Bride-weed, common linaria, flaxweed, yellow rod, yellow toadflax, wild snapdragon
Linaria vulgaris Mill.
Perennial Broad-leaved Weeds
Common toadflax is a native herbaceous perennial with an extensive spreading root system. It is found throughout Britain. Common toadflax occurs in hedgebanks, waysides, woodland clearings and waste grassy places on dry, well-drained gravelly, sandy or chalky soils. In the UK, common toadflax is absent from meadows, pasture and wetlands. It thrives on nitrogen rich soils and has a good tolerance of heavy metal contamination.
Common toadflax has a variable growth habit depending on the soil type, shade level and grazing regime. It sometimes develops peloric flowers. Several subspecies have been described and common toadflax can form hybrids with related species.
Common toadflax has a long tradition of medicinal use but is little used today. A yellow dye is extracted from the flowers. Common toadflax leaves are thought to have insecticidal properties. The foliage may be toxic to stock but it rarely occurs in pasture in the UK. In North America, where common toadflax is a prairie weed, cattle usually avoid it but will eat it in a dried state. It has been used to treat cattle that are unable to ruminate.
Common toadflax flowers from July to October. The flowers are insect pollinated but self-incompatible so seed set can be unreliable. However, a small proportion of seed can develop without cross-pollination. Ripe seed is produced from September onwards. There are around 70 winged seeds in each capsule. Seeds dehisce through pores in the seed capsule. Each flower spike can produce 1,000 seeds. The average seed number per plant ranges from 8,700 to 31,500. Seed colour varies from grey to black. Darker seeds are heavier and more likely to be viable.
In the laboratory, seed germination in the light was around 67% but there was no germination in the dark. Germination is increased by a period of dry storage and is promoted by chilling and alternating temperatures. Common toadflax seed can germinate when shed but in field studies seed sown in the autumn did not emerge until the following spring. The main periods of seedling emergence were April-May and August-October and flushes tended to follow soil cultivation. Seeds germinate shallowly at 0-20 mm deep in soil.
The roots of 3-week old seedlings are able to develop adventitious root buds and can produce shoots if separated from the main plant. The main shoot usually dies off and numerous secondary shoots develop, all of which flower in the first year. An extensive root system also develops during the 12 months after germination. The main root may penetrate 1 m down into the soil. Adventitious flowering shoots develop on the taproot and on the lateral roots and by the second year there may be 193 shoots spreading up to 46 cm from the parent plant. Common toadflax regenerates from adventitious buds produced on the taproot and lateral creeping roots. It forms a web of superficial roots. The horizontal roots may be several metres long. The aerial shoots are killed by frost but individual roots will live for at least 4 years and can withstand freezing.
Persistence and Spread
The seeds may persist in soil for several years. Common toadflax seed has been spread as a contaminant of crop seed and in baled hay. Wind, water and ants may disperse the seeds. Despite having winged seeds, 80% of common toadflax seeds fall within 0.5 m of the parent and the rest within 2 m. The seeds are oily and can float in water for an extended period. Seeds have been recovered from the crops of various birds, and seedlings have been raised from their droppings.
Common toadflax forms clonal patches but has a limited capacity to spread laterally by vegetative means. Regeneration is possible from root fragments as short as 1 cm, and is common from 10 cm fragments. Plants that develop from root fragments exhibit similar rates of early growth as seedlings.
Common toadflax is controlled by cutting, hoeing, pulling and removal of the creeping rootstocks during tillage. Seedlings should be destroyed and as much of the roots as possible removed by spring cultivations. Seedlings less than 2-3 weeks old are most susceptible to cultivations. Older plants should be hoed out to prevent seeding. Common toadflax is likely to be more of a problem under reduced tillage systems. In a summer fallow, tillage should begin in June and be repeated at 3-4 week intervals. Cultivations should be shallow to avoid the spread of root fragments. Further cultivations should be made when the shoots have made 8-10 days regrowth. Sweep-type cultivators are recommended. It takes 2 years of cultivations to eliminate the weed, with 8-10 cultivations in the first year and 4-5 in the second year. Mowing will prevent seeding but will not eliminate the weed.
Common toadflax is thought to be tolerant to burning because of its deep taproot and is quickly able to colonize open sites where burning or overgrazing has reduced the other plants. However, the superficial lateral root system is susceptible to root competition from other plants. Grasses will outcompete the weed in properly managed grassland.
Seed production is reduced by plant competition. Seeds also suffer predation by insects and this can have a drastic effect on seed numbers. The larvae develop inside the seed capsule while the adults feed on the buds, leaves and stem. Small root-mining moths overwinter as larvae on the roots of common toadflax and damage the plant throughout the growing season. Eggs are laid at the base of the shoot and the emerging larvae burrow into the stem and move down into the root. They tunnel into the root cortex where most development takes place but the larvae return to the stem base to pupate. Many European insects have been introduced into North America as biocontrol agents for common toadflax.
Updated September 2007