This is best started in April or May.
Herbicide – This is best when the plants are young. A garden herbicide with Glyphosate (contact herbicide) kills plants when they are small. Please follow the instructions on the packet and check after 2 weeks to make sure that all the plants are dying. Re-apply if necessary.
Cutting – Cutting the plant repeatedly throughout the season will stop it from flowering and it will die eventually. Wear long sleeves and gloves – keep the sap off your skin – and wash everything afterwards.
Digging – If you have only a few plants, dig them up, making sure you cut the tap root at least 10 cm (4 inches) below ground level and shake all the soil from the root. To prevent any possible spread, do not take the plant away (and do not put it in your brown bin, if you have one). Let it rot, with the roots exposed to dry out. Wear long sleeves and gloves – keep the sap off your skin – and wash everything afterwards.
Keep checking – Which ever treatment you use, check it, and make sure you have got them all. And do the same thing next year. Be thorough and there will be less and less!