Blooms! Better late than never, I guess. How long can you keep things outside in Denver? Or can you make yourself a mini-greenhouse with a window opening to keep it warm? I haven't spent a LOT of time in your area, but I know your winters are pretty frosty...
=^..^=
It can vary wildly; our winter weather tends to be more during true winter, late December onward, so sometimes the fall is mild enough you can leave things out for some time. We haven't hit the equinox yet though, so I still definitely have time. However, there is an odd tradition of snow on Samhain (Halloween), even though our snowiest month is March. So, probably October-ish? Maybe November? When we have sub freezing cold snaps, they seem to be in January.
The thing about Colorado is, the weather never stays for long. It can be below freezing, freezing, and 40F in three days, or 60*F and zero in the same week. ;)