You can cat it back any time , but better not in very hot weather ( I don't believe you get very hot temperatures up there so anytime would be good, as long as you can wait for a couple of weeks without flowers on them). I cut my greggiis in the fall and by April they've developed in very beautiful plants. I keep them a little on the dry side so as not to make them very lanky buy the end of summer.
DJR

What do you consider to be hot? It stays in the uppper 90's until around the first of October around here.

Mimidi, I just realized that Ala stands for Alabama (i checked you member page) instead of Alaska! that's why I wrote you don't get very hot... sorry, I don't know the us that well.
I suppose then that until october you can trim the plant but don't risk a drastic cut back until winter.

Not to worry Daniot. I will probably wait until around March and cut it back. That way if we get a hard freeze this winter it won't get hurt too bad.