Forcing is used to create early season rhubarb by preventing it seeing light, so the stems grow longer in search ... whilst this does create some early stems, it does stress the plant, and it is recommended that forced rhubarb is not cropped again that year. After March, there seems no reason to force, as the amount of natural growth is enough to harvest normal crops. I do not force, and enjoy copious amounts all through the late spring and early summer.