My suggestion would be to retain an attorney in the area that handles civil litigation.
The first action he would take is to write a "demand letter". A demand letter is pretty much what it sounds like, you demand certain action (whether correcting the "disaster" or refunding monies) within a certain period of time. You would be amazed at the number of people that will respond in a positive fashion to nothing more than a letter from an attorney.
If he fails to respond in a positive fashion, you should be prepared to file a lawsuit. I don't know that I would normally recommend filing a lawsuit over landscaping. But here, $17,000 is NOT a typical landscaping job. With that type of expense, a lawsuit is financially viable.
If you do file suit, you would certainly want pictures of the poor outcome of the landscaping. Additionally, it would certainly be helpful to have another landscaper in the area to testify that the job was sub-par. However, you need not worry about this unless you get to the point of filing suit.
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