This does not say book one on the cover, but does on the title page. (sigh) May have a connection to Zita the Spacegirl. And I'm pretty sure it's SF, not fantasy, though the character discuss the matter.
But Jack has to look after his autistic sister Maddie, who doesn't speak, while his mother works during the summer. At a flea market, they meet someone who looks familiar from Zita, who says Maddie told him Jack's name, offers them seeds, and goes off with their mother's car.
Maddie plants the seeds, producing wild and fantastic results. A home-schooled neighbor and medieval recreationist Lilly gets to see some of the plants.
And it goes on from there. Involving a dragon, a giant snail, someone thinking something is weedkiller, the seeds Jack refused to plant, a dispute between safety and wonder, and more.
Reaches the definite end of a phase at the end.