Almost all Azaleas (which are grown both as house and garden plants) produce harmful grayanotoxins which block normal muscle function, and can cause heart failure and damage nerve function. The toxin is present in all parts of the plant, including leaves, flowers and pollen.
Azaleas and their close relatives, Rhododendrons, should therefore not be fed to tortoises, and care should be taken that they are not grown anywhere near the tortoise enclosure in case leaves or flowers accidentally fall into the enclosure. Azaleas have smaller leaves than Rhododendrons and usually have only 5 stamens rather than 10 in the larger leaved Rhododendron shrub. See Rhododendron.