A little boy offers to help his neighbor take care of their houseplants while they’re away, and as each day passes the plants grow and grow, until they’ve filled up the house and are spilling out the windows and doors.  I would have seen it as a child in school or at the library in the early 80s, but the book may be from earlier.  I believe hardcover, with color illustrations ( I want to say ink and/or colored pencil?)  I’ve always been fascinated by plants and this book was my favorite!