I agree with Arthur.
Here we pick a warm day when the plants will be pumping the sap up pretty
quickly. Cutting the plants with a hoe or spade just below soil level seems
to really knock them back - my father says they bleed to death, by which he
means that the shock of losing the top and all the moisture on a warm drying
day is too much. Sometimes they regrow, but repeating this method seems to
work. Not practicable on a large scale, but ideal for getting odd ones in the
Hope this helps.
Croftway Nursery, Barnham, West Sussex, UK