Cirsium vulgare (Savi) Ten. (Cirsium lanceolatum, Carduus lanceolatum, Cnicus lanceolatum)
Spear thistle is a robust biennial or short-lived monocarpic perennial that can be a serious problem in grassland and waste and cultivated ground throughout the UK. It also occurs on roadsides, coastal dunes and in woodland clearings. It is frequent on unsown set-aside land and seems to have increased since the 1960s.
Spear thistle flowers in June and July but rosettes can survive for up to 4 years without flowering. The plants need to reach a threshold size before flowering is initiated. The flowers are pollinated by bees and other insects. Plants cut down in flower produce few seeds and although they appear normal none are viable. On intact plants the first ripe seeds are formed by the end of July. The average number of seeds per flower head is around 100 but there can be up to 340. Seed production per plant may vary from 1,600 to 8,400 seeds. More seeds are produced when ample soil moisture is available during the growing season. Seeds are dispersed during August and September.
Seeds have little dormancy and germinate readily in moist conditions at favourable temperatures in the light. Spear thistle seedlings may emerge in spring and autumn but the main period of emergence is March to April. A flush of emergence is stimulated by rainfall but there can be high seedling mortality if dry conditions then follow. Fewer than 10 seedlings are likely to develop from the seeds shed by an individual thistle plant. Spear thistle does appear to accumulate a persistent seedbank and seedlings may only emerge after fresh seed has been shed
Disturbance of the soil and vegetation has been shown to advance the germination of spear thistle. In pasture and other areas of dense vegetation seedlings emerge from small areas of disturbance like rabbit scrapings or the bare patches left when a parent plant dies. Seeds may germinate and become established in thinner parts of a grass sward. As the leaf rosette develops it physically suppresses the growth of the surrounding grass. Spear thistle rosettes grow better in grazed than ungrazed pasture because of reduced competition from neighbouring plants. Flowering and seed production is improved and seedling survival is greater.
The seed has little innate dormancy and is generally thought not to form a persistent seedbank. In light soils the seeds do not persist for longer than a year on the surface or in shallow layers of soil. In heavier soils, however, a small number of seeds may remain for 3 years or more. In all types of soil, seeds buried at 15 cm deep remain firm and viable for at least 3 years. Seeds do not germinate after 3 years in dry storage. Seeds submerged in water gave 1% germination after 3 months.
Spear thistle spreads only by seed. Unlike creeping thistle (C. arvense), the feathery pappus remains attached firmly to the seed as an aid to wind dispersal. Nevertheless, most seed is dispersed less than 2 m from the parent and only 10% travel more that 32 m after reaching higher air currents. Another important means of seed dispersal is in baled hay or as a contaminant in cereal, clover and grass seed. The seeds are attractive to ants that may further disperse the shed seeds.
Spear thistle reproduces only by seed and it is important to prevent fresh seeding. In arable crops, spear thistle seedlings are destroyed by surface cultivations in spring, and by hoeing or spudding as necessary. In grassland, young plants should be spudded out at the rosette stage or cut down when in bud to prevent seeding. Spear thistle can be pulled out when in flower, or the taproot may be cut below ground when plants are at the rosette stage using a thistle hoe. Manual pulling is easier in moist conditions in spring and autumn. Rosettes at 5-8 cm in diameter are more readily lifted and disposed of than large plants.