University of Illinois Extension


Barbara Larson
Unit Educator, Horticulture
Boone and Winnebago County Units

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Seed Starting

By the middle of February many gardeners are getting cabin fever and can’t wait to get outside into their garden. A good way to get over the winter hump is to start flowers and vegetables from seed indoors for later transplant. Growing plants from seed is inexpensive, easy, and efficient. The biggest advantage is a much wider variety of plants and cultivars are available from seed than from ready to buy transplants.

Materials needed to begin seeds indoors are not expensive or elaborate. The minimum supplies required are: clean containers with drainage holes (pots, flats, peat pots, plastic cups, or the bottom of milk cartons with drainage holes punched in), clean potting soil or seed starting mix (do not use garden soil from outdoors), light (either natural or shoplight with new cool white florescent bulbs), and water.

Transplants can be successfully grown in the natural light of a south or west window, but most gardeners prefer using artificial light to supplement the natural light. Artificial light alone is also very effective. The key to using artificial light is keeping the light source within two to four inches of the top of the plant. As the plants grow, the lights should be raised to maintain the 2 to 4 inch space between the plants and light. Cool white fluorescents provide adequate light for seedlings and will not get too hot. Make sure you purchase new fluorescent bulbs each year because fluorescents dramatically decrease in light output as they age.

Most germinating seeds and young seedlings prefer air and soil temperatures of 65 to 75 degrees Fahrenheit. The temperature should be lowered 5 to 10 degrees after the first set of leaves appears. Reduced temperatures promote shorter stockier plants.

Plants with a long interval between planting and flower or fruit production, such as impatiens, broccoli, and tomatoes, are ideal candidates for starting indoors.

Before starting seeds indoors planting date should be calculated based on optimal transplant time. Figuring the date to start seed takes a little arithmetic or, for the math impaired, counting weeks on the calendar.

Base your calculations on the average last spring frost date, which is April 25 in northern Illinois. [Editor’s note: Be aware that this date is an average, which means that 50 percent of the time , the last frost comes after this date. In recent years, it has been common to have frost even in mid-May. Take this into consideration when timing planting]. Southern Wisconsin gardeners or Illinois gardeners in colder pockets should use May 5 as their average frost date. Cautious gardeners use May 5, so they don’t have to worry about a late frost. Keep in mind that the plant’s tolerance to cold temperatures determines transplant date. For example, tomatoes are damaged by frost and must be transplanted on or after the frost-free date. In contrast, broccoli are very cold hardy and may be transplanted outside 4 weeks before the average last frost date.

Check the seed packet or catalog for the recommended sowing time and transplant time. Starting from the outdoor transplanting date, count back the number of weeks the plant will need to germinate and grow to the proper size in order to get the seeds sowing date. Here’s an example. Tomatoes need five to eight weeks to germinate and grow to transplant size. Counting back or subtracting seven weeks from the last frost date of April 25 gives a start date for tomato seeds of March 5. If you think late April is too early to put tomatoes outside, then push the seed sowing date back a couple of weeks to produce May transplants.

Plant the seeds in pre-moistened potting soil at the depth listed on the seed package or, in lieu of directions, at a depth two times the seed’s diameter. Because most people have difficulty thinning out extra plants, the seeds should be spaced an adequate distance apart to give the seedlings room to grow. (An easy way to thin seedlings is to cut off the extra plants at the soil line using small scissors.) Many gardeners cover the newly sown flats or pots with plastic until the seeds germinate so that moisture is maintained in the potting mix.

Seedlings should be watered sufficiently to keep the potting mix evenly moist, but not wet. Wet soil creates two problems: cold that inhibits germination and root growth, and dampness that is a good medium for fungal disease development.

Quarter to half strength fertilizer at should be added with the water once a week after seedlings have true leaves. Use a complete fertilizer with nitrogen, phosphorus, and potassium. Excess fertilizer will burn tender roots and promote weak spindly growth.



February - March 2004: Seed Starting | It’s Never Too Early to Prepare to Compost | Season Extenders | Winter Damage to Home Lawns

Past Issues

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