Just because you don't have a contract does not mean not getting a single monthly payment. In other words, you do not need a contract to get paid once per month. Just because you do not have a contract does not mean you bill after every visit, for every service.
I do not work with contracts. But, I send an Invoice once per month, the customer pays once per month. Each Invoice includes an itemized list of what was done that month (e.g. mowing, bush trimming, etc.).
Your plan forces all customers to accept a certain list of services, such as bush trimming, mulching twice per year, overseeding, etc. I can easily see potential customers wanting a menu selection, not a bundle that includes services they do not want. Residential customers, in my experience, like an ala carte approach. Usually, they want to have input on what is being done on their property. No, this may not be the case always. Commercial customers, on the other hand, want a "sign it, forget it" approach, letting the LCO make the decisions. Homeowners usually want to micromanage, whereas commercial clients want to turn over responsibility to others.
As somebody pointed out above, the cancellation is not easy to prevent. They may not even cancel, but rather just have somebody else start doing the work, or choose to do it themselves. Your Invoice for the first month you don't work would not be well received.
We understand your desire to lock in a residential customer, but that idea is not very realistic. This is why I don't even try to work this kind of relationship.
Also, remember that a contract is no assurance you will get paid -- a common reason stated for wanting a contract. The honesty of the customer will determine whether you get paid, or not. For these small amounts of money, adjudicating a dispute is most likely worth the time.
You would be better perceived by your customers if you promptly manage their Invoices and Payments. In other words, get a monthly Invoice to them promptly, and cash their check promptly. This sounds pretty simple, but often does not happen. [side note: a friend of mine hired an LCO to mow 2.5A of lawn for the 2012 season, weekly cuts. The weekly charge is probably close to $200 for this property. He told me today, no Invoice for ANY 2012 work has yet been received, despite many requests! The LCO has been working all season, without bothering to send an Invoice. He will not hire this LCO next year. The LCO may have great equipment, may lay down great stripes, may show up every week, ... but is a horrible business person.]