Dwarf Snapdragon |
| The Dwarf Snapdragon (Chaenorrhimun minus) is an annual herb, with ascending, much-branched stems, growing from a short taproot. The overall plant is glandular and pubescent (McGregor et al. 1986). Dwarf snapdragon ranges from two to ten inches tall, but sometimes reaches 16 inches. The leaf arrangement
and shape varies from opposite and oblong at the base of the plant, to alternate and linear toward the top of the plant. The leaf size ranges from 3/8 to 1 ¼ inches long, and leaves are petiolate. Singular flowers, with a pedicel, are found in leaf axils starting from near the base of the plant. The flowers consist of five sepals,
five petals, and four stamens in unequal pairs. The stigma is capitate (head-shaped). The flowers are bilabiate, with an arching two lobed upper lip, and a three lobed lower lip. The outer flower color and spur are purple. (Roche’ 1991). The overall flower shape resembles the closely related toadflax, or snapdragon. The flower color
can range in local populations from lilac and white with a yellow throat to mainly white with a purple stripe along the lower lip (Arnold 1982). The fruit is a capsule, and each capsule produces 40 to 60 dark brown and longitudinally ribbed seeds.
The Dwarf Snapdragon closely resembles species of Linaria (toadflax). In 1870, C. minus and closely related species with the morphological characteristic of the corolla (petals) being open at the throat, were moved and given a new genus name Chaenorrhinum. Loosely translated this name is derived from the Greek Chainen ‘to gape’ and rhis ‘snout’ - to gape at the throat (Arnold 1991). The Dwarf Snapdragon has the ability to grow in the sterile strip along the road shoulders of highways, causing speculation that this species is resistant to herbicides. This non-native plant is relatively new to the region, and it continues to spread. Controlling the small, satellite populations will prevent further spread. "I know of no other plant species that shows such a preference for railroad tracks" (Arnold 1981). Dwarf snapdragon has a narrow habitat requirements (Widrlechner 1982). This species is closely associated with railroads, and railroad right-of-ways, and is found in the dry, gravely soils and in the cinder ballast of railroad lines. It is seldom found far from the railroad tracks - the better soils support competitive vegetation. When established along abandoned railroad lines, dwarf snapdragon lasts for about five years before it is outcompeted by nearby vegetation. European floras mention it in dry, cultivated fields, waste areas, and along railroad lines (Arnold 1991; 1981). Dwarf snapdragon prefers sites that are well drained, with little or no soil moisture during times of stress. Local populations lead to secondary site infestations that include stream beds, rocky river banks, and roadsides. These sites often cross railroad right-of-ways (Widrlechner 1982). Dwarf snapdragon is also associated with "sterile strips" along roadsides. Dwarf snapdragon does not tolerate shade, and it is not competitive with dense vegetation. In the Pacific Northwest, dwarf snapdragon habitat includes winter wheat fields, gardens and a ski hill. (Roche’ 1991). The Dwarf Snapdragon is native to the Mediterranean region and southern Europe. There are 10 species in the Chaenorrhinum genus, and C. minus is the only species that is widespread throughout Europe and North America (Arnold 1982). The Dwarf Snapdragon was first recorded in the British Isles in 1638, 200 years before the railroads. Dwarf snapdragon was probably introduced to the United States as ballast in ships. It was identified in 1874 in New Jersey, and in 1881 in Canada. It continued to spread as far west as Kansas and Nebraska, and is found in Vancouver, B.C. (Arnold 1991). The rate of spread is 10 miles per season, or more (Arnold 1982; Widrlechner 1982). Northwestern locations include Idaho and Oregon, and in Washington the known locations are from Spokane and Whitman Counties (Roche’ 1992). More recently, dwarf snapdragon is reported from Kittitas, Thurston, and King Counties. Dwarf snapdragon was listed as a Class A noxious weed in Washington in 1988, and changed to a Class C in 1993. The designation was changed to a Class B for 1999, based on the distribution of this species. Considered a pioneer species (Arnold 1982), this annual will quickly inhabit well-drained or gravely soils, particularly the cinder ballast found along railroad tracks and "sterile strips" along highways. Plants germinate in the spring after the final frost, and produce flowers 30 - 45 days later. Areas of summer drought produce two flowering peaks - the main flower production before the drought, and another later in the season (Widrlechner 1982). Flower production can continue until mid-October (Arnold 1981). Flowers remain open for 24 hours, and the fruit will mature in two to three weeks, depending on the time of season. The faster maturation times occur in July versus longer maturation times in September (Arnold 1982). Dwarf snapdragons growing in clumps produced less flowers and seed than isolated individuals (Arnold 1981). The Dwarf Snapdragon is a summer annual that reproduces by seed and it is self-compatible. Cross pollination seldom occurs. The only observed pollinator was one species of "sweat" bee, at infrequent visitation rates (Arnold 1982). The mature capsules open at a terminal pore and dehisce seeds. This pore closes up if wet or during high humidity (Arnold 1981). The seeds are wind dispersed and are expelled over a period of time. Seeds are also dispersed by wind conditions created by moving trains. Acting like a tumbleweed, dwarf snapdragon breaks off at the stem and is carried by the trains themselves. (Widrlechner 1982; Arnold 1981). The seeds are short lived, with five to ten percent viability after overwintering. Seeds in the soil for two years had a 90 percent reduction in viability (Arnold 1981). There are no herbicide control methods listed in the 1998 Pacific Northwest Weed Control Handbook. However, railroad right-of-ways have been effectively controlling dwarf snapdragon with herbicide applications in Ohio (Widrlechner 1982). In Washington, herbicide control is used for roadside populations. However, plants escape this control measure, since they are able to produce early season flowers and seeds while only three to six inches tall. (Nielsen correspondence 1990). References: * Arnold, R. M. 1991. Dwarf Snapdragon -The Railroad Weed. Weed Technology. Vol. 5:467-8. *Arnold, R. M. 1982. Floral Biology of Chaenorrhinum minus (Scrophulariaceae) a Self-compatible Annual. The American Midland Naturalist. 108(2). Pp. 317-324. *Arnold, R. M. 1981. Population Dynamics and Seed Dispersal of Chaenorrhinum minus of Railroad Cinder Ballast. Herb Population Dynamics. 106(1). Pp. 80-91. *Gleason, H. A. and A. Cronquist. 1991. Manual of Vascular Plants of Northeastern United States and Adjacent Canada. The New York Botanical Garden. Bronx, NY. P. 477. *McGregor, R. L. (Editor) 1986. Flora of the Great Plains. University Press of Kansas. P. 762. *Muenscher, W.C. 1980. Weeds. Cornell University Press. Ithaca and London. Pp. 391-2. *Nielsen, R. S. 1990. Correspondence to the Washington State Noxious Weed Control Board. *Roche’ C. 1991. Dwarf snapdragon. Pacific Northwest Extension Publication. WSU Cooperative Extension. PNW378. * Roche’, C. 1992. Dwarf snapdragon. Weeds. Department of Natural Resource Sciences, WSU. Column published September 1992. P. 63. *Roche’, C. no date. Weed Alert. Dwarf Snapdragon (Chaenorrhinum minus). Dept. of Natural Resource Sciences Cooperative Extension WSU. *Taylor, T.M.C. 1974. The Figwort Family of British Columbia. Royal British Columbia Museum. Handbook No. 33. Pp. 49-50. *Widrlechner, M. P. 1982. Historical and Phenological observations on the spread of Chaenorrhinum minus across North America. Canadian Journal of Botany. Vol. 61. P. 179-186. Source: Washington State Noxious Weed Control Board |