Yard and Garden: Growing and Transplanting Indoor Seedlings

February 19, 2015, 9:28 am | Richard Jauron, Greg Wallace

Right now, the weather outside is cold and the ground is inhospitable to plants. However, this is an excellent time to begin growing seedlings indoors that can be transplanted outside for successful growth when the spring growing season begins.

Here are some tips from Iowa State University Extension and Outreach horticulturists on the proper way to handle indoor seedlings. 

What do I need to start transplants indoors? 

Successfully growing seedlings indoors requires high quality seeds, a germination medium, containers, lights and other supplies.  

Flower and vegetable seeds can be purchased at local garden centers. They’re also available from numerous mail-order companies.  

The germination medium should be lightweight, porous and free of pathogens. Excellent seed-starting media are commercially prepared soilless mixes, such as Jiffy Mix. 

Transplanting Seedlings

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

While seedlings can be grown in a sunny window, they often become tall and spindly because of insufficient light. For best results, grow seedlings under fluorescent lights. Expensive “grow lights” are not necessary. A standard fluorescent shop fixture with two 40-watt tubes (one cool white and one warm white) works fine.

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, salvia and impatiens; six to eight weeks - marigold, pepper and eggplant; five to seven weeks - zinnia, tomato, cabbage, broccoli and cauliflower; three to four weeks - cucumber, watermelon, muskmelon and squash.

How do I successfully germinate seeds indoors?

When sowing seeds in flats or trays, fill the container with the germination medium to within 1 inch of the top. Firm the medium, water thoroughly, then allow it to drain. Fine seeds and those seeds that require light for germination are sown on the surface of the medium and then lightly pressed into the germination medium. Cover all other seeds with additional medium to a thickness of one to two times the seed’s diameter. After sowing the seeds, water the medium by partially submersing the container in water. When the surface becomes wet, remove the container from the water and allow it to drain. Watering from below prevents the washing of seeds on the surface of the medium. The medium also can be moistened with a seedling sprayer. The fine mist from a rubber bulb seedling sprayer will not disturb the seeds or the medium.  

To insure a uniform moisture level during germination, cover the container with clear plastic food wrap. Flats also can be covered with clear plastic domes.  

Set the container in bright light, but out of direct sunlight. Extremely high temperatures may develop if the covered container is set in direct sunlight. These high temperatures may adversely affect germination. A medium temperature of 70 to 75 degrees Fahrenheit is adequate for the germination of most flowers and vegetables. Remove the plastic food wrap or dome as soon as germination occurs.  

Once the seeds have germinated, move the seedlings to an area with slightly cooler temperatures and direct sun or place under fluorescent lights. Transplant the seedlings into individual containers or cell packs when the second pair of “true” leaves appear.  

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