Molasses is useful to plants throughout their life cycle, but it is most valuable at the transition from the vegetative stage to the peak of bloom, when the need for sugars is highest.
Incorporating molasses into your regular feeding cycle is most effective and easiest when it is combined with other ingredients in a well-balanced liquid fertilizer.
All of the nutrients in the solution are more readily absorbed when molasses is in the mix and the sugars are available when plants need them most.
Highest benefit from molasses in your nutrient solution is in soil-based systems, though it works for hydro, too.
If adding your own molasses, be sure to check the pH of the solution before giving it to your plants – the sugars will pull it toward the acidic side.