The word arboretums is playable in Words With Friends™, no blanks required. Because it is longer than 7 letters, you would have to play off an existing word or do it in several moves.
Words With Friends™ Letter Score: 17
Highest Scoring Words With Friends™ Plays In The Letters arboretums
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Seven Letter Word Alert: (34 words)
aborter, abortus, arbours, armours, armrest, arouser, barters, boaster, boaters, borates, bromate, brumate, bursate, erratum, maestro, maturer, matures, mobster, mortars, obtuser, reboast, reburst, reroast, retours, roamers, roaster, rostrum, routers, smarter, sorbate, tourers, tremors, trouser, urostea