(Not In Watermelon Sugar, which is what search results keeps giving me.)
I read it in our school library in the late 1970s. Hardcover, so probably not a Scholastic book. Story seemed to take place in the late 1800s on communal farm somewhere in the US. The main character was a boy in his late teens who had grown up on the commune. His younger sister had reached the age of 15 and had been assigned to the laundry, which was not the job she had wanted. His older brother was placed in an arranged marriage, which was not what he wanted. Marriages were arranged according to age; basically you married whoever was closest to you in age who wasn’t your sibling or other close relative. There is a young girl who follows the main character around, and it finally dawns on him that she will eventually be married to him.
He goes into town to run an errand for the commune. While there he meets a man selling watermelon. The man recognizes that he’s from the commune and has no money of his own, but he gives him a slice of watermelon and some seeds. Technically, he is supposed to hand over the seeds to the commune, but he decides he wants to own something and he plants them in a secret place. When he is found out and threatened with expulsion from the commune, his brother tells him to go before he becomes trapped the way he now is.
Any ideas? I keep thinking the title had something to do with watermelon, but days of searching hasn’t gotten me any closer.