If you want to maintain healthy growth, you absolutely need to provide supplemental lighting that far North.

Please note that you don't actually want a ultraviolet light, but rather a more broad spectrum source. If you are interested in learning why, you can read about photosynthetically active radiation.

The ideal source of light is a High Intensity Discharge lamp (either mercury vapor or sodium vapor) and the second best is fluorescent grow lights. Don't waste your time or money on incandescent lights. There is quite a bit to learn about using artificial lights to grow plants. You would be better off doing some initial research on your own, and coming back to ask some more specific questions.

Okay, thanks

I've never used them, but I was under the impression that mercury vapor had a poor spectrum for plant growth, and high pressure sodium was more for flowering plants due to its red spectrum. Wouldn't metal halide be a better choice for foliage plants? I've been using a 250w metal halide over the winter and will be using a 400w next winter.

Metal halide lamps are really just mercury vapor lamps doped with some extra elements to tailor the spectrum to the specific application. You are correct that a metal halide bulb intended for use as a grow light will be better than a basic mercury vapor lamp. In practice though, there isn't much difference since modern mercury vapor lamps (as opposed to ones sold as metal halide) are doped to provide a broader spectrum. In particular, the inclusion of Scandium and Sodium fill out the red end of the spectrum.

I should probably note that my understanding of lights comes almost entirely from being a science nerd who is interested in light. I've got a very good understanding of the basic principles of these light sources and their spectra, but not necessarily how that would affect plant growth. The only indoors growing I've done was with fluorescent bulbs...

How much do these lamps cost?

Most people don't realize how expensive it is to reproduce sunlight. There's a lot of energy in sunlight, so it takes a lot of power and some specialized equipment to recreate it. While there are alternatives, here is what my setup cost:

$100 for electronic ballast
$120 for reflector/socket (less efficient reflectors can be found much cheaper)
$60 for bulb

This lights about a 3'x3'area. It's not cheap, but for high intensity light there are no cheap solutions. (Buying used may save you some money though.) You also have to consider long-term expense. An efficient light source and reflector might be expensive up front, but will allow you to use less wattage. Another advantage of high intensity lighting is that it's pleasing to be around during the darker months and can help with seasonal affective disorder if you spend time around it.

If you can keep your plants cool during the winter months, they may go more or less dormant and not require as much light. This would be the cheapest solution, but I would still put some fluorescent bulbs (with good reflectors) directly over them.