I have just heard back from my horticulturist brother (he trained at Kew)
and here is his reply:
"I can't think of a perennial cornflower with sharp leaves, they all have
fairly soft ones. I assume she means Centaurea (also known as knapweed) but
she should check in a flower book to be sure I am correct. A couple of
species spread by underground runners but most are not invasive.
"Spraying with Roundup (glyphosate) would control them and leave the site
ready for planting once they had dies - but she must take care not to get it
on desirable plants. Alternately, she could cover the site with heavy grade
black plastic and smother them by excluding light. This is a lot slower than
Roundup and is not possible if there are other, desirable, plants mixed in
with the cornflowers."
Or you can try the vinegar route.
Malvary in Ottawa (the Nation's capital), Canada
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