A greenhouse is a wonderful way to start spring and summer vegetables and get a jump on the early season. Greenhouses can also extend the growing season well into the fall. In warmer regions, you can continue growing summer crops right through the year with greenhouse protection. In all but the very coldest hardiness zones, you can grow cold-hardy leaf and root crops in the greenhouse for harvest in fall and winter if you apply proper timing and planning and utilize passive heating methods to fend off the worst of the cold weather.
Hang shade cloth on the inside of the greenhouse on the sunny side ten weeks prior to your average last frost date, to cast shade on one section of your potting benches for starting cold-weather crop seeds.
Fill 4-inch pots with potting mix. Water lightly, and plant small to mid-sized leaf crop seeds (spinach, lettuce, endive, mache) in the 4-inch pots according to seed packet directions for depth and spacing. Place the 4-inch pots and trays and set them on the potting benches in the greenhouse where they will be shaded by the shade cloth.
Mix equal amounts of compost and potting medium by volume, and fill half-whiskey barrels or large plastic tubs with this mixture. Fill the half-barrels or tubs in place, as they will be heavy to move after filling. Water the mixture lightly. Plant larger cold-hardy crops (brussels sprouts, broccoli) and root crops (turnips, beets) in these half-barrels or tubs, following seed packet directions for depth. Plant the seeds to the recommended final spacing rather than more closely-spaced recommendation, to avoid thinning if possible.
Water the planted seeds lightly as often as the climate conditions require to keep the potting medium moist but not saturated. Remove the shade cloth when the seedlings reach 4 inches in height; monitor the greenhouse temperature and open its doors and vents if early autumn days generate temperatures over 90 degrees Fahrenheit.
Plant a second round of all seeds four weeks prior to your average last frost date. Plant a third round two weeks after your average last frost date if you are in hardiness zone 7 or southward.
Spread a 6-inch layer of woodchips over the floor of the greenhouse about two weeks before your average last frost date, using a bow rake. Water the wood chips until they are moist but not saturated.
Paint milk jugs black using black spray paint. Fill the jugs with water. Stack concrete blocks and lumber planks to create shelves on the northerly or westerly side of the inside of the greenhouse. Stack the black-painted, water-filled milk jugs on these shelves.
Fill 1-gallon nursery jugs with mixture of potting medium and compost. Transplant seedlings from 4-inch pots into 1-gallon nursery pots when they have developed two sets of true leaves. Place nursery pots in trays on potting benches.
Water the fall greenhouse plants on warm days. Water carefully into the soil, without getting water on the leaves. Once a week, add a dilute solution of fish emulsion fertilizer to the watering.
Harvest fall-grown leaf crops when they reach the early stage of maturity, with head sizes about half the size you would grow them to in the spring or summer garden. Harvest fall-grown root crops at the 'baby' stage, about 2 inches to 3 inches in diameter for beets and turnips and about 3 to 4 inches long for carrots.