Hello John,

Thanks for reaching out to the UNH Extension Yard and Garden Infoline. It sounds a little bit like you've put your cart before the horse! That's OK though because having a greenhouse allows you to do many things and so you might want to experiment with how it best fits your needs. 

Starting seedlings is a great way to use an unheated greenhouse in our climate. First, I would utilize this chart to help you know about when you'll want to plant seedlings outdoors. I suggest working backwards from the planting date to tell you when you should be starting seeds in your greenhouse. We also have a resource on seed starting basics which might be handy. 

Keep in mind that even if you're starting seeds as it begins to warm up, your seedlings may still need a bit of cold protection at night like row cover or a light sheet or blanket draped over them to add a few degrees of warmth. 

Depending on the setup of the greenhouse, another thing that they can be good for is growing crops that prefer a warmer climate than we have here in New Hampshire. Think peppers, tomatoes, cucumbers, ginger, turmeric, figs, amaranth, etc. This requires either growing in containers or growing in the ground inside your greenhouse. You might find that tomatoes or peppers that grow acceptably outdoors might do especially well when they're grown in at greenhouse given the higher temperatures. Airflow is absolutely essential though, so keep door(s) open on really warm days and perhaps install a fan for better circulation. Tomatoes and some vining crops are particularly susceptible to powdery mildew. 

Greenhouses are great for extending the season if you plant crops inside. It will give those warm-weather crops the extra time and temperatures they might need to really produce, but it also can allow you to plant a spring crop earlier than outdoors. Think hardy crops such as spinach, lettuce, radishes, beets and brassicas like kale, broccoli, Brussels sprouts, cauliflower, cabbage and others. Furthermore, you could also experiment with a planting a fall crop of the same cold-hardy crops.

You might also experiment with not only season extension for cold weather crops, but trying to grow throughout the winter with those same hardy leafy greens. 

I hope this is helpful and please ask more questions if you have them!