I have a rather large fully covered front porch. It is not heated, but I am wondering if there is any way I might use that space as a greenhouse in the spring? It is north facing unfortunately, but gets a lot of light. I have thriving hydrangea limelights right outside the space.
Thanks for your question about using your north-facing covered porch as a greenhouse in the spring.
You don’t provide specifics about your plans for the space.
If it is to start plants from seed, it is likely too cool inside the porch, especially at night, since most seeds require consistently warm temperatures (often between 21 and 27 Celsius) to germinate. They benefit even indoors from a heat mat placed under a tray until they germinate, and after that they need about 16-18 hours of direct strong light from LED lights placed a few inches over the top (but no more bottom heat). Your north-facing porch, even though bright, may not provide enough light.
With larger seedlings of say, tomato and pepper plants, they generally need temperatures above 10C (see link below) to keep growing.
However, cool-season seedlings like lettuces, arugula, spinach and peas can handle some frost after they have germinated. Perennials are also able to tolerate frost and can be transplanted into the ground as soon as the ground can be worked. So your porch could be used for some cool-season seedlings that have already germinated, or to store hardy perennials (see below about tender perennials).
You might also try a space heater to warm up the space (but be aware of fire hazards and your Fire Code). Install a thermometer so you can keep track of temperatures. Since you say it is a large space and space limitations are not an issue, you could add artificial light through grow lamps (you can insert a grow-light bulb – available from home improvement stores – into a regular lamp) or get LED lights suspended on chains and attached to a power bar that includes a timer.
Many tender perennials (plants that grow in Zones 7-10) will survive the winter in an unheated garage, so you could use your porch to store them overwinter in a dormant state. Be aware that your plant will start waking up and needing nutrients and water if there is too much light. You could use some form of cover (e.g. an old blanket or sheet) to block light if you are not ready for the plant to wake up from its dormancy, but ensure there is still some form of air circulation.
The only way to find out what you can do with your space is to experiment.
You may find the following links helpful. Note that cold frames are generally placed in a southern exposure where they will get a lot of light. Your north facing porch most likely does not get enough light (without supplemental light) to be treated as a cold frame, but you may be able to obtain some guidance nonetheless from the links about cold frames, as they may tell you what your limitations (and possibilities) are. The same is true for the links about winter sowing.