By signing the contract, you are bound by its terms including the thirty day notice requirement.  If you don't give the thirty days notice, the landscape service could file a lawsuit for breach of contract.  The notice requirement is probably for the company to find another customer to offset the lost revenue when you are no longer paying for the landscape service.  It may also be to avoid a situation where the landscape service arrives at a customer's home and is then told on the spot that the customer no longer wants the service.