We do not have greenhouse-specific cultivation or crop-planning information for Maryland since they are relatively rare in home gardens, but can share some thoughts and resources below.

Greenhouses in the broad sense are varied structures built to accomplish different things, from protecting semi-tender or mostly-hardy plants from the brunt of winter weather (like a hoop house or cold frame does) to housing tropical plants intolerant of cool temperatures (like a heated conservatory). Your goals for using a greenhouse in relation to growing edible plants will dictate how suitable the type of structure you have is for the plants you want to cultivate. For food gardening, goals can range from simply starting vegetable transplants to get a jump-start on the growing season outdoors, to season extension (to keep producing a harvest just a bit longer into the autumn), to year-round food growing, to overwintering (or housing year-round) edible tropicals or half-hardy species like citrus, pomegranate, less cold-hardy fig varieties, and so forth.

The resources linked below from other Extension programs primarily discuss greenhouse construction and choice of design, but also do mention some considerations when it comes to heating, shading, and humidification, if applicable to your situation. There will probably be a lot of content overlap between them.

• The Hobby Greenhouse -- Oklahoma State University
• Hobby Greenhouse Design Notes [PDF]
• Hobby Greenhouses -- University of Georgia Extension
• A Small Backyard Greenhouse for the Home Gardener -- NC State
• Hobby Greenhouses -- Clemson Extension

If your goal is increased food production by way of shielding crops from cold in early spring and late autumn, it might be simpler to use a small high tunnel or hobby greenhouse. Either one can be a DIY project or purchased as a kit. In comparison, a more multi-purpose greenhouse, being more permanent, will need to be ventilated with a fan and probably heated. In addition to the resources below, you can also find how-to videos by searching the web for "garden" plus "high tunnel" along with search results narrowed to ".edu" pages for more science-based recommendations and approaches.

• Starting a Garden: Season Extension - Illinois Extension
• Extending the Garden Season with High Tunnels - Penn State
• High Tunnel Construction [PDF] - Mississippi State Extension

The simplest use of a greenhouse would be to consider it a large "cold frame," which are unheated structures meant to shield young seedlings and transplants from the brunt of the cold while allowing them to acclimate prior to planting outside. Otherwise, you can heat a greenhouse to accommodate cold-intolerant plants, but that tends to be expensive depending on how well-insulated it is along with the type of heat source. (We don't have information on local heating costs for the various heat source options.) Seed starting may also benefit from supplemental artificial light, depending on the plants involved and what time of year the process is begun, and this can be difficult to provide in a greenhouse where temperatures and high ambient humidity may impact the performance of the light fixture.

Some half-hardy edible plants prefer to remain cooler in their dormancy than others, which are adapted to stay more mildly warm and may grow for most of the season as long as there is sufficient light. For example, most Mediterranean herbs want a cool period with above-average humidity for winter.

One other aspect to keep in mind is that natural daylight in the winter is not long enough or intense enough here for most plants to continue growing, including plants grown for harvesting foliage (like salad greens and herbs) or fruit (like tomatoes). So, while plants may stay alive when the right temperatures are provided, they tend to stagnate in size and not actively grow (or may grow very slowly) until spring. For plants that don't have very high light requirements, the use of artificial light to supplement natural light might allow them to grow or bloom during this time, but it will be difficult to supply plants requiring strong light (like tomatoes) with enough light energy and daylength for them to continue bearing crops. Plus, pollination becomes complicated for fruit-bearing plants that require pollen transfer between flowers. This can be done by hand (like with citrus) but on larger or dense plants, that can be tedious or difficult to do well.

Examples of crops you might be able to plant in the greenhouse in October (or perhaps earlier), which will grow a bit and then stagnate until spring, include leafy greens like spinach, arugula, kale, and corn salad. There may be Extension web pages outside of UMD that cover hobby greenhouses and growing vegetables in them.

If you grow dwarf potted citrus trees, you can overwinter them in a cool greenhouse, and several of the cold-hardy Mediterranean herbs might do ok, though pest outbreaks are a risk. Spider mites, aphids, thrips, mealybug, scale, and whitefly are common greenhouse pests; mites in particular thrive in the lower-humidity environment of winter greenhouse enclosures. In that case, beneficial insect or mite releases might be of use to keep pest numbers suppressed, or you'd need to treat plants repeatedly after a population appears with either horticultural oil or insecticidal soap in a product formulated for use on edible plants, so that can factor into any operating cost tabulations in addition to heat and perhaps grow lights.

Cold-sensitive herbs like basil are not a good choice unless the greenhouse is maintained at temperatures closer to subtropical, such as those needed for tropical houseplants. Bear in mind as well that many herbs require excellent soil drainage, and cooler temperatures slow the rate at which soil moisture evaporates or gets used by the plant, so it could be easier to accidentally over-water them. (Using unglazed terra cotta pots instead of plastic pots can help speed evaporation.) Additionally, plants would need very little fertilizer since they aren't generally in active growth. The minimal or lack of active growth during winter will mean that harvesting herbs won't necessarily result in regrowth to replace the cut foliage until spring, so this could limit how much of a harvest is possible.

All of this information is not to suggest that you shouldn't bother experimenting to see what works, but we want you to be prepared for potential challenges.

Miri

You're welcome - glad it was helpful!