Determine when to plant seeds. First determine the date of the average last frost in your growing zone. This information is available through your local university Cooperative Extension. Check the seed packet for the number of weeks before the date of the average last frost that seeds should be started inside. Using this information, calculate a date to plant each packet of seeds.
Check the seed germination conditions on the back of the seed packet. Some seeds can germinate in cooler temperatures (as low as 45 to 50 degrees F), while others require warmth (no lower than 65 degrees F). Some seeds require darkness to germinate; others require light. Consider investing in a heating mat to place below seed trays to keep them warm, and mitigate fluctuations between day and night temperatures.
Set up your plant or aquarium lights when it is time to plant the seeds. Lighting should hang no more than several inches above the seedlings at all times. Hang the lights from chains to make it easier to adjust the height.
Use a timer to automatically turn the lights on and off, to ensure that seedlings get enough light. Most seedlings require about 16 hours of light per day. Placing seed trays near window can increase their exposure to light during the day, but it is essential that overhead lights are timed to extend the daylight hours further into the night.
Fill your trays with small biodegradable paper cups and fill cups with soil-free seed starting mix to within 1/4 inch of the rim. Or use re-hydrated pots pre-packed with soil-free seed starting mix. Both of these items are available at plant retailers. Do not use garden soil to start seeds inside; it is too dense for seedling roots to penetrate. Moisten the seed-starting mix well. Excess water in bottom of tray is fine; it will soak up through cups into mix.
Plant the seeds according to the instructions on the seed packet. Use your finger to poke holes in the soil-free mix at the depth required. Drop seeds in the holes, distribute more soil-free mix over the seeds, and tap down to ensure good contact between the seeds and the mix.
Label the trays according to the variety of seed and the date planted. Keep the seed packet nearby, as it contains helpful information you will need when transplanting seedlings outdoors.
Loosely cover the tray of planted seeds with plastic wrap. If using a seed-starting kit, cover with the clear plastic top included with the kit. When using plastic wrap, ensure that air can circulate beneath wrap; leave the ends loose, or prop with toothpicks.
Raise the plastic wrap as the seedlings grow by using larger props, such as wooden chopsticks or thin wooden rods. When using the clear plastic top, it can be helpful to prop open a crack, to ensure air circulation. Both methods help seeds stay moist, creating a greenhouse atmosphere. However, too much moisture can be harmful, so check the seeds frequently and occasionally remove the cover altogether for several hours to increase air circulation.
Keep the seeds moist by misting them with a spray bottle. Regular watering can can disturb the seeds, even washing them out of the containers. A spray bottle distributes moisture more gently. Do not let the seeds dry out. Especially with overhead lighting, which encourages evaporation, some seeds may need to be misted more than once a day.
Watch for green shoots. Some seeds will germinate beautifully; others will disappoint. Care for your successes by maintaining moisture. When seedlings are 2 inches tall, occasionally brush them lightly with your hand to increase stem strength. This mimics the effects of wind in an outdoor setting. Check the seed packet for instructions on when to begin the process of planting outdoors.