Answer:
In this story, the marigolds represent things which are important to people, that is all those things which matter to us, where we put our efforts and energy to make them more perfect and beautiful.
When the author says "I too have planted marigolds", she wants to express her compassion towards Miss Lottie, because she also has things which matter to her, the same as Miss Lottie.
In the end, the author wants to say that each of us has his/her own marigolds.
