CORVALLIS, Ore. — While winter lingers, gardeners can mix their own sterile planting medium and get ready to start seeds indoors.

Home gardeners can start vegetable and flower seedlings indoors from four to 12 weeks before the last average spring frost in their area, which means planning can begin now. Making planting medium at home can be more economical than buying a sterile mix at the store, said Brooke Edmunds, a horticulturist with Oregon State University’s Extension Service.

Edmunds said a good germinating mix must be fine and uniform, well-aerated, loose and free of pests, diseases and weed seeds. It also should be low in fertility and total soluble salts, yet able to hold and move moisture.

Why backyard soil won't work

Typical backyard soil is too compacted, full of weed seeds and not pasteurized, which can lead to seedling diseases and plant loss. Native soil often does not drain as well as seedling mixes and can form a crust that seedlings cannot push through.

A basic mix to try

Edmunds recommended a simple formula for a pasteurized medium:

• One-third pasteurized soil or compost
• One-third sand, vermiculite or perlite
• One-third coconut coir or peat moss

“Many people just use coconut coir or peat moss (both have environmental consequences) and half perlite, vermiculite or sand, and this combination seems to work well, too,” she said.

How to pasteurize soil or compost

To pasteurize a small amount of soil or compost in an oven:

• Moisten the soil or compost slightly.
• Place it in a heat-resistant container or pan and cover with a lid or foil.
• Put it in a 250-degree oven.
• Check the temperature with a candy or meat thermometer.
• When the mix reaches 180 degrees, keep it there for 30 minutes.
• Avoid overheating because it can damage the soil structure.

After pasteurizing, mix the soil or compost with peat moss or coconut coir in small amounts, then add sand, vermiculite or perlite. These ingredients are available at most nurseries and garden stores.

Clean containers before sowing

Before sowing seeds, wash pots, trays and flats. Then rinse them in a solution of 1 part chlorine bleach to 9 parts water to kill plant disease microorganisms that could weaken or kill young seedlings.

Learn more

For more information about starting seeds, consult the Extension publication Propagating plants from seeds.

Previously titled Set seeds on the right path with homemade planting medium