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!

On 03/13/2025 9:40 AM EDT Ask Extension wrote:

 

 

Hi John,

I think that some crops might benefit from being started indoors and then brought out to the greenhouse, especially the ones that take longer to germinate or need more time before transplanting- think onions, herbs, eggplant, etc. Other crops might do fine started in the greenhouse IF the weather cooperates and we have temperatures that don't get too cold in April when most seedlings should be started for a late May transplanting. If you have electricity in the greenhouse or have the ability to run an extension cord, you could put your starts on heat mats which go a long way to keeping them at the right temperature. As you mentioned, row cover is always handy to have for most nights early in April or anytime that the temps dip too low.

All that being said, if you have the ability to start seeds indoors and then bring them into the greenhouse later, that is certainly an option. Many crops need a certain temperature to germinate but are then somewhat hardy after they sprout. Brassicas and other leafy greens like lettuce and spinach are a good example of crops that can handle some cooler temps. 

In regard to figs, I would point you to a research article put out by UNH which tested varieties and methods to growing figs in NH. Find the article here. Surprisingly, the plants were all grown in-ground and they tested different winter protection methods. The ones grown in unheated high tunnels with two layers of row cover for the winter fared the best. 

You could keep figs in pots and keep them indoors during the winter. If you let them experience a light frost, they will drop their leaves, enter dormancy and then you can keep them in a dark, cool place until spring. An unheated garage or outbuilding is usually fine. 

The varieties that fared the best in the UNH trial were Ronde de Bordeaux, Takoma Violet, Malta Black and Saint Rita. I would also recommend having a look at Chicago Hardy which is hardy down to zones 5-6 depending on your source. 

I hope this helps!