My worst self-sowers have been Pennisetum alopecuroides 'Moudry' and Eragrostis spectabilis. I call them Ornamental Weeds.

H. sempervirens and Festuca glauca get haircuts before the seeds go viable. I leave one bed with a couple of each alone, to encourage volunteers, and then select the best for unique characteristics. Blue Fescue out-paces Blue Oat, in volunteer production, by 10:1.

I live in the PNW and have grown, sold and spec'd for clients ornamental grasses for over 20 years. During all that time I have never encountered any issue with blue oat grass seeding or spreading. And I have used it alot!

My biggest sinners in that regard are the bronze sedges (Carex flagellifera and buchananii), New Zealand wind grass (Anemanthele lessoniana) and Mexican hair grass (Nassella tenuissima). And the usual culprits - golden millet, ribbon grass, etc.

Thanks for response. Yes, I too think the fescues are the most "prolific" ornamental grasses. The carex sedges spread themselves around as well, but not nearly as boldly and I do like them. As for my blue oat grasses, I am going to clear them all out along with the fescues. Then I will add some new blue oat plants. I'm convinced my current plants have either reverted or engaged in promiscuity with other types of grasses. And each generation carries on the bad traits.

Reverted to what? A species cannot revert to something else, although a hybrid could. Helictotrichon sempervirens is not a hybrid......not even that many named forms. And intergeneric hybrids are not very common, so I doubt that is an answer either.

FWIW, other than the few grasses I mentioned, I have never encountered seeding issues with any other ornamnetal grasses in this area, and specifically with the fescues or oat grasses. Western Washington summers tend to be cool enough that many ornamental grasses do not set seed readily or that seed never has a chance to ripen.