September can be wonderful, with most of the mosquitoes and biting flies dead and cool, crisp days. The only thing you'll need to pay attention to are yellow-jackets and other wasps, which love to live in deadfall. If you're heading cross-country through forest in September, you're bound to run into these little beasts. I'm probably over-sensitive to this issue right now, since I got stung 12 times in the space of about 10 seconds last year at that time. If you stay on trail and then leave trail above timberline, you shouldn't have any issues, but I would otherwise stay out of deadfall areas where those critters love to live.