Thistles

Biology of Thistles

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

Creeping Thistle

Cirsium arvense

thistle creeping

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