Far be it for me to add fuel to the fire but if they had a clear instruction from the Owners to leave the garden alone and then later went ahead and ordered its destruction without holding a proper meeting, the question of personal liability might come into it.
Or you could let it ride and put it down to experience.
Or, at the next AGM, raise a motion for a special resolution allowing you to take over care of the garden again – now that everyone can see how they have destroyed it. At no cost or hassle to them, taking responsibility for your own OH&S, who’d vote against it (apart from the obvious)?