Evaluating flowering plant selection for pollinator habitat enhancement: Open-pollinated natives vs. native cultivars

Final Report for ONE12-169

Project Type: Partnership
Funds awarded in 2012: $14,850.00
Projected End Date: 12/31/2014
Region: Northeast
State: Vermont
Project Leader:
Dr. Leonard Perry
University of Vermont
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Project Information

Summary:

This research sought to improve flowering plant selection for pollinator habitat enhancement by comparing the straight species of native flowering perennials to native cultivars in terms of their ability to attract and support insect pollinators.

A controlled field study was used to determine if cultivars of native flowering plants (also known as “nativars” and are human-bred) are as attractive to beneficial insect pollinators as straight species or “true natives” (open-pollinated). Two field plots and two educational gardens were designed, installed, monitored, and maintained at the farms of our three farm partners.

A selection of native herbaceous flowers, and cultivars of the same species, were chosen following research about each plant’s form, habits, and availability. Effort was made to choose flowers that bloomed at different times throughout the season and varied in size, color, and flower structure. Fourteen straight species and 16 native cultivars were included in the study. One native cultivar of each species was selected for the study, except for Echinacea purpurea, which was paired with three cultivars.

Preliminary observations were made on pollinator diversity and abundance in and around the study plots in 2012. Between May and October of 2013 and 2014, weather, flower, and pollinator data were collected weekly at the field plots. This data included temperature, cloud cover, wind speed, flower bloom stage, flowers per plant, plant height, time of observation, and pollinator visits. Field methods for nectar sampling were tested during 2014 and utilized for three species.

Throughout the two seasons of data collection, the flowering plants were monitored to determine the mean rate of insect pollinator visits. Of the 13 plant pairs being evaluated, seven of the native cultivars attracted significantly fewer bee pollinators than the straight species. (Two additional Echinacea purpurea cultivars also attracted significantly fewer native pollinators than the species.) There was no significant difference in pollinator visits in five of the pairs. One native cultivar, Veronicastrum virginicum ‘Lavender Towers’ attracted significantly more native bee pollinators than the straight species.

Opportunities for learning about pollinator-friendly plants and landscapes were provided to the farming, landscaping and gardening communities throughout the project. Educational signs were created and installed at the research plots and display gardens and over 20 educational seminars and lectures were given to horticulturalists, agriculturalists, landscapers, students, and home gardeners. We used multi-media to connect with interested parties, including a website, podcast, YouTube videos, and a television appearance. (See the “Publications and Outreach” section for a complete list of educational outreach efforts.)

Introduction:

Ecosystem sustainability and the success of many agricultural crops in the northeast rely upon the mutualistic relationship between plants and pollinators. This research evaluated the difference in pollinator preference between straight species of native flowering plants and native cultivars. Understanding a pollinator’s preference between natives and their cultivars is important for pollinator habitat restoration projects aiming to maximize the floral resources available for insect pollinators.

Thirty-five percent of food crops worldwide require animal-mediated pollination (Klein et al. 2007), making sustainable pollination services integral to global food supply. Like most of America, the northeast relies heavily on the services of a single domesticated species, the European honeybee (Apis mellifera). Beekeepers have struggled in recent years to maintain healthy populations of honeybees. Declining honeybee populations and rising costs for employing their pollination services have led farmers to reevaluate the potential role of native bees for pollinating their crops. The USDA and NRCS are actively promoting the restoration of pollinator habitat to both agricultural and human-dominated landscapes. The Horticultural Research Institute (HRI) and the horticulture industry in general are also supporting and promoting the use of pollinator-friendly plants in the landscape

Restoring pollinator habitat with plants indigenous to the region is generally recommended because native plants are evolutionarily adapted to coexist with native pollinators and are well adapted for the local soil and climate conditions. However, no previous research has focused on the ecological differences between straight species of natives and native cultivars, which are more readily available for purchase at garden centers. This research sought to identify the potential tradeoffs of using native cultivars in place of open-pollinated “true” natives for the restoration of pollinator habitat.

