I just did a ton or research on molasses for my own set up- check the journal for my specific details. But from what i read you can begin using molasses at the very start of flowering. Molasses has a bit of phosphate and sulfur that is good for the plant- it also has the ability to break down bad mineral deposits and acts as a carbohydrate and stimulates growth acting as a sugar cranking out photosynthesis. I just put my first mix of it in today- first day flowering... And will be watering with that up till my final flush.