Even if you have a management agreement that delegates all functions to a managing agent, the executive committee or the owners corporation as a whole can still exercise any of their functions that they care to exercise.
If you were to employ a gardener, it would be wise to ensure they have their own workers compensation and liability insurances. If you find a gardening contractor you like, it would keep the accounting simple and transparent if you were to instruct them to send their monthly account to the managing agent and for the managing agent to pay that account.