The most distinguishable feature of garter snake is a light shaded stripe running along its back.
Garter snake in house siding.
They prefer to live in a warm and damp environment.
Yards and gardens easily become their cozy homes so they can feed themselves on earthworms tadpoles insects mice lizards and other reptiles found in this area.
While garter snakes are generally solitary creatures they generally hibernate in large numbers to prevent heat loss and keep their bodies warm.
Gaps between siding and stone veneers brick or any similar material should be repaired to prevent moisture damage and stop snakes from gaining entry.
Visually inspect your siding for openings through which the snakes can find access.
Lifting up the loose edges allows for some of the snakes to drop out.
These are individuals that needed to be relocated.
That one does not look like any of those so most unlikely it is poisonous.
The snakes climb up into the openings and use the space behind the siding.
Due to their combined smell a large group of garter snakes in or under your home can make your residence uninhabitable.
I had a family of garter snakes living inside my siding exactly same situation as you had when i put the deck on i removed siding to attach the ledger and half a dozen snakes came out of the lower trim piece just like you describe.
Garter snakes living in vinyl siding removal.