Many home gardeners like to get a head start on the gardening season by starting transplants indoors. As with plants in the home landscape and garden, home gardeners occasionally encounter problems when starting transplants indoors. Solutions to these problems are easy if the responsible factor can be identified. Horticulture specialists with Iowa State University Extension and Outreach share information on how to grow transplants indoors.

When growing indoors, poor or erratic germination of seeds may be caused by improper planting (for example, planting too deeply), uneven moisture and cool temperatures.

How deep to seed?

Fine seeds and seeds that require light for germination should be sown on the surface of the germination medium and then lightly pressed into the medium. Cover large seeds with additional gemination material to a depth of one to two times the seed’s diameter.

Uniform moisture levels are required for optimum seed germination. To maintain uniform moisture levels, place clear plastic food wrap over the containers after the seeds have been sown. Flats can also be covered with clear plastic domes. Remove the plastic food wrap or dome as soon as germination occurs.

Most seeds germinate best when the medium temperature is 70 to 75 degrees F. Placing containers in a warm location in the house, such as near a heat register, usually provides suitable germination temperatures. In cool environments, containers can be placed on electric heating mats to ensure warm medium temperatures.

When growing transplants indoors, tall, spindly growth is a common problem. Poor (insufficient) light, excessive watering, high temperatures, crowded conditions and excessive fertilization are factors that contribute to spindly growth.

Good growing conditions should produce short, stocky transplants. Immediately after germination, move the seedlings to an area with a temperature of 60 to 70 degrees F and place them under fluorescent or LED lights—a sunny window usually doesn’t provide enough light. A standard fluorescent fixture containing two 32- or 40-watt tubes works fine. Position the fluorescent lights no more than 4 to 6 inches above the seedlings. Leave the lights on for 12 to 16 hours a day.

When should seedlings be placed in pots?

Thoroughly water the seedlings when the soil surface becomes dry to the touch. Seedlings growing in flats should be transplanted into individual pots or cell packs when the second pair of “true” leaves appear. Fertilization should not be necessary if using a commercial potting mix containing a slow-release fertilizer. Fertilize the seedlings with a dilute fertilizer solution if the seedlings are exhibiting nutrient deficiency symptoms, such as yellow-green foliage or stunted plant growth.

How do I avoid damping-off?

If seedlings collapse and die shortly after germination, it is usually due to damping-off. Damping-off is caused by several different fungi. Environmental conditions associated with damping-off are a poorly drained potting soil, overcrowding and excessive watering. Damping-off can be prevented by using clean containers, a well-drained potting mix and by following good cultural practices. Wash previously used containers in soapy water, then disinfect the containers by dipping them in a solution containing one-part chlorine bleach and nine parts water.

Sow seeds thinly to avoid overcrowding. Flower and vegetable seeds need an evenly moist potting mix for good germination. After germination, allow the potting soil to dry somewhat between waterings. Transplant seedlings into individual pots or cell packs when the second pair of true leaves appear.

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