Greenhouses provide ample space for starting home or market garden seeds, but greenhouse seed starting has its challenges. Seedlings need warm soil temperatures and considerable amounts of light to sprout and grow stout transplants, so supplemental heat and artificial lighting are likely to be required for early seed starts in zones 7 and northward. Proper humidity and ventilation also are necessary for successful seed starting in your greenhouse.
Spread a 6-inch-thick layer of wood chips on the floor of the greenhouse using a metal rake, if you are starting seeds at a time and location where outdoor nighttime temperatures are below 40 degrees Fahrenheit. Paint plastic milk jugs black, fill with water, replace caps, and set along the inside edge of greenhouse walls if additional heat is required. Install an electric greenhouse heater if wood chips and milk jugs do not keep nighttime temperatures inside the greenhouse over 40 F.
Stack concrete blocks into pillars 36 inches high, 4 feet, 6 inches apart. Lay plank boards over block pillars for shelves. Stack a second tier of blocks over the boards centered over the pillars, to a height of 24 inches from the shelf. Place additional plank boards over these for a second shelf above the first.
Hang florescent light fixtures from their chains from the upper shelves. Place 1 cool and 1 warm florescent tube in each fixture. Plug the fixtures into a multiplug extension cord. Turn all the lights on and leave them on 24 hours a day until seeds germinate.
Open a bag of soil-less seed-starting medium and add water. Let the water soak in to the medium for several hours. Fill 4-inch pots and 2-inch cell packs with damp starting medium using a trowel; do not compact the medium, just level it off at the top of the pots with the edge of the trowel.
Place the pots and cell packs in trays and set them on shelves under the lights. Adjust the height of lights to be just over, but not touching, the surface of seed-starting medium. Let the pots and cell packs sit for several hours to warm the soil before planting.
Slide one tray at a time out from under the lights and plant seeds according to directions on the seed packet as to depth and spacing. Use 4-inch pots to start peppers, eggplants, tomatoes, squash, perennial flowers and other large plants; use 2-inch cell packs for salad greens and annual flowers and bedding plants.
Use masking tape and a permanent marker to mark each tray as to its contents. Water lightly, then place a sixteenth of an inch layer of sand on the surface of the seed-starting medium. Slide each marked, planted tray back under the lights as you finish with it.
Monitor greenhouse climate to maintain warm, but not overly hot, temperatures. Turn on your electric greenhouse heater or add more black-painted milk jugs if temperatures threaten to go below 40 F. Open the greenhouse doors and any vents when daytime temperatures in the greenhouse exceed 60 F. Install and turn on electric box fan or barn fan if temperatures in the greenhouse exceed 85 F.
Maintain high humidity, but provide adequate ventilation. Water the wood chips on the floor, or the dirt floor of the greenhouse if you did not use wood chips, if the air seems dry and the seed trays are drying out frequently. Water the seed trays every third day by sliding them out from under the lights and adding rainwater.
Once the seeds sprout, connect lights to a timer and set it to run lights 18 hours per day. Continually adjust the height of the lights so that the tubes sit just above the top of the plants.
When seedlings have their first set of true leaves, begin to cut back the amount of time the lights are on by one hour each week. Fertilize the seedlings weekly with diluted liquid fish emulsion.
When seedlings reach 6" high and danger of frost has passed, stop using florescent lights and set seedlings out in full sun for one hour per day for three days, then increase by two hours each day for four more days. Transplant the seedlings to a garden location.