Most mating occurs in April and May, and the female stores the sperm during hibernation until the following spring, when it allows its eggs to be fertilised. The male Plains gartersnake finds a receptive female by following its sex pheromone, and small mating balls may occasionally be formed with between four and six males and one female. Once it has mated with a female, the male may place a copulatory plug into her cloaca, to attempt to prevent any other males from fertilising the eggs (4). The female Plains gartersnake gives birth to live young (3) after a 9-week gestation period (4), with clutches usually containing 9 to 30 young (4). Generally, larger females have clutches with a higher number of young than smaller females (4). Sexual maturity is attained in the second or third year of life (2).
The diet of the Plains gartersnake consists mostly of earthworms and frogs, with toads, leeches, slugs, carrion (4), fish, amphibians and small mammals such as mice and shrews also being taken secondarily (3) (4). The diet of this species is seasonally variable and depends on the availability of prey in the area (4). A mildly venomous species, the Plains gartersnake immobilises its prey by injecting it with a toxic secretion, although this is not harmful to humans (3).
The active season of the Plains gartersnake runs between mid-March (4) and October (3) (4), although this is dependent on which area the individual occupies, and some populations may become active later in the year. During the period of activity, the Plains gartersnake is diurnal (4).