Calling Mr. Sunshine

Just put an interrupt light (heard it called Gas Light technique here) on your girls at night, you don't need 18 hours of light to keep them in veg. Just have a 60W bulb come on during the night for one hour and they will stay in veg. This is a very inexpensive option.

Yes artificial lighting works great to make up for the lack of light... If you're planning for next year.. look up @Garden Boss he's the man and has a great thread that outlines the basic principles of growing outdoor monsters...definitely one of my favorite growers and people on riu...