Thanks for writing. It largely depends on your climate, the type of crops you want, and how well insulated the greenhouse is. In your area, if the greenhouse is well insulated, you should have no problems growing cool-season crops such as lettuce, spinach, and other greens.
If you'd like to grow warmer-weather crops than that, you may need to invest in a greenhouse heater. Putting in a brick floor in the greenhouse may also help a little; on sunny days the bricks will absorb heat from the sun and then radiate it back out as the temperature cools.
---Justin, Senior Garden Editor, BHG.com