The message
from Roy Bailey contains these words:

Don't you know the old rhyme about getting rid of thistles?:

Cut them in May, they'll grow next day
Cut them in June, they'll grow again soon
Cut in July, they'll surely die
Cut in August, die they must.

I hadn't heard that before; but the bad news is, neither have thistles.

The implication is that if cut after they flower/seed, the thistle's
tap root can't survive. It does, many thistles are perennial and severed
ones can easily grow new leaves in early autumn.

Janet.