As residential developments continue to sprawl into previously natural and agrarian landscapes, the ornamental plants installed in home gardens are of increasing importance for pollinators and other wildlife. It is critical for the sustainability of pollinators and biodiversity in general that we better understand the interactions between plants and pollinators in our built landscapes as well our agricultural landscapes.  This research sought to identify the potential tradeoffs of using native cultivars in place of open-pollinated “true” natives for the restoration of pollinator habitat.

Project Objectives:

The primary objective of this research was to improve flowering plant selection for pollinator habitat enhancement by comparing “true” native plants (open-pollinated) to native cultivars (human-bred) in terms of their ability to attract and support insect pollinators.

This research project also aimed to disseminate information to horticulturalists, agriculturalists, and home gardeners about the importance of native pollinator habitat and methods for establishing and/or restoring habitat to support beneficial native pollinator populations.

To accomplish the primary objective of this research, a controlled field study was used to determine the abundance of native pollinators visiting the straight species of native flowers versus native cultivars within pollinator habitat gardens in agricultural landscapes of Vermont. Two field plots and two educational gardens were designed and installed in 2012 at three farm partner sites. Between May and October of 2013 and 2014 data were collected weekly at the field plots. This data included temperature, cloud cover, wind speed, flower bloom stage, flowers per plant, plant height, time of observation, and pollinator visits.

The second objective of this research was to educate horticulturalists, agriculturalists, and home gardeners about the importance of native pollinators and how to restore pollinator habitat in their landscapes. This objective was achieved through educational signage, a research website, public lectures, academic lectures, field days, a television appearance, YouTube videos, a podcast, and forthcoming written publications. (See Section 6 for a complete list of educational outreach efforts.)

Cooperators

Click linked name(s) to expand
  • Annie White

Research

Materials and methods:

Plant Selection. The project began with an informal survey of plant nurseries in New England to assess native plant availability and the prominence of straight species versus native cultivars. Most nurseries in New England selling native plants, sell native cultivars, and no straight species.

A selection of native herbaceous flowers and cultivars of the same species were chosen following research about each plant’s form, habits, and availability. Effort was made to choose flowers that bloomed at different times throughout the season and varied in size, color, and flower structure. Thirteen straight species and 15 native cultivars were included in the study. One native cultivar of each species was selected for the study, except for Echinacea pururpea, which was paired with three cultivars because of the high use and availability of Echinacea cultivars.

Plant List:
Achillea millefolium
Achillea millefolium 'Strawberry Seduction'
Agastache foeniculum
Agastache foeniculum 'Golden Jubilee'
Aquilegia canadensis
Aquilegia canadensis ‘Corbett’
Asclepias tuberosa
Asclepias tuberosa ‘Hello Yellow’

Baptisia australis
Baptisia australis ‘Twilite Prairie Blues’
Echinacea purpurea
Echinacea purpurea 'Sunrise' Big Sky
Echinacea purpurea 'Pink Double Delight'
Echinacea purpurea 'White Swan'    
Helenium autumnale
Helenium autumnale ‘Moerheim Beauty’
Lobelia cardinalis
Lobelia cardinalis ‘Fried Green Tomatoes’
Monarda fistulosa
Monarda fistulosa 'Claire Grace'
Penstemon digitalis
Penstemon digitalis ‘Husker Red’
Symphyotrichum novae-angliae
Symphyotrichum novae-angliae 'Alma Poetschke'
Tradescantia ohiensis
Tradescantia ohiensis 'Red Grape'
Veronicastrum virginicum
Veronicastrum virginicum 'Lavender Towers'

Plants were purchased as landscape plugs from North Creek Nursery in spring 2012 and transplanted into an organic potting soil from Vermont Compost in quart-size pots. The potted plants were irrigated and allowed to mature outdoors through the summer before being planted in the research plots in the early fall of 2012.

