The Common Lilac (Syringa vulgaris) was one of the first food trials we conducted since one of them is growing in my yard.
Leaves: Unsafe. The crabs never touched these, not even to move them out of their way. We do not recommend putting any green portions in the tank.
Bark: Unsafe. We would not feed the bark from the stems. The stems may be suitable for climbing toys, but we would not recommend their use since there are so many other woods suitable for this purpose.
Blossoms: Safe. The crabs are attracted by the strong (nearly over-powering) fragrance of these blossoms and they eat lilac blossoms fairly well. My crabs never gorge themselves on these, but they do eat them. This is a great item to put inside the tank when you're having odor problems. They can be dried or frozen for later use. If mixing with other foods, we recommend removing all green parts since other portions of the plant may be poisonous.