Mighty Jack by Ben Hatke
Jack might be the only kid in the world who's dreading summer. But he's got a good reason: summer is when his single mom takes a second job and leaves him at home to watch his autistic kid sister, Maddy. It's a lot of responsibility, and it's boring, too, because Maddy doesn't talk. Ever. But then, one day at the flea market, Maddy does talk—to tell Jack to trade their mom's car for a box of mysterious seeds. It's the best mistake Jack has ever made.
In Mighty Jack, what starts as a normal little garden out back behind the house quickly grows up into a wild, magical jungle with tiny onion babies running amok, huge, pink pumpkins that bite, and, on one moonlit night that changes everything…a dragon.