Mighty Jack Mighty Jack Series 1 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.