Thistles

Why Control Thistles?

Thistles need controlling because:
● They compete with grass for space, light, nutrients and water
● They are unpalatable to stock
● They lead to greater incidence of Orf
● Reduce the available grazing.

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):
A perennial that grows from seed or from root sections in the soil. Once established, the root mass can be greater than the plant above ground, competing effectively with the grass.

Spear Thistle (Cirsium vulgare):
A biennial that grows from seed, and in the first year often goes unnoticed, since it produces only a small rosette. In the second year the plant can grow to over a metre in diameter before flowering, posing a serious economic threat.

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 5 m²
• In the second year the rooting area can extend to 80 m²
• 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 70 cm in length