A person cannot delegate their legal responsibilities. Assuming that the ordinance is otherwise valid, the landlord/owner shares responsibility with the tenant and can be independently cited for violation. In a well-drafted lease, you could have cut the grass and charged the tenant as additional rent. You may also be able to claim that the tenant is in breach of the lease and evict them. However, you can't escape responsibility for the condition of the land by blaming the tenant.
See also: http://info.corbettlaw.net/lawguru.htm
