Native Thistles: a Conservation Practitioner's Guide
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Native Thistles A Conservation Practitioner’s Guide Plant Ecology, Seed Production Methods, and Habitat Restoration Opportunities James Eckberg, Eric Lee-Mäder, Jennifer Hopwood, Sarah Foltz Jordan, and Brianna Borders Native Thistles A Conservation Practitioner’s Guide Plant Ecology, Seed Production Methods, and Habitat Restoration Opportunities James Eckberg Eric Lee-Mäder Jennifer Hopwood Sarah Foltz Jordan Brianna Borders The Xerces Society for Invertebrate Conservation www.xerces.org The Xerces® Society for Invertebrate Conservation is a nonprofit organization that protects wildlife through the conservation of invertebrates and their habitat. Established in 1971, the Society is at the forefront of invertebrate protection, harnessing the knowledge of scientists and the enthusiasm of citizens to implement conservation programs worldwide. The Society uses advocacy, education, and applied research to promote invertebrate conservation. The Xerces Society for Invertebrate Conservation 628 NE Broadway, Suite 200, Portland, OR 97232 Tel (855) 232-6639 Fax (503) 233-6794 www.xerces.org Regional offices from coast to coast. The Xerces Society is an equal opportunity employer and provider. Xerces® is a trademark registered in the U.S. Patent and Trademark Office © 2017 by The Xerces Society for Invertebrate Conservation Acknowledgments We are grateful to Chelsea McGinnis for her assistance in compiling floral visitor records. The following individuals, organizations, and databases greatly enhanced our understanding of insect use of native thistles: Discover Life, Heather Holm and members of Pollinators on Native Plants Facebook Group, Steve Hendrix (University of Iowa), Crystal Boyd (Minnesota Department of Natural Resources), University of Minnesota Insect Collection, Jim Sogaard, Sam Droege (USGS), Illinois Wildflowers, Florida Wildflower Foundation, Ladybird Johnson Wildflower Center, Nancy Adamson (Xerces Society), Karin Jokela (Xerces Society), Theodore Burk (Creighton University), and Jarrod Fowler (Xerces Society). We also thank the following individuals and organizations for their contributions to native thistle seed collection and propagation: Keith Fredrick (Minnesota Native Landscapes, Inc.), USDA–NRCS Plant Materials Program, Bob Allison (Cardno Native Plant Nursery), Steve Thomforde (Great River Greening) and Peter Dziuk (Minnesota Wildflowers). We are grateful to F. Leland Russell (Wichita State University) and Jeff Taylor (Kansas State University) for providing insight and data on native thistle ecology. We thank the following individuals for their thoughtful review of this document: Joel Douglas, Wayne Duckwitz, Nancy Jensen (USDA–Natural Resources Conservation Service), Keith Fredrick (Minnesota Native Landscapes, Inc.), and Dan Shaw (Minnesota Board of Water and Soil Resources). This material is based upon work supported by the U.S. Department of Agriculture- Natural Resources Conservation Service- Conservation Innovation Grant number 69-3A75-12-253. Any opinions, findings, conclusions, or recommendations expressed in this publication are those of the author(s) and do not necessarily reflect the views of the U.S. Department of Agriculture. Additional support provided by Cascadian Farm, Ceres Trust, Cheerios, CS Fund, Disney Conservation Fund, The Dudley Foundation, Endangered Species Chocolate, Gaia Fund, General Mills, Irwin Andrew Porter Foundation, J. Crew, National Co+op Grocers, Nature Valley, Sarah K. de Coizart Article TENTH Perpetual Charitable Trust, Turner Foundation, Inc., The White Pine Fund, Whole Foods Market and its vendors, Whole Systems Foundation, and Xerces Society members. Editing and layout: Sara Morris, The Xerces Society. Citation Eckberg, J., E. Lee-Mäder, J. Hopwood, S. Foltz Jordan, and B. Borders. 2017. Native Thistles: A Conservation Practitioner’s Guide. Plant Ecology, Seed Production Methods, and Habitat Restoration Opportunites. 92 pp. Portland, OR: The Xerces Society for Invertebrate Conservation. Photographs & Artwork Cover: front—Monarch butterfly (Danaus plexippus) nectaring on native tall thistle (Cirsium altissimum); back—brown-belted bumble bee (Bombus griseocollis) foraging on field thistle (Cirsium discolor). (Photographs by Jennifer Hopwood and Sarah Foltz Jordan, The Xerces Society.) We are grateful to the many photographers for allowing us to use their wonderful photographs. The copyright for all photographs is retained by the creators. None of the photographs may be reproduced without permission from the creator. For a complete list of photographers, please see the Additional Acknowledgements section. If you wish to contact a photographer, please contact the Xerces Society at the address above. Contents 