How much molasses you should use, normally depends on the particular strain that you’re growing and its nutrient requirements. The right dosage can also depend on your plant’s age and may be different depending on environmental factors, such as temperatures and light intensity.
When you’re starting out feeding molasses, it is recommended that you start with lower dosages at first. A good starting point can be 4–5ml of molasses per litre of water. Later, when your plants are flowering, you can increase the dosage a bit, since your cannabis plant will need more potassium. Although you can use molasses during all stages of growth, including the vegetative phase, you will likely notice the most benefits during flowering.