>
> > It is not possible to exterminate them with any control measures.


A good biocontrol will bring them into "balance."  In California, we were
overrun with yellow star thistle (a close relative).  Vast areas of pasture
land were nearly 100% thistle.  But it was a great honey crop.

Biocontrols released about 20 years ago have reduced thistle populations to
occasional plants or stands, following an El Nino year in 1999 when the
thistle grew 4 feet tall and blocked off country roads.  That event was the
last hurrah for star thistle in these parts.  Has never come back to any
extent.

Randy Oliver

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