In the beginning, Lizbeth was a childish girl who went to bully (or as they said it, "prank") this old woman named Miss Lottie. She threw rocks at some marigolds, something she took good care of, just for the fun of it. Deep down, she knew it was wrong but didn't want the other kids to really think of her as a loser.
During the middle of the story, Lizbeth overhears her parents in a slight argument. Her dad isn't working to support the family because there are hardly any jobs available, and her mom is. She realizes they have both switched roles, basically, and she doesn't think her family will be the same again so she runs off to Miss Lottie's to take out her anger on some poor flowers.
At the end, after taking out Miss Lottie's marigolds, she realizes that the marigolds meant something to Miss Lottie. They weren't something she could just destroy and tear apart on her own accord. Lizbeth grows up in a sense, realizing that stuff aren't just things that are there to look pretty, but they can symbolize things and mean stuff to others.