It all depends on what's under the 9" of compost/loam. Helictotrichon sempervirens will grow well in almost any kind of soil as long as it drains well, particularly in winter. If your underlying soil doesn't drain well, the 9" layer will remain too moist in the winter, which will make the blue Oats suffer.

Thanks donn.

I suppose at some theoritically deep thickness of newly installed well-drained soil, the underlying soil would be of no consequence ... but 9" falls short of the mark. Fair enough response. I just put some water in the dug-out trench to test the underlying soil drainage. Not great, but the water had all drained within about 5 minutes.

My pit is probably more like 12" at this point. Guess I'll have to wait and see what happens.

Just curious - why is wintertime the toughest with regard to drainage?

"Just curious - why is wintertime the toughest with regard to drainage?"

That's a good question, and one I'll have to research. I don't know the horticultural reason, but many perennials, including many grasses seem to fail in cold wet winters when drainage is insufficient. I don't know if it's root rot, or something to do with the relationship between moisture and cold, but I'll bet the answer is out there somewhere.

Maybe it's a frost issue. Water trapped by poor drainage, freezes and damages the roots?

Hi-
Here in the hot inland valleys of southern california the blue oat grass does well if the crown of the grass is left 1"-2" above the soil level at planting and a 2" layer of mulch laid up to but not touching the base of the plant.
Once per week watering even in 100+ temps.