that were almost eradicated in the last century by chestnut blight.
HERBACEOUS NATIVE CULTIVARS AND POLLINATORS
The next area of research to cover is the comparison of native plants vs. cultivars of natives with regards to pollinators. Several studies have conducted comparative field trials, the first undertaken by Dr. Annie White of the University of Vermont (2016) in replicated research gardens at two sites in northern Vermont (see photo of Maidstone Plant Farm at top of article). The field trial included 500 plants, 14 native species with native cultivar pairings. For a description of the study, a listing of species and their pairings, and study results for particular plants, see White’s blog (3/01/16), From Nursery to Nature: Are native cultivars as valuable to pollinators as native species? You may also wish to see a video of Kim Eierman’s interview with White on her study and results.
What her studies have revealed is that the more manipulated the cultivars became, the less attractive they became to pollinators. Therefore, if considering native cultivars for use in a pollinator garden, open pollinated seed-grown “selections” or “sports” (naturally occurring mutations) are the best choices. Cultivars that differ significantly in color and morphology from the native species should be used cautiously. (Morphology is a study of the form of things. In plants, it includes the roots, stems, leaves, flowers, and fruits.) White hypothesized that color differences and decreased nectar and pollen production in hybridized cultivars are the leading factors. However, she also cautioned that cultivars should be evaluated individually. White found that about half of the cultivars in her study were comparable to the native species, and about half were inferior. It’s important to note that although the cultivars in the study were sometimes less attractive to pollinators than the native species, the cultivars were still visited by pollinators. This suggests that cultivars provide valuable floral resources in the landscape. For example, all pollinators combined exhibited a significant preference for Agastache foeniculum over the cultivar Agastache ‘Golden Jubilee’. However, ‘Golden Jubilee’ had a higher mean pollinator visitation rate than all but two other native species in the study. There were also cases when the native cultivar was equally attractive, such as Aesclepias tuberosa and the cultivar Aesclepias tubersosa ‘Hello Yellow’, a naturally occurring mutation in the natural population. One native cultivar selection, Veronicastrum virginicum ‘Lavendelturm’ (Lavender Towers) attracted significantly more total pollinators than the native species and had a longer bloom time.
Tallamy’s research showed that making a woody plant more compact did not change its attractiveness to leaf-feeding insects. But White found that compact form for herbaceous plants does seem to have an influence on pollinator attraction, because compactness often equates with fewer flowers per plant and fewer floral resources.
A comparison of the straight species, Echinacea purpurea and three Echinacea cultivars focused on altered traits of color, compactness, double-flowered, hybridization, and sterility.
Results of the study showed pollinator preference for the straight species, E. purpurea, followed by E. purpurea ‘White Swan’, an open-pollinated seed cultivar of E. purpurea. The interspecific hybrid, Echinacea ‘Sunrise’, a cross between two different species of Echinacea, was less attractive to pollinators. The double-flowered cultivar, E. purpurea ‘Double Pink Surprise’, was the least attractive. The reproductive organs (stamens and carpels) in double-flowered varieties have been modified into additional petals, thus rendering the plant sterile or near sterile, and reducing the quantity and/or accessibility of floral rewards. These results are consistent with the recommendation to use open-pollinated cultivars that are true to seed. Choosing a cultivar that’s as close to the native species as possible—in morphology, bloom time, and color—is going to increase the likelihood that it’s a comparable substitution.
White also studied patterns of nectar production in two native species of Lobelia, L. cardinalis and L. syphilatica, and four native cultivars. One cultivar had nectar equivalent to the species, but the rest of the cultivars had less. One cultivar had only 20% of the nectar available from the species. This means that when pollinators are attracted to the cultivars, they may have to work harder to get energy rewards equivalent to their visits to native species. White is studying not just the quantity of nectar produced by flowers but also the quality. In this video, using Monarda fistulosa, she shows how she measures both the standing nectar, which is the amount of nectar at any time, and the secretion rate, which is how quickly the plant can reproduce nectar. With sugar content ranging from about 15-75%, not all flowers are equally valuable to pollinators. She will study the differences in nectar production between native species and cultivars, and she plans to measure pollen in future research as well.
ADDITIONAL RESEARCH ON POLLINATOR ATTRACTION
At the University of Delaware, Keith Nevison, a graduate student (2016) with Dr. Deborah Delaney, conducted field trials at Mt. Cuba. He compared insect attraction, nectar quality, and floral characteristics between U.S. Eastern Phlox species and associated cultivars. In total, 6 straight species, and 10 cultivars were evaluated for factors having the greatest influence on insect visitation. Results suggest that certain Phlox cultivars, especially those selected from the wild, are more attractive to insects than their straight species counterparts. Cultivars Phlox ‘Jeana’ and ‘Lavelle’ were far more attractive to pollinators than the straight species Phlox paniculata. This is presumed to be due to the ease with which the insects were able to get at the nectar in the narrow-shaped flowers. Both cultivars also had high nectar volume and sucrose content, making them ecologically beneficial to feeding pollinators. For the majority of Phlox cultivars, however, insect attraction and nectar quality did not differ significantly in comparison to their associated straight species. In the case of Phlox paniculata and its cultivars, the narrowness of a flower’s corolla, in particular, has a strong influence on insect attraction. Like White, Nevison cautions that native cultivars need to be considered on a case-by-case basis. In addition, more experimentation in different parts of the country over a longer time frame may yield different results.
