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An Assessment of Exotic Plant Species of Rocky Mountain National Park

Linaria vulgaris P. Mill
Butter and eggs, yellow toadflax, wild snapdragon (Scrophulariaceae)


Current level of impact
Known locations in RMNP: Upper Hollowell Park, common around Beaver Point, utility area. Found especially around old homesites.
Assessment: Several widespread and dense populations in park. When added together, populations would cover an estimated area of 11-50 hectares. Found in high quality areas with no known disturbance for last 100 years. Potential to invade and modify/replace existing native communities.

Distribution
Origin: Eurasia, escaped ornamental.
Geographic distribution: Throughout most of temperate North America. In Colorado found from 6000' to 8500', mostly on western slope but also found on eastern slope.
Ecological distribution: Waste places, pastures, and roadsides. Cultivated fields, meadows, gardens. Locally abundant and often found around sites of former homesteads. Occurs in a wide variety of habitats, but is generally limited in wet or dark conditions.
Soils: Occurs on mostly sandy or gravely soils. Common on chalky soils.

Reproduction
Perennial herb, reproduces by seeds and creeping rhizomes. Extensive, creeping rhizomes. Production from shoots can be large, and may begin at seedling stage. Plant is self-incompatible and insect pollinated. Flowers June-August.
Vegetative: Vegetative reproduction is important because of low seed viability. Vegetative reproduction may begin as soon as 2-3 weeks after germination, and is possible from root fragments as short as 1 cm.
Seed production: Range from 1,500-30,000 seeds/individual, but it is often difficult to define an individual because of colony forming habits.
Seed dispersal: Seeds are winged which may allow for long distance dispersal. Seeds may also be dispersed by water and ants.
Seed longevity: Seeds may remain dormant for periods up to 8 years.
Germination: Germination success is often below 10%, although it is highly variable.

Competition
In North America, Linaria vulgaris is generally considered a strong competitor, but is considered a weak to moderate competitor in Europe. Linaria vulgaris is able to quickly colonize open habitats, and is capable of adapting growth form to a wide range of conditions. Generally does well in moist areas with high fertility but will be displaced by other species when growing in less favorable conditions. Plants growing in dry soils tend to be stunted, but are relatively persistent.
Level of impact: Persistent, aggressive invader and is capable of forming colonies through adventitious buds from creeping root system. Seeds generally germinate in the top 2-3 cm of soil, and also at the soil surface.

Competition
Once established, high seed production and ability to reproduce vegetatively allow for rapid dispersal and high persistence.

Control
Extensive root system makes this species difficult to control.
Chemical: According to some studies, permanent long term control cannot be achieved by herbicide treatments. L. vulgaris is resistant to many chemicals such as 2,4-D amine and MCPA salt. Glyphosate, amitrole, diquat, and picloram can be used for spot treatment. Glyphosate applied at early bloom provided some current season control, but was followed by abundant regrowth the following spring.
Mechanical: Generally the keys to controlling Linaria vulgaris are to prevent seed production and promote root starvation which can be accomplished by mowing and tillage (Saner et al. 1995). Tillage has been somewhat successful in controlling L. vulgaris in crops. Tillage should begin on summer fallow in June and continued at 3-4 week intervals. In post season cultivation, tillage could begin as soon as crops are harvested and should continue at 3-4 week intervals until shoots are killed by first frost. Mowing may also be used to help decrease seed production but generally will not eliminate L. vulgaris stands.
Cultural: Grasses can be used to compete with L. vulgaris. Disturbed areas can be seeded with vigorous, well adapted grasses.
Biological: Biological control has been relatively successful for Linaria vulgaris. In general, the root system seems to be the most promising target that is susceptible to damage. Weevils (Gymnetron antirrhini) larvae develop inside of the fruit, and adults feed on leaves, buds and stems. Harris (1961) concluded that G. antirrhini is the most important agent for biological control of L. vulgaris in the eastern provinces of Canada, British Columbia, and the northwestern U.S. Other species that may be used to help control L. vulgaris include another weevil (Mecinus janthinus), leaf feeding moths (Calophasia lunula), and root mining moths (Eteobalea serratella). Mecinus janthinus larvae bore in stems, and the adults feed on shoots. Celophasia lunulahas become established in Ontario, but other more northern releases have failed. Eteobalea serratella winter as larvae in roots and damage plants throughout the growing season.
Other: Fire is not a recommended method of control for L. vulgaris.

Notes: Generally considered only moderately poisonous to livestock.

References:

Baig, M.N., K.N. Harker and A.L. Darwent. 1994. Tillage enhances yellow toadflax 
    (Linaria vulgaris Mill.) control with glyphosate. Weed Science Society of 
    America 34:16 (Abstract #47).

Carder, A. C. 1963. Control of yellow toadflax (Linaria vulgaris) by grass 
    competition plus 2,4-D. Weds 11:13-14.

Harris, P. 1961. Control of toadflax by Brachypterolus pulicatius (L.) (Coleoptera: 
    Nitidulidae) and Gymaetron antirrhini (Payk.) (Coleoptera L. Curculionidae) in 
    Canada. Canadian Entomology 93:977-981.

Morishita, D.W. 1991. Dalmatian toadflax, yellow toadflax, black hembane, and 
    tansy mustard: Importance, distribution and control. In James, L.F., J.O. 
    Evans, M.H. Ralphs and R.D. Child (eds.) Noxious Range Weeds. Westview Press. 
    Boulder, Colorado. 466 pp.

Saner, M.A. D.R. Clements, M.R. Hall, D.J. Doohan, and C.W. Crompton. 1995. The 
    biology of Canadian weeds. 105. Linaria vulgaris Mill. Canadian Journal of 
    Plant Science. 75:525-537.

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