Starting seeds in a greenhouse is a great way to get a jump start on your summer garden. By allowing plants to germinate in the greenhouse versus planting directly in the garden, you allow yourself to have an earlier and possibly larger harvest. Generally, seeds should be started in a greenhouse four to six weeks before the average last frost in your area. Last frost dates in Virginia vary from early April to mid-May depending on location.
Set up benches in your greenhouse to put your seed trays on. Use a concrete block on each end with a length of 2 by 12 board between the blocks for an easy and inexpensive bench.
Fill your seed trays three-quarters of the way full with a high quality potting mix.
Plant your seeds in the seed tray. Seeds should be planted a depth that is equal to their width. Cover the seed with a light top dressing of potting mix or compost.
Water the seeds with a frequency that keeps the soil moist but not soggy. A good guide is water little, but often. Germination times will vary depending on the type of seed but generally most plants are up in a few weeks.
Ensure that your greenhouse doesn't fall below freezing at night. Use supplemental heating to keep the temperature up on cold nights. Place 55-gallon drums filled with water throughout your greenhouse. You should use 2 1/2 gallons of water per square foot of wall and ceiling space. These will moderate the temperature swings in the greenhouse.
Vent your greenhouse on warm, sunny days. Some greenhouses come with automatic venting systems that will vent out hot air when a certain temperature is reached. Most home hobby greenhouses do not have this feature though, and you must manually vent it by opening the vents or greenhouse door.
Provide a fan for air circulation in your greenhouse. Box fans work great for this. By keeping the air moving you will significantly reduce the chance of fungus and disease harming your plants.
Plant your seedlings outside once the danger of frost has passed. Plant them in your garden as deep as they were in their pot. Water them in thoroughly and enjoy an earlier harvest.