It might be enough to attract a hummer but not enough to keep any. I would most certainly add a feeder. Either 4 parts water to 1 part sugar or 3-1 either will be ok.

I have never used petunias myself but most everyone I have seen who tried it says hummers wont use so a far better bet is a feeder. Get the 16oz first nature feeder cheap and easy to clean and sold at most department stores. Also dont use any red coloring as the red dye is potentially harmfull to hummers and its not needed. As far as plants go salvias such as lady in red and black and blue are very good draws for hummers.