Trees aren’t poisonous to cats, but tree fertilizer and pine sap can be. Cats ingest sap by chewing on a tree’s needles or branches, making it extra crucial for your tree to be cat-proof. Never add things like aspirin to your Christmas tree water. In the case that your cat drinks this water, it can be extremely toxic. If you think your cat has eaten or ingested something poisonous, call the ASPCA National Animal Poison Control Center immediately at 1-888-426-4435.