A greenhouse extends the growing season beyond the average date of last frost in the spring to first frost in the fall. That extension could be a month on either side, such as March and October, if the greenhouse doesn't have a built-in heater. Heated greenhouses allow plants to grow throughout the year. Even the most frost tender plant can be grown in a greenhouse, states Charlie Ryrie in his book, "Country Gardens." Keep your greenhouse set at the temperature and humidity levels that your plants prefer. Tropical plants need a warmer, moister environment while vegetables do well with cooler temperatures.
Sterilize used potting soil by spreading it in a baking pan and placing in a 200 degrees F oven. Turn off the oven and let the soil set for 30 minutes. It should smell earthy, but not nasty, as it gets hot. The sterilization kills the fungus that causes dampening off disease. As an alternative, you can use new potting soil.
Fill the pots with the potting soil to within 1/2 inch of the top. Place seeds three to a pot equidistant from each other and the sides of the pot. Cover with 1/4 inch of soil. The exception would be large seeds, like beans and peas. Plant them 1/2 inch deep. Water until the soil is saturated. Then keep the soil moist but not soggy. Place the pots on a tray and the tray in a part of the greenhouse that receives bright light.
Thin to the strongest seedling in each pot after the seeds have sprouted and reached 4 inches tall. If the light in the greenhouse isn't strong enough, the seedlings will start to grow spindly and leggy. If that's the case, place the seedlings under grow lights, but keep them in the greenhouse. Transplant to bigger pots when the seedlings are 6 inches high and again when they're 12 inches high.
Adjust the temperature inside the greenhouse to the level the plants prefer. Some, such as orchids, prefer temperatures during the day of about 75 degrees F, according to Gustav Schoser, author of "Orchid Growing Basics." Other more tropical plant varieties prefer warmer temperatures in the 80s.
Control the temperature by shutting off or lowering the heating source, or venting the roof in the greenhouse. Hot air rises. Opening the roof vents allows the hot air to escape to be replaced by cooler air coming in through the windows. Increase the heat by turning up the thermostat on the heater.
Wrap black plastic around 5-gallon water jugs and place them where they're in the sunlight. Black absorbs heat. The water will heat up and slowly release warmth back into the greenhouse during the evening and night.
Increase air circulation with fans or by taking advantage of natural breezes by opening windows on opposite sides of the greenhouse.
Keep humidity between 75 to 80 percent for tropical plants. A hygrometer measures relative air humidity. Increase the humidity with a misting system, placing the plants on trays filled with rocks and water or adding a humidifier to the greenhouse.
Light the greenhouse with sunlight coming through the clear panels of the roof. During fall, winter and early spring, supplement waning sunlight with grow lights.
Inspect the plants for signs of insect infestation. Take prompt action.