Study Sites: Research plots were established at River Berry Farm in Fairfax, Vermont, and Maidstone Plant Farm in Maidstone, Vermont. Educational pollinator gardens were established at Full Circle Gardens in Essex, Vermont, and at Maidstone Plant Farm. The educational gardens were established in 2012 and maintained through the 2013 season. The research plots were established in 2012 and maintained through the 2014 season.

The study sites were chosen for the diversity in their immediate surroundings (fruit and vegetable crops, plant nursery), diversity in the greater surroundings (agricultural, and residential/forested), and their ability to support the educational objectives of the study.

Experimental Design: The two research gardens are 3,000 sq. ft. each with 15 native herbaceous flowering perennial species represented. Within each species, one straight species and one (or more) native cultivars of the same species are represented. Six plants of each plant type were grouped in a randomized complete block experimental design with three replicates at each site, totaling 1,008 plants in the study.

Data Collection: The study sites at River Berry Farm and Maidstone Plant Farm were visited a minimum of four times per month between late May and early October of 2013 and 2014.

Pollinator visit data was collected in 2013 during favorable weather conditions for maximum pollinator activity: >60°F, <50% cloud cover,

Pollinator visits to individual flowers were observed and recorded during 5-minute visual scans of each 1x1.5 meter (6 plants) block from a distance of 1 meter. If a pollinator was present or landed on a flower in the block during the 5-minute scan, it was counted as one visit. If a pollinator moved from flower to flower within the block, it was still counted as one visit. In some cases, a pollinator may have been counted twice (or more) if it left the block and returned a second time within the 5-minute scan period.

To minimize error in the data collection, a single graduate student collected the data in 2013. In 2014, two interns were trained alongside the graduate student to collect data.

Measures were taken to minimize the impact of the observers on the pollinator activity. Upon approaching a block for observation, the observer would sit or stand quietly for one minute prior to starting the five-minute scan. The observer avoided wearing perfumed cosmetics, bug repellent, shiny jewelry or glasses, and colorful or bright clothing. Bright white paper is very attractive to some pollinators, so data sheets were printed on dark green paper.

All pollinators were classified into 14 visually identifiable groups: honey bee (Apis mellifera), bumblebee (Bombus sp.), other large bees, other small bees, green sweat bee (Halictidae), blue orchard bees (Osmia sp.), butterfly/moth (Lepidoptera), wasps (other Hymenoptera), beetles (Coleoptera), bugs (Hemiperta) and flies (Diptera).

Additional data were collected during each observation outing on air temperature, cloud cover, wind speed, flower bloom stage, number of flowers per plant, and plant height.

Nectar Analysis: At the visually-observed peak of each plant’s bloom period in 2014, microcapillary tubes (0.5 μl, 1μl and 5 μl) were used to extract nectar from flowers. This method (Comba et. al. 1998 and Morrant et. al. 2009) proved very difficult given the small flower sizes and morphology of the flowers and was only somewhat successful for Penstemon digitalis, Monarda fistulosa and Lobelia cardinalis. On three separate days in July and August, a 24-hr. study was conducted to quantify the standing crop and secretion rate of nectar and to measure the sugar content of the nectar in these species. Twelve hours prior to the first sample, mesh bags were tied over 10 flowering stalks of the straight species and 10 flowering stalks of the cultivar to prevent nectar loss to insects. At two-hour intervals from sunrise to sunset, nectar was extracted using the appropriately sized micropipette. Flowers were tagged and re-bagged for resampling to measure secretion rate. If >1 μl could be collected, the sugar content was measured using a handheld refractometer.

Data Analysis: The data was compared using analysis of variance and mean separation to determine if pollinator preference differed between straight species and native cultivars. Without significant variations between sites, the sites were analyzed together. To answer the primary research question of whether beneficial insect pollinators were equally attracted to straight species of native flowering plants and cultivars of the same species, the mean visitation rates of all bee pollinators were compared for each straight species/native cultivar pair. Additional and more rigorous analyses are underway to investigate the preferences of each pollinator group and to incorporate the data on time of day, weather conditions (temperature, cloud cover, and wind), and differences in floral characteristics (i.e. flower color, flower size, number of flowers per plant, pollen availability, plant height, etc.). These additional analyses will be completed in 2015 and discussed in the forthcoming peer-reviewed journal article.

