Can I garnish my ex-wifes taxes for back child support owed of 1,800.00??

If you have a Court Judgment against her, yes. If not, just an Order which she has ignored, no.

You can but will need a specific court order for this

So will I have to file another order in FOC??

What state is this because many states will help you collect child support and do tax intercepts.

In my state tax intercepts don't start until they are $2,000 in arrears. Though you may be able to get a judge to sign off on it because it is so close. Is she making regular payments now?

If you go to your state child support enforcement office and apply with them, they may have ways to collect on the arrears including, tax intercepts, suspending drivers license, garnishments. In the state of Georgia, it is an automated process for tax intercepts every year. I think the minimum arrears are $500 for state and $1000 for federal.

Child support enforcement can be a useful tool to keep the financial part out of the parenting of the child. Let them deal with it and the parents can then work together for the betterment of the child.