"Old witch, fell in a ditch, picked up a penny and thought she was rich!"
"And I too, have planted marigolds"
Lizabeth being the child she was, taunted Mrs. Lottie. She had destroyed her flowers, sang a mean song directed at the old woman all due to her friends teasing her and pressuring her. In the end, she did all of these things because she wanted to prove to them that she was just as "good" as them.
Her parents argue one night, which makes her emotions go wild. She gets up in the middle of the night and venture out to Mrs. Lottie's place with her brother Joey and then once again, destroys the beauty that Mrs. Lottie spent time on. Meanwhile Joey begs for his sister to stop, Mrs. Lottie appears.
After some time passes, Lizabeth changes and transforms into an adult. She says that she also planted marigolds. By this I think she meant, that she learned and grew up and wasn't so childish anymore.