Gardening does not have to stop in winter. With a bit of planning, using heat-sinks like painted milk jugs and passive heat sources like decomposing wood chips, you can grow cold-hardy vegetables can grow all year round in your greenhouse even in harsh climates.
Stack concrete blocks in pillars about 36 inches high and lay board planks across them to create a raised bench for your winter vegetable growing.
Spread a 6-inch layer of wood chips on the floor of the greenhouse with a metal rake when outdoor nighttime temperatures drop below 40 degrees Fahrenheit. Place a 5- to 10-gallon sized pile of extra woodchips in each corner of the greenhouse or under the growing benches if space allows. Water the woodchips weekly.
Paint plastic milk jugs with black paint. Fill milk jugs with water and set them in inside the greenhouse walls. Alternatively use concrete blocks and wood plants to construct temporary shelving up the north wall of the greenhouse and fill the shelves with tightly-spaced, painted, filled milk jugs.
Mix one part compost and two parts potting medium. Water the mixture and fill seed trays and 1-gallon nursery containers lightly with mixture using a trowel.
Select cold-hardy species and varieties for cost-effective winter vegetable growing in a greenhouse: kale, spinach, endive, mache, miners lettuce, radicchio, radishes, turnips, kohlrabi, and others which can tolerate a light freeze. Avoid heat-loving vegetables which require more than 6 hours of sunlight, such as tomatoes, peppers, eggplants, and melons, unless your greenhouse is located in a warm climate, zone 7 or southwards.
Plant seeds for cold-hardy leafy winter vegetables like kale, endive, spinach, and corn salad in seed trays beginning 6 weeks prior to your average first frost date, following seed packet directions for seed depth and spacing. Plant seeds cold-hardy root vegetables like radishes, turnips, and kohlrabi in individual 1-gallon nursery containers, again following seed packet directions for depth and spacing.
Water with rainwater as necessary to keep soil from drying out. Add liquid soluble fertilizer once every other week.
Stand bamboo poles up between seed trays and pots. Drape floating row cover over the bamboo poles, so that it sits above the emerging vegetable plants but not touching them. Lift cloth to water and tend to plants then re-cover the plants.
Throw space blanket or insulated blanket over the greenhouse on the coldest nights to avoid hard freeze, if the greenhouse is small enough to cover. Remove the blankets when sun reaches the greenhouse. Avoid opening the greenhouse door unnecessarily when daytime temperatures remain below 10 degrees Fahrenheit.