Most Phalaris cultivars are rampant spreaders, some even considered invasive in colder climates than yours. Wide spreading growth is certainly not uncommon and is often a good reason for growing these grasses in containers. I'm not sure how lacking a period of dormancy will affect them afa cutting back, but I'd tend to do so in late winter for optimum results. Root pruning to control spread or dividing can be done at that time also.

I had a client (z5) who had to have strawberries 'n cream, as this is a very sturdy grass it was placed between the street curb and walk. 4'w x 80' L.

We placed several at 48" spacing and two years later the whole 4'x was almost completely filled in.

Year 3 we noticed straw 'n cream coming up on the other side of the side walk. The roots went under a 4' wide walk and sprouted. End of year 3 there were no strawberries 'n cream left.

The salt from the winter de-icing didn't even damage a blade of this grass.

The shovels we had ate all the strawberries 'n cream in sight.

Best cutting back job I ever did.

Jake