Why does seeing red and yellow leaves panic Luke Garner from among the hidden?

In "Among the Hidden" by Margaret Peterson Haddix, Luke Garner panics at the sight of red and yellow leaves because they signify the arrival of fall, which brings the impending danger of his family's garden being harvested. This season change indicates that he will soon lose his hiding place and be exposed as an illegal third child in a society that prohibits such individuals. His fear stems from the threat to his safety and existence, highlighting the constant anxiety he faces due to his status as a hidden child.