There is no lawn left, it’s all native plants and wild flowers which have a tendency to grow taller than 12 inches and that is their sticking point: height. 🤣
Discussion
They don't allow trees? Their ordinance is an infringement. Fighting stiff like that either takes a few hundred k in fiat, or nearly complete knowledge of law as compared to the legal system.
Part of the "law vs legal" route is to get them to admit that they claim you've broken contract (which is the case here, as there is no true victim). They're threatening you with a fine with the concept that they (the municipality) are a victim of breach of contract. Once they admit that, request that they produce the contract that you agreed to. This all has to be done with correct procedures, but it is possible.