Perry's Perennial Pages

Evaluating Flowering Plant Selection for Pollinator Habitat Enhancement: Open-Pollinated Natives vs. Native Cultivars

Annie White (graduate assistant) and Dr. Leonard Perry, University of Vermont

The objective of this study is to compare open-pollinated native flower species to native cultivars, in terms of their ability to attract and support native pollinators.  Species richness and abundance of native pollinators will be measured to assess flower attractiveness and nectar and pollen availability will be measured to determine the plantís ability to support pollinators.

A native plant is defined by the USDA and NRCS as a plant that is a part of the balance of nature that has developed over hundreds or thousands of years in a particular region or ecosystem. Only plants found in this country before European settlement are considered to be native to the United States.  This research will focus on flowering herbaceous plants native to the New England region.

The project will begin with an informal survey of plant nurseries in Vermont to assess the current interest and commitment to growing and selling native plants; issues with native plant availability; prominence of true native (open-pollinated, local genotype) plants versus native cultivars; and which species are most commonly grown and sold.  A minimum of ten native herbaceous perennial species will be selected for the study and will be compared to common Ďimprovedí cultivars of the same species.

Cultivars will be chosen for the study based upon their popularity in the retail garden market, bloom period (early, mid, and late-season bloomers), and method in which the cultivar was established.  When possible, one cultivar will originate from a natural selection and the second cultivar will originate from a controlled cross. Each pollinator garden will therefore contain a total of 30 plant types (ten plant species with one open-pollinated type and two cultivars). Nine individual plants will be planted for each plant type, forming an approximately one by one meter plant block within each garden. Each garden will be approximately 37 square meters (400 square feet).  Plants will be obtained in quart or gallon-sized pots from plant suppliers. Efforts will be made to obtain open-pollinated native plants with genotypes most local to the study sites in northwestern Vermont.

Using the selected species, pollinator gardens will be designed for three study sites. Two of the study sites will be situated on the properties of cooperating farmers, River Berry Farm in Fairfax, Vermont and Full Circle Gardens in Essex, Vermont, and a third study site will be located on the University of Vermont campus.  The study sites were chosen for the diversity in their immediate surroundings (fruit and vegetable crops, plant nursery, and urban campus), diversity in the greater surroundings (agricultural, residential/forested, and urban/suburban), and their ability to support the educational objectives of the study. The diversity in sites for the pollinator gardens will allow for comparisons not only between open-pollinated natives and native cultivars (and the method of cultivar selection), but also between site variables such as competing nectar/pollen availability and landscape surroundings.

To control for environmental variables, each pollinator garden will contain both the true native and the native cultivars.  The design of each garden will vary slightly based upon the site constraints and opportunities to display the plant material to the public. The pollinator gardens will serve to provide valuable data and will also serve to educate visitors to the sites. The farmstand and greenhouse at River Berry Farms, the retail plant nursery at Full Circle Gardens, and the University of Vermont campus are open to the public and have regular visitors, who will be able to tour the perimeter of the pollinator gardens and learn about the value of native plants and pollinators from interpretive signage.  Each plant species and cultivar will be clearly labeled and additional educational signage will be designed and installed.

During the first growing season, ongoing maintenance of the study gardens, including watering and weeding as needed will be performed.  Preliminary field surveys will be conducted in year one to assess species richness and abundance of native pollinators including bees, butterflies, and hummingbirds.  This preliminary data will be used to identify the predominant pollinator species in the research area.  In consultation with UVM entomologists and statisticians, the most rigorous method for grouping and quantifying these pollinators will be employed.  For example, due to the large abundance of native bee species and the difficulty of identifying them in the field to the species level, it may be deemed appropriate to group native bees into four sub-groups based upon morphology, field-recognizability and pollination efficiency. Such groups could include bumble bees, other large bees, green bees, and small bees, similar to methods utilized by Winfree et al. 2007.

During the second growing season, field data will be collected by the project manager under the guidance of the principal investigator.  Each garden will be visited weekly throughout the blooming season (early-June through late September). Each visit will rotate between morning, midday, and evening.  Pollinator visitation rates will be measured for native and native cultivar flowers as pollinator visits per flower per unit time. Pollinator visits to as many flowers as can be viewed simultaneously within each approximately one by one meter plant block will be recorded during 60 second scans. Each plant block will be scanned three times per garden visit.

At the visually-observed peak of each plantís bloom period, nectar volume and sugar concentration will be measured using a microcapillary tube and a hand-held refractometer.  Pollen mass will be measured manually. A minimum of 8 samples will be taken per plant type at each site, totaling 24 nectar/pollen samples per plant type.

Collaborating entomologists, plant physiologists, and statistical consultants from the University of Vermont will review the proposed data collection methods and provide guidance throughout the study to ensure the research methods are rigorous and yield statistically valid results. The data will be statistically analyzed to determine if native cultivars differ from open-pollinated native flower species in their ability to attract and support native pollinators.

(This project funded through the SARE on farm/partnership research program, 2012-2014, and with support from Greenworks, Vermont-- The Vermont Nursery and Landscape Association.)



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