For most transactions, the law does not require a signed written contract, so they can take legal action on an oral or verbal agreement, or one evidenced by a quote or proposal or an unsigned contract.If the contract did not guaranty results, then if they did the things called for in the contract, they are entitled to be paid even if you lawn did not improve. In the absence of a warranty or guaranty, it is no defense to nonpayment to say that the treatment was not successful. If there was some warranty or guaranty, you can enforce it as per its plain terms.From what you write, it would appear that if there is no guaranty or warranty, they could sue you.