Feed your agapanthus with a potassium-rich liquid plant feed (such as comfrey or tomato fertiliser) every week during spring and summer. Potassium encourages flower production which means that your plants will create more blooms all summer long. Make sure you don't overfeed your plants. One a week is sufficient. It's nothing to do with restraining the roots and you'll know that if you observe nature. Agapanthus growing directly in the garden flowers brilliantly and the roots aren't restrained. F