Flowering phenology in a common garden among invasive Canada thistle, Cirsium arvense, and native Cirsium species of the Upper Midwest 2025

Abstract

Native thistle (Cirsium) species occupy a critical, but often misunderstood position in landscapes across North America. Of significance, native thistle flowers produce a high sugar nectar and pollen source for native pollinators, including a variety of butterflies, bees and other insects. Additionally, native thistle flowers and vegetation provide a food source for insect defoliators and seed feeders and the seeds provide a food source for numerous species of birds. The University of Minnesota herbarium lists six thistles in the Cirsium genus as native to Minnesota. These include three biennials; tall thistle (Cirsium altissimum), field thistle (Cirsium discolor), and swamp thistle (Cirsium muticum), and three perennials: Flodman’s thistle, (Cirsium flodmanii), wavyleaf thistle (Cirsium undulatum) and Hill’s thistle (Cirsium pumilum var. hillii). Although not endemic to Minnesota, Pitcher’s thistle (Cirsium pitcheri), is native to the dune ecosystem of the Great Lakes and is a threatened species. Canada thistle (Cirsium arvense) the ubiquitous invasive perennial, present in the Upper Midwest, is native to Europe and the Mediterranean and has been introduced world-wide. It is considered as one of the worst weeds of agricultural and natural systems. The objective of this study was to compare the flowering phenology of native thistles from the upper Midwest in relation to Canada thistle in a common garden where all thistles grew under the same environmental conditions. The phenology of flowering times could be of interest in native pollinator research or host range studies of biological weed control agents. The length of time between first bud appearance and first flower (defined as the time when ray florets were expanding from the receptacle) was approximately one month for the perennial Flodman’s and Canada thistle. In contrast, the length of the bud stage for the biennial tall, swamp and field thistles ranged from one-and-a-half to two months. Biennial thistle plants formed buds in late-May to early-June, but did not flower until late-July to early-August. The lengthy bud stage of biennial thistles overlapped with bud production and flowering of Canada thistle.