Research results and discussion:

Pollinator visits: Throughout the two seasons of data collection, the flowering plants were monitored to determine the mean rate of insect pollinator visits. Of the 13 plant pairs being evaluated, seven of the native cultivars attracted significantly fewer bee pollinators than the straight species. (Two additional Echinacea purpurea cultivars also attracted significantly fewer native pollinators than the species.) There was no significant difference in pollinator visits in five of the pairs. One native cultivar, Veronicastrum virginicum ‘Lavender Towers’ attracted significantly more native bee pollinators than the straight species.

Native cultivars that attracted significantly fewer bee pollinators than the straight species:
Achillea millefolium 'Strawberry Seduction'
Aquilegia canadensis ‘Corbett’
Baptisia australis ‘Twilite Prairie Blues’
Echinacea purpurea 'Sunrise Big Sky’
Echinacea purpurea 'Pink Double Delight'
Echinacea purpurea 'White Swan'   
Helenium autumnale ‘Moerheim Beauty’
Symphyotrichum novae-angliae 'Alma Poetschke'
Tradescantia ohiensis 'Red Grape'   

Native cultivars that attracted the same number of bee pollinators as straight species:
Agastache 'Golden Jubilee'
Asclepias tuberosa ‘Hello Yellow’
Lobelia cardinalis ‘Fried Green Tomatoes’
Penstemon digitalis ‘Husker Red’
Monarda fistulosa 'Claire Grace'

Native cultivar attracted significantly more bee pollinators than the straight species:
Veronicastrum virginicum 'Lavender Towers'

Additionally, differences in bloom time, bloom duration, plant height, flowers per plant, and flower color were observed between the straight species and native cultivar. This data suggests that many native cultivars exhibit different characteristics than the true native equivalents, attracting a different, and sometimes smaller, set of pollinators.

Many native cultivars are selected for a more compact growth habit that is favorable in ornamental gardens and landscapes. However, these compact varieties typically produce fewer flowers per plant, ultimately providing less nectar and pollen resources, and attracting fewer pollinators per plant. The majority of the native cultivars that attracted significantly fewer bee pollinators per plant than the straight species also had significantly fewer flowers per plant.  

When choosing and purchasing plants for pollinator habitat enhancement, farmers, landscapers, and gardeners should be aware that most native species sold in commercial nurseries are native cultivars, not “true” natives. This research shows that there can be significant differences between straight species and their cultivars, but the differences vary and are species/cultivar specific. This research should be expanded to study more species and more cultivars of each species.

If using a cultivar of a native plant in a garden with the goal of maximizing floral rewards for pollinators, the best option is to choose a cultivar that is closest in form and in color to the straight species.

Nectar Analysis: Our field technique proved to be largely ineffective for the low nectar volumes (< 1 microliter) and low nectar production rates of native wildflowers. The technique is successful for measuring nectar standing crop in Monarda sp. and standing crop and secretion rate in Lobelia cardinalis. There was no significant difference in the standing crop of nectar available in Monarda fistulosa and Monarda fistulosa ‘Claire Grace.’ Lobelia cardinalis and Lobelia cardinalis ‘Fried Green Tomatoes’ had significantly different bloom periods, which didn’t allow for a side-by-side comparison of nectar production, which is largely influenced by weather conditions.

Research conclusions:

Throughout the study period, the target audiences (farmers, horticulturalists, and home gardeners) were engaged and educated about the importance of native pollinators, the differences between open-pollinated native flowers and native cultivars, and how to enhance pollinator habitat in both agricultural landscapes and home gardens. We have impacted these audiences through our educational signage at the pollinator gardens, our website, public speaking engagements, academic lectures, podcasts, television appearances, and written publications.

