The poisonous part of rhubarb is the leaves. They contain high levels of oxalic acid, which can cause nausea, vomiting, diarrhea, and other health problems. When rhubarb leaves freeze, they release oxalic acid, which is a toxin. This acid can then move down into the stalks of the rhubarb, making the entire stalk poisonous. However, if you cut off the leaves before freezing the rhubarb, the toxin will not be able to move down into the stalks and the rhubarb will be safe to eat. It is important to never eat rhubarb leaves and to harvest rhubarb stalks before they have a chance to freeze overnight.