20 November 2016
I thought it might be the cooler weather affecting the coloring as it does some plants. Or, it might be the shorter day length that was causing the change, much like a poinsettia depends on longer nights for its red blooms. I discovered the reason my marigold change color this year by accident.
It is neither the day length nor the temperature that causes the loss of red. The plant simply gets tired. This marigold seed blooms true to its parent. As summer progresses I deadhead the spent blooms, leaving them around the ground near the plant. Some of the seed heads sprouted and grew new plants. Since we had a long growing season this year, getting our first killing frost only last week, these volunteer spouts had enough time to bloom.
There is nothing I can do other than to sow successive plantings throughout the summer, something that is not going to happen. We will accept the change as a natural pattern.