Hi Adrienne -
Good growth depends on the light intensity, the duration of the light and the plants' requirements. Different kinds of glazing transmit differing amounts of light. Glass transmits about 90% of
daylight which equates to 10,000 foot candles. Some plants from tropical under stories can survive in less than 300 foot candles. Flowering plants typically perform better in 500 foot-candles and
twelve hours of light per day. For food plants, 1000 foot-candles and 12 hours are necessary. Regions with short winter days will require supplemental lighting for good growth. The low outside
temperatures will require more supplemental heating. You may also need to add humidity and circulation to help with yield and quality. Different seed companies advertise vegetable varieties better
suited for greenhouse growing and can help with temperature, light and humidity requirements. Greenhouses can overheat in winter and if the ventilation is automatic, the cold air pulled in can be a
shock to plants if done too quickly. Many of the ventilation systems offer 2 speed or variable speed fans so you can set them lower in winter. There are a lot of good books and links for greenhouse
growing. Here's a good one on growing tomatoes:
Please try my links page or books page for more information.
I started a list of seed companies on my supplies page and there are a lot of suppliers for hydroponics systems that work great for tomatoes.
I hoping to do a page on tomatoes soon and now I know where to send them for recipes (http://tomato.org).
Hope that helps and thanks for visiting,