Also at the University of Delaware, Deborah Delaney and graduate student, Owen Cass, are looking at how well various plants attract insects by assessing floral traits such as color, nectar and pollen quantities, and the nutritional qualities of nectar and pollen. They are comparing differences among cultivars within two genera: Coreopsis and Monarda. Plans are to use the diverse selection of flowers present at Mt. Cuba Center to develop a pollen library for commercial and hobbyist beekeepers. Study results have not yet been published.
OTHER WILDLIFE CONSIDERATIONS
It is important to note that although the larger fruits produced by some cultivars, such as the highbush blueberry, may be more attractive to insects, larger berry size may be problematic for some birds. In addition, although more compact form on woody plant cultivars did not deter insect feeding, a more compact form might negatively affect bird nesting. This reminds us to consider ways in which cultivar characteristics might impact wildlife in addition to insects.
BIODIVERSITY AND NATIVE CULTIVARS
A major consideration when using human-bred and hybridized native cultivars in the landscape is the loss of genetic variation naturally found in open-pollinated plant populations. Humans reproduce most hybrid varieties through vegetative propagation, either by tissue culture, or by cuttings and divisions, making hybrids genetic clones of each other. This sameness can make our planted landscapes more vulnerable to disease, pests, or other disruptions.
There is also the potential for cultivars to hybridize with surrounding populations of native species. According to the Maryland Cooperative Extension, studies have shown that, in some cases, cross-pollination with cultivated varieties resulted in the loss of the wild species. This loss has ramifications for all the species that interact with the native plant. Or, cross-pollination with strong cultivars can make wild species stronger. Although this could benefit the plant species, increased vigor could also make the wild relatives more effective at competing with other plant species, putting the balance of the ecosystem at risk.
Sterile cultivars of native plants can’t cross-pollinate with their wild relatives, so they pose no risk to wild plant populations. Sterility is a two-edged sword, however, because sterile cultivars may have reduced pollen and nectar production. As White points out:
Breeding for sterility can inhibit flowers from setting seed, hence resulting in longer bloom duration. This could be a benefit to pollinators if the flowers continue producing ample nectar and pollen, but this is often not the case. Degrees of sterility can vary among cultivars, along with quality of nectar and pollen production, making it important that floral resources for pollinators are evaluated on a plant-by-plant basis. To our knowledge, nectar and pollen production have not been studied in Echinacea cultivars, but in other species, male-sterile cultivars have significantly decreased nectar and pollen flow.
Additionally, gardeners need to consider the impact of sterile, non-seed-producing cultivars on seed-eating bird populations.
In 2017, a diverse group including representatives such as ecologists, geneticists, public garden professionals, government organizations, and research/cultivar developmental entities, met to discuss the development of an analytical tool that would evaluate which native species and/or native cultivars might be appropriate to different planting objectives and to different planting sites. Sites were identified as: 1) large, undisturbed sites identified as candidates for restoration, that were in proximity to wild plant populations or, 2) small, highly disturbed sites that were isolated from wild plant populations, such as urban gardens. For the large undisturbed areas, native species were recommended as essential for restoration. For the small sites, the group proposed a lower risk/higher rewards trade-off analysis for including cultivars that met various criteria. For information on their recommendations, see this report.
A CALL TO HOME GARDENERS
Four botanic gardens (Chicago Botanic Garden, Denver Botanic Gardens, North Carolina Botanic Garden, and San Diego Botanic Garden) are asking home gardeners to join a study across the country to investigate whether cultivars of native plants or “nativars” (a popular term, rather than a scientific term, often used in casual reference or in marketing) provide the same pollinator service as native species. Since spring 2018, participating public gardens have been planting popular native species with matched cultivar pairings to compare pollinator use. In addition, home gardeners are being asked to watch such comparisons in their own gardens, and to complete weekly 10 minute observations while flowers are blooming, and record the number and type of pollinators that visit. Data will be uploaded via the Budburst Data Portal. The Nativars Research Project runs through fall 2022. For information on the native plants and native cultivars recommended by geographic area, pollinator identification, guidelines on how to participate, and to register with Budburst, go to this link.
At first, we were so happy to have a “simple” way to help the environment: plant native plants. But then the cultivars started multiplying, and the choices became more difficult. Were we even asking the right questions? Fortunately, there is good research, and more is coming. Studies have shown that cultivars aren’t always good, and they’re not always bad. In fact, goodness and badness can be partly defined by our gardening priorities. Researchers have concluded that cultivars of native plants should be evaluated on a case-by-case basis. The research is providing us with a framework for analysis: does the native cultivar exhibit characteristics similar to the species plant, or has the cultivar been altered too far from its origin? Annie White advises, “Choosing a cultivar that’s as close to the native species as possible—in morphology, bloom time, and color—is going to increase the likelihood that it’s a comparable substitution.” This means, of course, that we must begin by knowing the species’ characteristics, and how they’ve been altered in the cultivar.
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Feature Photo of Plant Trial Gardens, Maidstone Plant Farm, University of Vermont, Dr. Annie White.