I'm not following your math. Each push/pull is 10', and it's 2d4 per 5', so shouldn't that be 2d10+12d4 (41)?
It comes online at level 3 too, at which point it is 1d10+8d4 (25.5). Not bad for a level 3 PC, although IMO it actually works better if you have someone else cast the Spike Growth so you can start grating on round 1.