Some garters if deprived of reproduction and/or brumation for a period of time, will find a way to make it happen anyway. It's survival. I mean, what if the climate suddenly warmed in concinnus territory, and the snakes no longer needed to brumate? many would die off, but the remainder would eventually reproduce, brumated or not. The point is, garters are obviously adaptable or they wouldn't be so diverse and plentiful. The concinnus pair i had for about 20 years would sometimes attempt(unsuccessfully) to breed even if not brumated. All they needed was an awareness of seasonal change.

Heck, that 6 year old female pueblan milk snake I had has never been bred. They are known to produce fertile eggs anyway, at around 8-10 years old. They'll do it.

If you want babies from you concinnus, it's best to get both male and female through a long hot summer, fed very well, then brumate them. They are originally from NW Oregon/SW Washington. If they are great shape, don't go easy on them for brumation. Go 45-50 degrees for 4 months minimum. They'll make babies after that. Even when successful breeding took place, litters were small when I went easy on them. When I stuck them with very long and cold winter rest, and they came out without losing much weight, my litters were huge. 35-45 babies. I found that you cannot skimp on the female's brumation. She must stay cold for several months. The male doesn't seem to make much difference. He can skip it, or only have a very mild winter season. The female seems more fertile if she doesn't get treated like a pet. She must get somewhat harsh and long brumation if she's healthy and strong. Big litters follow.