Gather Your Equipment
You'll need a few things to start seeds indoors or in a greenhouse. Have everything set up ahead of time to make seed starting easy.
Containers. You can purchase plastic flats that can be reused year after year, although they should be cleaned in a diluted bleach solution. You can also reuse containers like egg cartons, yogurt cups, or small plastic pots. If you are using plastic cells within the flats, use smaller cells for smaller seeds/plants and larger cells for larger, fast-growing plants.
Potting mix. Seeds are ideally started in a soilless mix or a compost-based potting mix. Get something designed for seed starting.
Seeds. Not all seeds need to be started indoors or in the greenhouse. Some can be direct sowed in the soil outdoors. Generally, you'll start squash, tomatoes, peppers, and melons indoors. Herbs and flowers also can do well with being started and transplanted. Follow the guidelines for each variety.
Labels. You will definitely want to label everything clearly.
Light source and shelving. If not in a greenhouse or high tunnel, you will need to start seeds on a table or shelf with fluorescent lighting to help your plants grow. Plants need about 14 hours of direct sunlight to grow properly.
Prepare Potting Mix
Put your potting mix into a large container, and loosen it with your hands or a spade. Use water to dampen the mix while loosening. It should be moist but not soggy and there should be no dry lumps. When you pick up a handful, it should stick together fairly well, but slowly break apart.
Fill Planting Container
Fill your planting tray and cells with the mix. Pack soil in gently to reduce the air space, but not too dense. Fill to just below the lip of the tray, so water sinks down into the mix instead of flowing over the top.
Plant the Seeds
1. Make indentations in the soil for your seeds. As a general guideline, plant seeds roughly twice as deep as the diameter of the seed (or follow seed packet information). Put one to two seeds in each hole. You can always thin the weaker seedling later by pinching or snipping it at the base of the stem.
2. Cover the seeds with additional potting soil. Again, don't let soil get too close to lip of tray.
3. Water the seeds in gently, sprinkling carefully.
4. Cover each tray with a propagation dome (a clear plastic cover that usually comes with propagation trays). The propagation dome helps to hold in moisture. Remove the propagation dome once seedlings emerge. If you don't have one, drape clear plastic over whatever containers you are using.
5. Label your seedlings carefully!
Care for Your Seedlings
Keep the fluorescent lights on at least 14 hours a day.
Keep your plants warm and free of drafts. The air temperature should be between 65 and 75 degrees F. Plants like peppers and tomatoes like a little more heat, so you can use a heat mat under the tray to keep them warmer. This will also dry them out faster; water more frequently.
Moisten with a mister or sprayer as needed so the seeds don't dry out. "Moist but not wet" is the level of moisture you're looking for.
When seedlings emerge, remove the propagation dome or plastic and make sure to get them under lights.
Potting Up and Hardening Off
Once the true leaves have emerged, your seedlings can remain in their original containers, but if you know it will be a while before the garden is ready for them and they're beginning to get big, you can "pot up" your plants by moving them into a larger pot. Seedlings should be a couple of inches tall and have several sets of true leaves before potting up into three- to four-inch pots.
Once temperatures are warm enough, your hardy, healthy young plants are ready for the cooler temperatures, direct sunlight, wind and other elements of the outdoors - but first you must harden them off. The hardening off process takes three to five days. You can put them on the porch or in another shady spot for three to four hours the first day. Make sure they are protected from the wind and bring them in if temperatures dip. You can increase the number of hours and amount of sun each day. After transplanting during a cooler, shady part of an overcast or gently sunny day, water them well.
