Below is great information about the last effective bloom date for cotton from the Jeff Davis Plot Points Extension blog.
Every year I get asked about late planted cotton that is still active blooming come September. Will these blooms make a boll that is harvestable? Last year I was able to participate in a multi-county study collecting data on late cotton blooms. The data was presented in a poster at a Beltwide this past January. You can look at it here: READ MORE
If you have any questions please contact your local county Extension agent.