Adu first scored in the 28th minute when he collected the rebound of his own shot and launched a low ball past Dynamo goalkeeper Talley Hall.
He also broke a 1-1 tie early in the second half on a penalty kick for the Union (8-14-6).
Josue Martinez capped the scoring with his first career goal.
Boniek Garcia tallied in the 44th minute for Houston (12-8-10), which remained in fifth place in the Eastern Conference, just one point ahead of Columbus for the final playoff spot.
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