Start Seed Planting Indoors with Oven-Sterilized Soil

Start planting seeds indoors now to take advantage of the entire growing season. Some flowering plants may need to be planted 6, 8 or even 10 weeks before the last frost date for your area. Perennials may not have enough time to flower the first year unless you help them out by starting their seeds early.

Any container can be used for starting seeds that provides at least two inches of soil depth and that has drainage holes. Old containers, flats, cut-off milk jugs and plastic bottles are just fine, provided that you clean them with warm, soapy water. If you want to be really careful, take a few extra minutes to disinfect your containers with a 10% bleach solution (1 part bleach plus 9 parts water).

Put your soil in the containers to a depth of at least two inches. The kind of soil that you use for starting seeds does make a difference. Regular potting soil is too dense and allows for little water drainage. Potting soil or dirt dug up from your garden will work fine if you add vermiculite and perlite in approximately equal parts. As an alternative you can use equal mixtures of garden soil, sphagnum peat moss and sand. The vermiculite, perlite and sand all serve the purpose of increasing drainage, lightening the soil and allowing for good oxygen exchange.

Actually, potting soil isn’t even a requirement. You can mix equal parts of vermiculite and perlite, or sphagnum peat moss and sand, instead. Going a different route, you can try rock wool for starting your seeds. Rock wool is used for plants in hydroponic setups and may not be easily found in your local gardening center.

Sterilization of the soil can improve results significantly. Some folks opt to purchase the more expensive, commercially prepared seed starting potting mixes, which are available in specialty garden shops. Commercially prepared soils are conveniently sterile for starting seeds, but more expensive than doing it yourself.

Sterilizing soil basically involves heating the soil long enough to destroy the disease-causing bacteria and fungi, but not too long for toxins to develop in the soil. You can do it at home in your conventional oven, too.

Porous containers, like clay pots, can be sterilized in the oven the same time as your soil mixture, but make sure to disinfect your clay pots first with a 10% bleach solution.

We can get by without sterilizing the soil, but if you want a better result bake the covered soil at 180-200 degrees for about 30 minutes.

Follow these steps for oven sterilization of soil and containers:

  • Heat oven to 180-200 degrees F.
  • Use metal containers or clay pots to hold the soil for sterilizing. No plastic containers.
  • Fill containers with moist soil mixture no deeper than 4 inches.
  • Cover containers tightly with aluminum foil.
  • Heat at 180 degrees for 30 minutes. Use a meat thermometer poked into the soil.
  • Do not let temperature go over 200 degrees.
  • Allow the soil and containers to cool to room temperature.
  • Leave the foil on the containers until you’re ready to use the sterilized soil.

While the oven is baking your soil, you’ll want to open a window or turn on a fan to get rid of the odors being released!

You can use the microwave for sterilizing small batches of soil but you’ll have to use plastic containers and not metal ones or aluminum foil. The key here is to hold the temperature at 200 degrees for 20 minutes, so if you have a programmable oven you’re good to go. Oh, poke a hole in the lids of your plastic containers to allow steam to escape while the soil is getting zapped.

Now that your soil and containers are ready, let’s plant seeds!

Read the seed packets. Seed packs are usually full of great information that pertains to the species or variety of seed you’re planting. Things like how far apart to plant the seeds and whether the plants do well in full sun or partial shade should be on the seed packet.

Place your planting containers on a tray or in a shallow baking dish that will hold water. When it comes time to water your seedlings, gently pour lukewarm water into the tray or dish. Watering in this manner will not disturb the seeds as watering from the top often does.

Place seeds on the soil and cover with a light amount of your sterilized soil, no deeper than 2-3 times the width of an individual seed. Certain seeds, like the lettuces, do not germinate well with a soil covering. In those cases just press down on the seeds. In the garden we step on the lettuce seeds and they’re planted!

Cover the container with plastic wrap, a plastic bag or a piece of glass in order to retain moisture. Until the seeds actually germinate don’t put the containers in direct sunlight which could heat up the soil too fast and possibly dry it out.

To add a little warmth to the soil you can place the containers on a heating mat or other radiator.

Remove the plastic or glass coverings when the seeds germinate and place under florescent lights. Sunny windows are not sunny every day and those cloudy days can lead to some spindly growth. Place the lights about 3-4 inches above the seeds and raise the lights as the plants get taller. Lights need to be on about 16 hours every day, so use a lamp timer to make this a no-brainer.

Water seedlings from the bottom by adding water to your tray, but remove any standing water after about five minutes. Refrain from fertilizing seedlings until they grow at least two sets of true leaves.