Since "ignorance of the law is no excuse," it doesn't matter that the city didn't give you notice to correct the violation. The city code on the bill is the law that makes this legal. You can request a hearing with a judge, fix the issue, bring pictures with you to show the judge that you fixed it, and maybe he/she will dismiss it or lower the fine.
I agree with the previous answers and there must be more facts to properly answer. Was the offending shrubbery removed by the city? Did the notice have a date for compliance or the imposition of the fine? The notice will have an appeal procedure, listed to have the administrative decision reexamined. Follow the procedure to the letter and pay the fine under protest, ( to avoid other charges if you lose). You should take everything to a lawyer to help you with the appeal if you have trouble.
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