Are you sure it's a concinnus? Too many people misidentify or mislabel any garter snake with red on the sides.

I can tell you this: Of the native garters in my area, there are T. sirtalis (concinnus) and T. ordinoides and they are always found sharing habitat and dens. Hybrids between these two NEVER occur in nature. I've kept both species together right out of the fridge. The males were going nuts over the smell of their own females but were NOT interested in the other species at all.

IF that "concinnus" really is a concinnus I can almost guarantee it would have bred with it's own species (T. sirtalis) before it would breed with another species.

sirtalis is "compatable" with, and will breed with T. radix and perhaps a few others but for the most part, they stay with their own kind. That might not sound like much, but there are a lot of different T. sirtalis out there. There's eastern garters, CA red-sided garters, san francisco garters, etc. ALL are T. sirtalis so crossing those subspecies are not hybrids, they are intergrades.