How to Start Seeds for an Outdoor Greenhouse


Get a head start on your garden by germinating seeds indoors and then transferring them to an outdoor greenhouse as seedlings. Use a spot near a sunny window or use grow lamps to provide light. Putting seedlings outdoors hardens them off by exposing them to outdoor sun and temperatures in the protected greenhouse environment. They will then be ready to go into your garden when the danger of frost is passed.

Growing Seeds

Step 1

Use plastic trays with inserts divided into various sizes. Fine seeds like lettuce, begonia and poppy go in 72-cell inserts while larger ivy or petunias grow in the 24-cell dividers. Plant sweet pea, squash or sunflower seeds in individual peat or newspaper pots. They require several inches of soil depth and do not transplant well, so you plant the entire pot. Time your germination periods so the seedlings will be ready to transplant into the garden at the same time.

Step 2

Fill trays with a soil-free planting medium such as sterile Canadian sphagnum moss, which has fungicidal properties. Commercial mixes have limestone and fertilizers that last up to six weeks. Seeds need good moisture retention balanced with good drainage. Sand or vermiculite helps water drain.

Step 3

Moisten the soil before planting and use fresh seeds. Sow very fine seeds by folding a piece of paper in half and using the crease to contain them while you push them in place with a finger or pencil. Cover them with a very light dusting of sand or soil. Poke holes in the soil to the recommended depth for larger seeds and cover with soil. Keep the soil warm in a cool basement or garage with an electric warmer mat underneath the trays.

Step 4

Set the pots and trays in a sunny space or under grow lights. Cover with plastic. Conditions are too wet if the cover is cloudy or dripping with water. Remove it as soon as the seeds sprout. Keep the soil moist, not wet. Test this by squeezing a bit in your hand. If it clumps together in a ball, it is moist; if water drips out, it's too wet.

Step 5

Use a China marker pen to label plastic pots and trays or a permanent marker to make labels for peat or newspaper pots. Attach the seed packet to identify trays, but mark individual pots so you'll know what's in them if they get misplaced.

Step 6

Transplant plants grown in small cells to larger pots within a few weeks of germination. Move the seedlings to the outdoor greenhouse when they have three adult leaves. For the first few days, protect the seedlings from the sun. They will adapt to outdoor light quickly in the warm greenhouse.

Tips and Warnings

  • Seedlings drown in soggy soil or are killed by a fungus, called damping off, that grows in moist conditions.

Things You'll Need

  • Peat pots/plastic trays
  • Inserts
  • Germination mix
  • Seeds
  • Labels


  • University of Kentucky College of Agriculture: Starting Plants From Seed at Home
  • Ontario Hort Matters: Damping-off of Vegetable Transplant Seedlings
Keywords: grow seeds indoors, germinating garden plants, moving seedlings outside

About this Author

TS Owen spent her career in journalism, winning the national Koop science writer award and penning articles in "Newsweek" and the "San Francisco Chronicle." She also served as an editor for a variety of publications in the San Francisco Bay Area and Banff, Alberta. Owen has a master's degree in English education.