My Crocosomia have started blooming -- impressively. These are
new to me, planted last year, and I wonder if I should deadhead.
Will that extend blooming?

These things spread so, I wonder if they're potentially invasive.

Aren't Crocosmia incredible? Mine are blooming right now as well, and
are putting on a dazzling display. This is really my first season
with them (as I was on vacation when they bloomed last year), so I
can't advise as to dead heading. But I can tell you that how invasive
they are depends upon the variety. I have Crocosmia Lucifer, a deep
red-orange flower, and they aren't invasive at all. Though at 4.5",
they did get a lot taller than I thought they would. I've heard that
some of the other Crocosmia are rather invasive. If they get out of
hand, you might replace them with the Lucifer.

-Fleemo