When was the last time you decided, mulched or fertilized your rhubarb? Most perennial plants like rhubarb will grow weaker once the plant becomes mature if the roots are not divided or refreshed with a manure or compost mulch or given a dose of slow release plant food such as Osmocote. I would wait until early next spring to divide your rhubarb plant cutting the mother plant into several sections all with some roots attached. Keep and replant the young outer sections but discard the center part if it looks dry and dark. Rhubarb responds best when fertilized in early spring. One more thought. I have seen rhubarb in my own garden decline due to a mole tunnel below the main root system. My plant eventually grew new roots and recovered but you may want to check near the root zone for burrowing rodents. Keep Growing, Marianne Binetti