Thistles

Biology of Thistles

There are 150 species of thistles worldwide, with 20 species in the UK. The two most common and damaging are creeping thistle - Cirsium arvense - and spear (Scotch) thistle - Cirsium vulgare.

Creeping Thistle

Cirsium arvense

Seed

• Creeping thistles can produce 40,000 seeds per plant
• Viable seed sets 6 days after the flower opens
• Seed is viable in the soil for 10-21 years

Root

• Vertical roots are used for storage of nutrients
• Horizontal creeping roots spread the infestation
• Adventitious buds give rise to new infestations
• Fragments of root can lay dormant for many years

Seedling

• In the first year a creeping thistle seedling can produce a root system covering 5m²
• In the second year the rooting area can extend to 80m²
• The roots can weigh up to 2 tonnes per hectare
• The roots extend over a vast area - over 12 metres per year

Spear Thistle (Scotch)

Cirsium vulgare

• Spear thistles are typically biennial; completing their life cycle in two years
• They grow from seed, forming a rosette and a tap root that is 70cm in length