CORVALLIS, Ore. – Home gardeners can start vegetable and flower seedlings indoors from four to 12 weeks before the last spring frost in their area. A good way to get motivated for growing seedlings or “starts” is to make up some planting medium at home.
Mixing your own seed-starting mix is more economical than buying sterile potting mix at a garden store.
A good germinating medium must be fine and uniform, yet well-aerated, loose and free of pests, diseases and weed seeds, explained Barb Fick, consumer horticulturist with the Oregon State University Extension Service. The planting medium also should be low in fertility and total soluble salts, yet capable of holding and moving moisture by capillary action.
But beware, Fick warned. Soil straight from your backyard just won’t do the job.
Typical backyard soil is too compacted, full of weed seeds and it is not pasteurized, causing seedling diseases and death. And native soil does not often drain as well as seedling mixes. It can develop a crust that prevents seedlings from pushing though the soil.
Fick’s recipe for a good basic pasteurized soil for growing vegetable seedlings is a mixture of one-third pasteurized soil or finished compost, one-third sand, vermiculite or perlite, and one-third peat moss.
“Many people just use half peat moss and half perlite, vermiculite or sand, and this combination seems to work well, too,” said Fick.
To pasteurize a small quantity of soil or finished compost in an oven, put the slightly moist soil or compost in a heat-resistant container or pan. Cover with a lid or foil. Place in a 250-degree oven for at least a half-hour. Use a candy or meat thermometer to ensure that the mix reaches a temperature of 180 degrees for a full half-hour. Avoid overheating it, as the structure of the soil may be damaged, rendering it useless as a seedling soil ingredient. Sand, vermiculite, peat moss and perlite are available at most nurseries and garden stores.
Another task to complete before the start of seedling growing season is to clean your pots, trays and flats. Then rinse the containers in a solution of one part chlorine bleach to 10 parts water to kill remaining plant disease microorganisms that could weaken or kill your tender young seedlings. Avoid recontaminating your planting medium or tools.