The Cardinals, who are chasing the first-place Cubs in the National League Central and the Rockies in the wild-card standings, have won five of their last six and nine of 12.
Weaver (6-1), a rookie right-hander who has boosted the St. Louis rotation, allowed one unearned run, struck out six and didn't walk a batter.
Weaver was 3-0 with a 1.71 ERA in four appearances, including three starts, in June. He's now 3-0 with a 0.99 ERA through his first three starts in September.
Rookie left-hander Amir Garrett (3-8) was charged with four runs in four innings. He struck out seven and walked five.
