Is bimonthly or bi-monthly correct?
I can find both in the examples.
I think this is a trivial matter in the sense that no one will fault you if you spell it bi-monthly or bimonthly. Anyway, the entry is under bimonthly at dictionary.com and Merriam-Webster bimonthly. The examples given at dictionary.com are from works that are at least one hundred years old. If we look at some search results, we see that bimonthly is more popular.
So all in all, I would write bimonthly.