The results of the quantitative research will encourage further research into the ecological differences between straight species and native cultivars when choosing native flowering plants for pollinator habitat enhancements. This research showed that there can be significant differences between straight species and their cultivars, but the differences vary and are species/cultivar specific. This research should be expanded to study more species and more cultivars of each species. ‘Improved’ cultivars of native plants are sometimes better suited visually for a designed landscape than a straight species, but it is important that we fully understand the ecology of these plant selection decisions.

This research has sparked an ongoing dialogue in the horticulture industry and a considerable amount of interest among growers and gardeners alike. Gardeners who have attended our seminars express an interest in planting more pollinator-friendly plants and growers are eager to meet this demand for plant material. (See the "Farmer Adoption" section for specific actions taken by our farm partners.)

 

Participation Summary

Education & Outreach Activities and Participation Summary

Participation Summary:

Education/outreach description:

  • Maintained educational signage at both the educational pollinator gardens and the research plots at Maidstone Plant Farm, River Berry Farm, and Full Circle Gardens. The signage explained the importance of native pollinators, the purpose of this research project, and offered instructions to home gardeners on how to create pollinator habitat in their own gardens. A similar handout was printed for garden tours and lectures.
  • Created a website (pollinatorgardens.org) to share information about our research with all audiences.
  • An educational tour of the pollinator habitat garden at Full Circle Gardens in Essex, VT was given to members of the Vermont Nursery & Landscape Association at the August 2012 summer twilight meeting. 12 attendees.
  • Gave a one-hour educational lecture on “The Buzz on Pollinator-Friendly Landscapes” at the 2013 Vermont Flower Show. 60 attendees.
  • Presented 12 guest lectures in 2012-2014 on plant selection for pollinator habitat to University of Vermont college classes. The courses included “Fundamentals of Landscape Design,” “Ecological Landscape Design,” “Landscape Design for Pollinators” and “A Bug’s Life” and the Plant & Soil Science Department Seminar. 300 attendees.
  • Presented three seminars on Pollinator-Friendly Landscaping to master gardener groups in Burlington, VT (2014) and Jericho, VT (2014) and the Hardy Plant Club in Vergennes, VT (2015). 80 attendees.
  • Served as the teaching assistant to River Berry Farm Owner and SARE Farm Partner, Jane Sorensen, for the creation of a new continuing education course at the University of Vermont on “Landscape Design for Pollinators.” Students worked on landscape design projects to enhance pollinator habitat for real clients, including 10 homesteads and eight farms in Vermont. During class field trips (2013 and 2014), students visited the research plot at River Berry Farm and learned about the pollinator research underway.
  • Taped an “Across the Fence” television interview about our research on pollinators and perennials. The segment aired on WCAX-TV October 7, 2013. (http://www.youtube.com/watch?v=RUk9APL4EZs.)
  • Information about this pollinator research, including a link to the Across the Fence episode and the research website was shared in GreenTalks, a sustainability e-newsletter from Ball Publishing with a world-wide distribution of nearly 30,000 (2013).
  • Provided a tour of the research plot at River Berry Farm at a Vermont Land Trust event, “What’s the Buzz on Pollinators? Habitat Enhancement on your Land” on June 30, 2014. The event was well attended by local farmers and landowners. 25 attendees.
  • Presented our research efforts to the Northern New England Pollinator Habitat Working Group (2014), a multi-state, multi-disciplinary working group established to share information, assess critical research and outreach needs, and identify emerging issues in conservation, maintenance, and enhancement of pollinator habitat across northern New England. 15 attendees.
  • Recorded a podcast with Garden Speaker and Founder of EcoBeneficial, Kim Eierman, and later hosted Eierman and a film crew at River Berry Farm to create a series of YouTube videos about the research (2014).
  • Interview 1  39:56 minutes
    www.youtube.com/watch?v=HTmuDcEzTOw 
  • Interview 2  11:07 minutes
    www.youtube.com/water?v=xEkGjnbDvHE 
  • Interview 3   3:52 minutes
    www.youtube.com/water?v=lcgHBrIHALc 
  • Interview 4  8:33 minutes
    www.youtube.com/water?v=h0rHknFB_Rk

  • Two journal articles are in preparation and will be submitted to peer-reviewed journals in 2015.

