Don’t let dogs eat rhubarb leaves. Cooked stalks are OK, but they won’t like them! All parts of the rhubarb plant contain oxalate crystals, most concentrated in the leaves. Oxalate crystals causes pain and irritation when chewed or swallowed. This irritation, plus the unpleasant taste, usually prevents dogs from eating them. If they’ve eaten them in any large quantity you’ll need a trip to the vets. In small amounts they’ll probably vomit and get rid of them that way.