There is strain from Lucky 13 seeds called okanagan grape, its suppose to bred for northern climates and they say she finishes around mid september, it may be worth looking at..
Here is info
Okanagan Grape
regular
Indoor / outdoor
Lineage: Grand Daddy Purple x Early Purple
Variety: 100% Indica
Potency: High
Yield: Above Average
Okanagan Grape was developed as the solution to the short Okanagan
summer, and is a non auto flowering, short-season purple indica that initiates flowering up to several weeks before regular strains, and ripens to maturity in just 7 weeks. Okanagan Grape provides incredible bag appeal with large and dense purple buds that smell of grape and cherry cheesecake when grown in organic soil without the use of chemical fertilizers.
Indoor avg. flowering 7 weeks
Outdoor harvest September