Ask the ISU Extension Experts about Starting Seedlings Indoors

Will a fluorescent light fixture provide sufficient light for seedlings? 

A standard fluorescent shop fixture containing two 40-watt tubes will provide sufficient light to grow seedlings indoors. For best results, place one cool white and one warm white tube in each fixture. Place the fluorescent lights no more than four to six inches above the seedlings. The lights should be on for 12 to 14 hours each day. 

What type of germination medium should I use to start seedlings indoors? 

The germination medium should be lightweight, porous and free of pathogens. Excellent seed-starting media are commercially prepared soilless mixes, such as Jiffy Mix. Use a high quality, well-drained potting mix when transplanting seedlings into individual pots or cell packs. 

When should I sow my flower and vegetable seeds indoors? 

The growth rate of the seedlings and the outdoor planting date determine when to sow seeds indoors. The crop time (number of weeks from sowing to planting outdoors) for several popular flowers and vegetables are as follows: 10 to 12 weeks - geranium; eight to 10 weeks - petunia and impatiens; six to eight weeks - marigold, pepper and eggplant; five to seven weeks - tomato, cabbage, broccoli and cauliflower; three to four weeks - cucumber, watermelon, muskmelon and squash.  

Which types of containers can be used to start seedlings indoors? 

Various containers can be used to germinate and grow transplants. Gardeners can purchase flats, trays, cell packs, pots, compressed peat pellets and other commercial products. Cut-off milk cartons or plastic jugs, paper cups and egg cartons also can be used to start seeds. Previously used flats, trays and pots should be cleaned and disinfected before re-use. Wash the containers in soapy water; then disinfect them in a solution of one part chlorine bleach and nine parts water. Holes should be punched in the bottom of milk cartons, jugs, paper cups and similar containers to allow for drainage. 

Do I need to purchase a heat mat to successfully germinate seeds indoors? 

Most seeds germinate best when the medium temperature is consistently 70 to 75 F. Placing containers in a warm location in the house, such as on top of a radiator or near a heat register, usually works fine. In cool environments, electric heating cables or mats can be used to ensure warm medium temperatures. 
To ask the ISU Extension garden experts questions, call the Hortline at 515- 294-3108, Monday-Friday from 10 a.m. to noon and 1 to 4:30 p.m., or e-mail