The Cardinals' 17-7 start is their best since 1944.
Wellemeyer (2-2) allowed two runs on six hits, walked two and struck out three.
Washington finally got to Wellemeyer in the seventh. Jesus Flores tripled off the wall in center and Willie Harris followed with a homer to right.
Ryan Zimmerman singled in the third, extending his hitting streak to a Nationals-record 20 games.
Pujols gave the Cardinals a 1-0 lead in the first on his ninth homer, a towering shot down the left-field line.
A two-run homer by Duncan extended the lead to 3-0 in the fourth. Pujols hit a leadoff double before Duncan sent a 1-0 fastball from Zimmermann (2-1) into the stands in right-center.
In the sixth, Ludwick followed Pujols' third hit of the game with his sixth homer, ending an 0-for-13 skid.
Zimmermann, one of the top young pitching prospects in baseball, allowed five runs and eight hits in 5 2-3 innings. He walked none and struck out six.
Thurston lining a homer inside the right-field foul pole in the seventh.
Notes: Zimmermann surrendered one home in his first two starts over 11 1-3 innings before allowing three to the Cardinals. ... St. Louis 2B Skip Schumaker went 0 for 5, ending his hitting streak at a career-best 11 games. ... St. Louis starting pitchers have allowed only five homers this season.
