My lease agreement clearly states that the tenants are responsible?
I have seen far to many tenants let the landscape turn brown, uncut, weeds, bushes and trees not kept up so I totally get why you would put that in the lease. Tenants nowadays dont seem to just know to keep that stuff up like they used to. When I got a service to do landscaping that included the lawn, sprinklers, prunning, trimming, weeding, etc. If your tenant wants to hire a service to do that its fine but the tenant pays. If the lease specifically says the tenant is responsible for maintaining the landscape your property manager or your tenant should not be expecting you to pay for it. So I think you are totally right to dispute it.