 If you liked
Ueki no Housoku
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 ...then you might like
Phi Brain: Kami no Puzzle
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Phi Brain's plot progression mirrors that of the Law of Ueki, and the characters from both of these anime seem to hold many similarities.
The concept isn't all that similar, but both have somewhat of an aura that makes them feel somehow related. If you like one of these anime, then I strongly recommend the other.