Soak all seed trays with 10 percent bleach solution for 15 minutes. This isn't necessary with newly purchased seed trays, but is a necessity for reusable seed trays to disinfect the plastic to remove disease and insects. Wash each tray with dish detergent and rinse completely.
Fill each seedling tray pocket with potting soil up to 1/4 inch from the pot rim and moisten lightly with a spray water bottle. If you're using peat pellets, follow instructions to moisten each pellet to expand the growing medium. After expansion, tear open the top netting and score the peat with your fingers to loosen the growing medium. Place the pellets in a container for easy transport.
Designate one container per type of flower or make labels to identify which plant occupies a seedling container. Cut the top edge of the seed packet to retain important growing information. Read the seed packet for specific seed instructions. Pour the seeds into a clean, dry paper cup.
Use a cotton swab to capture one or two seeds at a time for placement in the growing medium or peat pellet. Dump larger seeds into your hand and place in seedling trays with your fingers. When using potting soil, sprinkle a light 1/4-inch layer of potting soil over the seeds. For peat pellets, tuck loose peat around the seed after placement in the pellet. Soil must be in contact with the seed to germinate.
Mist the seeds lightly with the water bottle and cover with the clear seedling tray dome. If you're using other containers such as egg cartons or plastic pots, cover each container with a loose fitting section of plastic wrap to retain moisture and warmth in the container. Place the seed trays in a warm, 70-degree F location. Seeds need warmth---not light---to germinate.
Mist the soil regularly and do not allow it to dry out. Healthy seedling should appear in seven to 10 days for annuals or 14 to 21 days for perennials. Remove the plastic cover as soon as seedlings appear. Continue watering to keep soil uniformly and consistently moist, and provide bright light from a window or growing light to strengthen the little plants.
Transplant seedlings to larger pots when two sets of healthy leaves appear on the plant. Give the seedlings room to grow with 3 to 4 inches of space between each plant. Monitor moisture levels regularly and don't allow the soil to dry out. Let the plants mature for a few weeks until they reach 4 to 5 inches high.
Harden off the seedlings by placing the containers in a sheltered location outside after the last frost. Protect the plants from harsh sun and heavy rain. Gradually introduce the seedlings into the sunshine over the next three to four days. Hardening off helps plants adjust to the extreme temperature changes experienced in the outdoor environment.
Dig small holes in your garden to place the seedlings. Carefully remove each seedling from the transplant pot by capturing the soil around the plant stem. Do not pull by the plant stem. Set the seedling into the soil at the same depth as the growing container. Fill in around the plant with garden soil.
Mix a solution of half strength water-soluble fertilizer in the watering can. Apply liberally to the seedling to encourage further foliage growth and flower bud production. Feed the plants regularly with full-strength fertilizer after two to three weeks to induce blooming.