Seed Starting Indoors

barbQuestion: I plan to grow many of my vegetables from seed this year. I’d like to make my own potting soil. What do you suggest as a recipe for a seed starting mix? Should I buy specialty grow lights? S.R.

Answer: Potting soils for seed starting must have good drainage to help prevent rotting and damping off disease. For years I’ve used a combination of 1/3 peat moss or sphagnum moss; 1/3 compost; 1/3 vermiculite or perlite. I use a five gallon bucket for measuring – one bucketful for each ingredient – and mix everything together in an old rigid plastic children’s pool. After mixing the dry materials together, I stir in a bucket of water. The dry peat or sphagnum needs a couple of days to absorb the water. The mix should well hydrated before planting your seeds.

Most seed starting mixes are soilless but some gardeners prefer soil-based mixes. A combination of 1/3 compost; 1/3 topsoil; 1/3 vermiculite or perlite provides enough drainage for seed starting. Unless you purchase pasteurized topsoil you should pasteurize a soil-based potting mix. Place moist potting soil in a roaster and cover with the lid or foil. Monitor the temperature with a food thermometer in the center of the potting soil, similar to the way you use a meat thermometer in a roast. Put your roaster of soil into a 200 to 250 degree Fahrenheit oven. When the thermometer reaches a temperature of 140 degrees Fahrenheit, start timing. Thirty minutes at 140 degrees will kill disease causing fungi.

You can successfully grow transplants in the natural light of a south or west window, but most gardeners get higher quality transplants using artificial lights to supplement or replace natural light. The key to using artificial light is keeping the light source within two to four inches of the top of the plants.  As the plants grow, the lights should be raised, or the plants lowered, to maintain 2 to 4 inch spacing.  New energy efficient T5 and T8 fluorescents provide plenty of light for seedlings and will not get hot. The new fluorescent bulbs don’t fit in the old shoplight fixtures but a new set-up is relatively inexpensive. If you can afford them, LED grow lights are the best lighting option for plant growth. LEDs produce more light energy per watt and are much more efficient than fluorescent bulbs.


Barb Larson is horticulture educator for Kenosha County University of Wisconsin Extension. Barb has a Master’s of Science in horticulture from the UW-Madison. If you have a plant or gardening question, email Larson at  or call her at 262-857-1945.