Nassella tenuissima is a cool season grass, which goes dormant during the summer, after it blooms. I should think you could give it a haircut now, and it'll regrow foliage when the weather cools in the fall. I have about a dozen clumps, and as far as I can tell, have gotten no self-seeding yet, so I'll leave them be unless they get ratty looking as the summer goes on.

I no longer sell this grass anymore as its propensity to self-seed has made it close to an invasive species here. IME, no other OG, with the exception of Millium effusum and ribbon grass, poses quite as aggressive a behavior in the PNW as does this grass.