Yard and Garden: Starting Seeds Indoors under Lights

Starting seeds indoors requires proper lighting and setup

March 15, 2022, 1:18 pm | Aaron J. Steil

AMES, Iowa -- Growing vegetable and annual plants from seed often requires starting them indoors and transplanting them outside once the danger of frost has passed. While it’s important to use fresh seed, the right soil mix and containers, as well as appropriate levels of water and fertilizer, it is essential to have abundant light for the germinating seedlings to grow high quality plants. In this edition of Yard and Garden, Iowa State University Extension and Outreach horticulturists answer questions on how to start your seedlings under lights indoors.

Why is starting seed under supplemental lights beneficial?

Seedlings.Without adequate light, seedlings become lanky. Tall, spindly seedlings do not transplant well into the garden. For most homes, even the brightest windowsill does not provide enough light and the light is one-sided, causing seedlings to quickly bend toward the window, making them stretch even more. By utilizing a supplemental light source, such as a fluorescent or LED fixture, you can provide enough light to produce high quality, compact and stocky seedlings that transplant well into the garden.

What type of lights should I use?

For most home gardeners, a plug-in shop fluorescent or LED light fixture is the perfect type of light to use for seed starting.  When shopping for fluorescent lights, look for high-output, full-spectrum (sometimes called “daylight”) bulbs. When shopping for LED lights, purchase full-spectrum fixtures that emit a high number of lumens (at least 3,000 lumens). Cost is often an important factor. When you consider costs remember to factor the cost of the fixture and the bulb. Fluorescent bulbs dim over time and should be replaced every one to two years, which is a cost that should also be considered.

Plants do not use all wavelengths of light equally. The blue wavelengths of light are needed for good foliage and root growth and the red wavelengths are important for flower and fruit development.  Grow lights provide abundant light in the red and blue wavelengths and very little of the green wavelengths, which is why they sometimes look pink or purple in color. Grow lights can be a good source of light for seedlings but are often more expensive and not necessary. A full-spectrum bulb will provide red and blue wavelengths of light at a level adequate for seedling growth at a lower cost.

How do I set up the lights for my seedlings?

Supplemental lights need to be oriented relatively close to the seedlings to be effective and therefore need to be adjustable in height to accommodate the growing plants. For most fluorescent fixtures, the seedlings should be 4 to 6 inches from the light source. LED fixtures can often be set a little further away at about 8 to 12 inches from the plants. Hang light fixtures from adjustable cables or chains so they can be moved up as the plants grow or place seedling trays on blocks or bricks that can be removed to lower trays as they grow.

Provide adequate air circulation around the seedlings to dissipate heat produced by the light fixture.  Place lights on a timer set to turn fixtures on for 14 to 16 hours a day.

How do I know if I am providing the right amount of light for my seedlings? 

Careful observation of your seedlings will help you determine if the light levels are appropriate.  Seedlings that are not receiving enough light will stretch and lean towards the light source. Use a brighter fixture that provides more lumens, replace fluorescent tubes with new brighter bulbs, or simply move the fixture closer to the seedlings to increase the light levels.

If seedling leaves turn yellow or brown and burn starting with the upper leaves, they may be receiving too much light. You can swap the fixture for one that provides fewer lumens or simply adjust the fixture so it is further away from the seedlings. The lumens per square foot shining on the plants drops significantly by pulling the fixture just a few inches further away from the plants.  

Are lights needed for germination?

Some seeds need light in order to germinate, but many do not. Seed packages will usually indicate what your particular selection requires. If lights are needed for germination, place trays under lights immediately after sowing. If light is not required, turn the supplemental lights on after germination occurs.

More information about starting garden transplants from seed can be found from the Iowa State University Extension and Outreach store. See Starting Garden Transplants at Home.

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