The art of growing seedlings indoors

Starting seedlings indoors can be a fun, rewarding hobby for many home gardeners. Growing high quality seedlings indoors isn’t difficult. Successfully growing seedlings indoors requires suitable germination materials and just a little skill, note horticulture specialists with Iowa State University Extension and Outreach. To have additional questions answered, contact the Hortline at 515-294-3108 or [email protected].

When can I sow 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 indoors to planting outdoors) for several popular flowers and vegetables are as follows:

10 to 12 weeks—geranium;

8 to 10 weeks—petunia, impatiens, salvia and vinca;

6 to 8 weeks—marigold, pepper and eggplant;

5 to 7 weeks—zinnia and tomato;

4 to 5 weeks—cabbage, broccoli, cauliflower and kale; and

3 to 4 weeks—cucumber, watermelon, muskmelon and squash.

How do I start?

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 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 can also be moistened with a mist sprayer. The fine mist from the sprayer will not disturb the seeds or the medium.

To ensure a uniform moisture level during germination, cover the container with clear plastic food wrap. Flats can also 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 favorable for the germination of most flower and vegetable seeds. In cool locations, place containers on an electric heat mat to ensure optimal germination temperatures. Remove the plastic food wrap or dome as soon as germination occurs.

My seeds have germinated. Now what do I do?

Once the seeds have germinated, move the seedlings to an area with slightly cooler temperatures and direct sun, or place under fluorescent or LED lights. Transplant the seedlings growing in flats into individual containers or cell packs when the second pair of true leaves appear. Containers with two or more seedlings should be thinned to one seedling per container. Destroy the weak seedlings by cutting them off with a razor blade.

Short, stocky, dark green seedlings are the best quality transplants. For best results, grow seedlings under fluorescent or LED lights. It isn’t necessary to have “grow lights” or a fancy light stand. A standard fluorescent fixture with two 32- or 40-watt tubes works fine. Fluorescent lights should be positioned no more than 4 to 6 inches above the plants. They should be lit 12 to 16 hours per day. Plants grown in a window often become tall and spindly because of inadequate light.

Thoroughly water the seedlings when the soil surface becomes dry to the touch. If using a commercial potting mix containing a slow-release fertilizer, fertilization should not be necessary. An application of a dilute fertilizer solution once every two weeks should be sufficient for those potting mixes that don’t contain a slow-release fertilizer.