The start of the gardening season is evident when seed packets, potting soil and seed-starter trays make an appearance in stores everywhere. The seed packets have instructions about when to start the seeds, the planting depth and general lighting requirements. The instructions do not explain the mechanisms of the seed-starting trays, including when the lid comes off or ventilation requirements for the seeds.
Remove the seed tray lid for a while every day to allow the propagation chamber to ventilate. The lid traps heat in the soil and creates a humid environment. Too much moisture in the chamber causes the seeds to rot and bacteria to grow. Air circulation for about an hour each day helps keep the humidity level low enough that the bacteria does not breed in the environment. The air circulation also keeps the air temperature in the chamber low enough that the seeds do not cook before they germinate.
When the seeds germinate and start poking through the soil, remove the lid on the seed tray. The clear lid intensifies the light, causing stress to the seedlings due to the increase in temperature. Often, a seed tray is over-planted and the seedlings are in constant contact with each other when they start growing. Removing the lid allows the air to circulate through the newly formed stems and leaves, which keeps mold or fungus from growing where the seedlings are in contact.
A planting tray that is too moist while the seedlings grow can develop a fungus referred to as damping-off disease. The fungus attacks the stems of the seedlings, causing them to fall over and die. After the seeds germinate, removal of the lid reduces the risk of damping-off disease. If you do not use a heat mat for seed germination, there may be times when the air temperature is too cold for the new plants. If the seed tray lid does not restrict the plant growth, you can cover the plants for a few hours until the air is warm enough for the plants.
After you remove the seedlings from the seed-starting tray, clean the tray for the next use. Wash the tray and lid with warm, soapy water and rinse to remove the soap residue. Dip the tray and lid in a solution of one part bleach to nine parts water to sterilize each piece. This removes any bacteria that may have started to grow during the germination process. Allow the seed tray and lid to dry before storing them.