What flowers are toxic to dogs? Azalea, Buttercups, Chrysanthemums, Gardenias, Gladiolas, Hibiscus, Hyacinth, Hydrangeas, Mums, Primroses, Rhododendrons, and Sweet Peas are popular garden items that are poisonous. Thats just another good reason to keep your dogs away from your neighbors award-winning flower bushes.