What is the best way to get rid of sow thistles in the garden. Physically digging them out does not seem effective- they seem to multiply after attempts to manually weed! Roots seem to go deep, break off easily and branch off in multiple directions.
Sonchus arvensis L. or Perennial Sow Thistle is listed as a noxious weed in Ontario and is on the Invasive Species lists in several provinces. The plant reproduces by seed as well as by buds that form on spreading, brittle underground roots. It is a notoriously difficult weed to control.
Each plant can produce up to 4000 seeds that may remain dormant in the soil for 5-6 years and broken root fragments can send out new shoots. Optimum conditions for growth are moist, slightly alkaline to neutral soils. The plant does not thrive in acid or highly alkaline soils.
Persistence is required to reduce spread. Recommended methods of control are:
- removal of flowers to prevent the plants from seeding
- regular removal all shoots and vegetation to deplete the nutrient reserves in the root system
- digging and removal of as much of the root system as possible
Repeated removal of shoots over a period of 80 days is said to exhaust the rootstocks and regular defoliation will help to control the plant population. An application of mulch and a dense planting of favourable plants can help to reduce spread.
Unfortunately, elimination is very difficult to achieve.