1. Introduction 1 2. The Natural History of Thistles 3 What Are Thistles?, page 3 Table 2.1: Genera Often Referred to As "Thistles", page 3 Classification of a Cirsium sp., page 4 Thistles of North America, page 4 Native Thistle Diversity and Distribution, page 5 The Complexity of Native Thistle Diversity, page 5 Habitats, page 6 Life Cycle and Growth Forms of Native Thistles, page 7 Identifying Common Native Thistles of North America, page 7 3. The Conservation Value of Native Thistles 13 Flowering Characteristics and Value to Pollinators, page 13 Importance of Native Thistles to Pollinators, page 13 Importance of Native Thistles to Insect Herbivores, page 16 Table 3.1: Known Native Insect Herbivores of Native Thistle Species, page 17 Importance of Native Thistles to Songbirds and Other Wildlife, page 18 Native Thistles Role in Suppressing Invasive Thistles, page 19 4. Native Thistle Decline & Conservation 21 Rare and At-Risk Species, page 21 Table 4.1: Threatened and Endangered Thistles in North America, page 21 Impact of Non-Native Invasive Plants on Native Thistles, page 22 Impact of Non-Native Invasive Insects on Imperiled Thistles, page 22 Conservation of Widespread Native Thistles, page 24 5. Invasive Thistles 27 Canada Thistle Invasion: History and Impacts, page 28 Managing Canada Thistle Invasion, page 29 Managing Invasion of Biennial Thistles, page 31 6. Native Thistle Propagation & Seed Production 33 Seed Collection: Locating Source Populations, page 33 Germination, page 35 Field Establishment, page 37 Site Selection, page 37 Plant Density, page 37 Planting Methods, page 38 Table 6.3: Planting Method—Drill Seeding, page 38 Table 6.4: Planting Method—Transplanting, page 39 Table 6.5: Planting Method—Hand-sowing, page 39 Calculating Your Seeding Rate and Production Field Size, page 40 Additional Considerations of Planting Methods, page 42 Maintaining Production Fields, page 45 Calculating Target Number of Transplants and Size of Planting Area, page 45 Thistle Pathogens, page 48 Known Diseases of Native Thistles, page 49 Table 6.6: Known Pathogens of Native Thistles*, page 49 Mitigating Losses Due to Insects, Birds, and Diseases, page 53 Managing Invasive Thistles: Preventing Spillover of Herbivory, page 54 Seed Harvesting and Drying, page 55 Seed Cleaning, page 56 Seed Viability, Testing, and Storage, page 60 7. Native Thistle Marketing & Habitat Restoration Opportunities 61 8. Establishing & Maintaining Native Thistles in Conservation Plantings 63 Initial Planting and Establishment, page 63 Maintaining Native Thistles in Conservation Plantings, page 64 9. Conclusion 67 Appendix A: Thistle Species Native to the United States, Canada, and Mexico 68 Appendix B: Floral Visitors of Native North American Thistles 76 Appendix C: Thistle Identification Resources 84 Literature Cited 85 Photography Credits 91 1 Introduction Native thistles are a largely misunderstood and wrongly maligned group of wildflowers. These diverse plants fill a variety of significant niches along more esteemed wildflowers including the coneflowers, prairie clovers, camas, and compass plant. While so many of those native wildflowers have been embraced by restoration practitioners, ultimately finding a place in our gardens and restored natural areas, appreciation for our native thistles never really caught on. This is too bad. With sublime blue-green foliage, interesting stem and leaf architecture, and pink blossoms, our native thistles are every bit as resplendent as countless other native plants. More significantly, these plants play important roles in our ecosystems. In great grasslands and prairies, alpine meadows and silty Midwestern river bottoms, the seeds of our native thistles help sustain enormous flocks of songbirds such as goldfinches and indigo buntings. The nectar of these plants fills the stomachs of countless flower visitors, including the enormous black and gold bumble bee (Bombus auricomus), while the foliage of thistles feeds both people and rare butterflies alike. Edible thistle (Cirsium edule), for example, is a staple food of the Salish people of the Pacific Northwest, while swamp FIGURE 1.1: Great spangled fritillary butteflies (Speyeria cybele) have been thistle (C. muticum) is a caterpillar host plant for the documented to visit several native thistle species, including tall thistle (C. endangered swamp metalmark butterfly (Calephelis altissimum), shown above. muticum). As with so many of our other native prairie and meadow species, thistles have been a direct casualty of habitat loss, first beginning with the conversion of native plant communities to intensive plow- based agriculture, then continuing with urbanization and the development of cities and roads. Most significantly, the later invasion of non-native thistles and the lack of discernment between superficially