The Englishman was making his fifth start in as many weeks and his debut appearance north of the border, where he closed with an 8-under 64. At 20 under, Poulter finished alone in third place and one shot out of the playoff where Jhonattan Vegas beat Charley Hoffman.
Poulter started the day five shots off the lead, but vaulted into contention and at one point set the clubhouse mark after rolling in a number of birdie putts without dropping a shot.
"The best thing I did today was start holing a few putts," Poulter told reporters.