Donations have been granted and are still being sought for garden maintenance (including new plants and equipment), which is another measure/activity outside of the traditional ones, but which can have a positive impact in various places in the park. Having said so, a lot has already been done in this respect anyways (ever since they got some additional public funds for gardening)
I became interested in the above post about Berlin Tierpark.
Many zoos have lush gardening and tending all the plant life in general spaces and in animal exhibits probably takes lots of time and money. Gardening, at the same time, is a rather routine task and zoo people are not the best in it.
Do zoos search or use donations of gardening centers and companies? Plants, work, maintaining or even designing and executing whole parts of the zoo? Are volunteers sought specifically for gardening?