Project Outcomes

Project outcomes:

The choice between utilizing a straight species or a native cultivar when selecting native flowering perennials for pollinator habitat enhancement is more about availability than economics. The propagation of open-pollinated native species is a specialized field, requiring access to local seed sources and demanding a strong understanding of seed pre-treatment requirements, which frequently vary from species to species. These propagation challenges, combined with a desire for more predictable plant habits, have led to the selection and breeding of native cultivars. Native cultivars now dominate the native plant market, particularly in New England. Landowners seeking to restore pollinator habitat to their agricultural or garden landscapes will find native cultivars more readily available than straight species. Although more difficult to obtain, there is typically no cost difference between straight species and native cultivars. North Creek Nursery, a large wholesale native plant nursery in Pennsylvania, offers straight species and native cultivars at similar prices. The cultivars are sometimes more expensive because they include royalty fees for the patented plant stock.

Farmer Adoption

Throughout the span of this research project, national awareness about the importance of preserving insect pollinators in our landscapes increased. This project aimed to complement the movement by educating farmers, growers, landscapers, and home gardeners about creating pollinator-friendly landscapes with pollinator-friendly plants, while scientifically evaluating the differences between straight native species and native cultivars in terms of attracting insect pollinators.

As a result of the public interest and curiosity about pollinator habitat, in 2013, River Berry Farm began identifying and highlighting with signage perennials for sale in their greenhouse nursery that are pollinator friendly in. In 2014, River Berry Farm expanded their plant list to include a large selection of native plants that are beneficial to pollinators.

Full Circle Gardens is promoting pollinator-friendly plants in their retail business, including "Annie's Ten Top Pollinator Plants" featured on our website and discussed in our seminars. They have also increased the number of native plants available for purchase.

This research prompted Maidstone Plant Farm to recognize the pumpkin yield benefits to preserving pollinator habitat around the pumpkin field. They also adopted more bee-friendly pest management strategies for growing their chrysanthemum crop.

As this research becomes more widely disseminated in journals, its impact will continue to spread beyond the research farms and Vermont gardeners. It will help farmers aiming to enhance pollinator habitat on their farms to make better-informed decisions when selecting flowering perennials species and choosing between the straight species and native cultivars.

Assessment of Project Approach and Areas of Further Study:

Areas needing additional study

This was the first scientific research project to evaluate straight species of native flowering perennials to native cultivars and has laid the groundwork for future studies. This study evaluated 14 species and cultivars, and because the results suggest that every species and cultivar is different, many more species and multiple cultivars of each species should be evaluated. It would also be valuable to the horticulture industry to study non-native introduced species and cultivars, as they comprise the majority of the plants in the industry.  

Additionally, the challenges experienced in collecting nectar samples in the field highlight the need for a methods study on field techniques for sampling nectar in flowers with very low nectar volumes. It would also be useful to evaluate methods for laboratory analyses of nectar quantity and quality and pollen nutrition, which may yield more accurate data. This information would help us understand not only how attractive a flower is to a pollinator, but also how beneficial the floral reward is to the pollinator.

Citations

COMBA, L., S.A. CORBET, L.V. HUNT AND B. WARREN. 1999. Flowers, nectar and insect visits: evaluating British plant species for pollinator-friendly gardens. Annals of Botany 83:369-383.

KLEIN, A., VAISSIERE, B.E., CANE, J.H., STEFFAN-DEWENTER, I., CUNNINGHAN, S.A., KREMEN, C., AND TSCHARNTKE, T. 2007. Importance of pollinators in changing landscape for world crops. Proceedings of the Royal Society B: Biological Sciences 274:303-313.

MORRANT, D.S., SCHUMANN, R. AND PETIT S. 2009. Field methods for sampling and storing nectar from flowers with low nectar volumes. Annals of botany 103:533-542.

Any opinions, findings, conclusions, or recommendations expressed in this publication are those of the author(s) and do not necessarily reflect the view of the U.S. Department of Agriculture or SARE.