Notes on the nomenclature, characteristics, status, and biology of Field Thesium, Thésium des Champs (Thesium ramosum Hayne; Thesiaceae/Santalaceae), a potentially serious invasive plant in Alberta

Abstract

Field Thesium (Thesium ramosum Hayne; Thesiaceae/Santalaceae) is an alien species in Canada, previously misidentified as Thesium arvense Horvátovszky or Flaxleaf (Thesium linophyllon L.). It is a hemiparasitic herb characterized by its many 25–50 cm long aerial stems that grow indeterminately from a caudex. Its narrow leaves extend along each aerial stem from their base into the paniculate inflorescence. The flowers are white, 4–5 mm wide, with five corolla lobes; they are perfect and occur singly, subtended by a three-parted bract at the tip of a narrow pedicel, with 60–90 such flowers along each inflorescence. Its roots develop profuse haustoria that attach to host plant roots. Thesium ramosum is compared to the related native genera, Comandra and Geocaulon (placed in Comandraceae or Santalaceae), which share features but differ by having determinate growth and being unbranched. Thesium ramosum is widespread from western Europe to western China, but in North America it is known from only three western states and Alberta, where it has established in Fish Creek Provincial Park and elsewhere in Calgary. Worldwide, many species in the genus Thesium are notable invasives and T. ramosum has the potential to be a high risk invasive in North America. Observations in the park show that it can spread rapidly and parasitize many host species. It does not have federal or provincial control status in Canada, but because it is parasitic and has potential to become widespread, it is regulated in the USA by the United States Department of Agriculture.