How to Use Grow Lights for Seed-Starting

If you want to start seeds indoors but lack a sunny spot, grow lights are essential. Join Briana Yablonski to learn how to select a grow light and set it up for optimal plant growth.

Close-up of young seedlings of Chard plants in starting trays under LED lamps. Chard plants are characterized by their large, glossy, and deeply veined leaves that grow in a dense rosette. The leaves come in bright green with contrasting yellow and purple veins and stems.

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If you’re anything like me, the first warm day of the year has you itching to get your hands in the soil. But don’t let that warm spell fool you—the cold days probably aren’t gone for good! While you should wait until warmer weather arrives before planting warm-weather crops like tomatoes and zinnias outdoors, you can utilize warm indoor environments to get a jump on your growing season.

Since seeds and seedlings require light to germinate and flourish, grow lights are a helpful and often necessary tool for indoor seed-starting. Even if you have a bright window, adding supplemental lighting will ensure your seedlings can access the light they need. By the time warm spring days arrive, you’ll have mature seedlings ready to pop in the ground.

However, choosing and setting up grow lights can be intimidating! Which one do you choose? How long do you leave it on? Stick with me to learn all you need to know about using grow lights for seed-starting.

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The Benefits of Using Grow Lights for Seed-Starting

While you may be tempted to place your tray of seeds on a sunny window and hope for the best, investing in a grow light will pay dividends. After all, plants obtain their energy from light! If you’re still wondering if you should add grow lights to your list of gardening supplies, consider these benefits.

Allows You to Start Seeds Anywhere

Close-up of many trays with young tomato seedlings on a rack under ultraviolet phytolamps. Young tomato seedlings exhibit a delicate and slender appearance, with tender stems and small, oval-shaped leaves that emerge in pairs along the stem. The leaves are light green in color and have a slightly fuzzy texture.
Grow lights provide flexibility in seedling placement for optimal growth.

If your house is anything like mine, the brightest spot doesn’t always coincide with the best location for seed-starting. Sure, that spot beside the south-facing window in the living room is bright, but that’s also where your lemon tree overwinters, and the cats lounge their days away. But that dim corner in the spare bedroom? Well, that would be the perfect spot for tender seedlings if it wasn’t so dark.

Utilizing grow lights allows you to choose where you grow seedlings. That means you can place them in low-traffic areas of your home that are safe from curious pets and kids.

Helps You Get a Jump on the Growing Season

Close-up of many starting trays with small pepper seedlings under pink LEDs. Pepper seedlings are characterized by their slender stems and delicate, lance-shaped leaves that emerge in pairs along the stem. The leaves are green and have a slightly waxy texture.
Start your own seedlings to save money and offers variety.

Starting your own seedlings offers many advantages. It helps you save some money, cultivate tomato varieties you can’t find locally, and learn the joys of watching plants grow from seed to flower. And if you start seeds indoors, you can end up with healthy plants that are ready to transplant outdoors when the weather is warm enough.

For example, let’s say you utilize a grow light to start tomato seeds indoors in late March. If everything goes as planned, you’ll have healthy tomato seedlings that are ready to plant outside by mid-May.

Encourages Healthy Transplants

Close-up of young pea seedlings in starting trays and pots on a windowsill under LED lamps. Pea seedlings are recognized by their slender, vining stems and delicate, pinnately compound leaves that emerge in pairs along the stem. The leaves are green and comprised of several small leaflets arranged opposite each other along a central stalk.
Insufficient light leads to weak seedlings.

There’s no denying that plants need light to thrive! While your seedlings may be happy growing on a windowsill for a few weeks, they’ll eventually show signs of a lack of light. Spindly stems, faded leaves, and elongated plants are all signs that your plants are begging for more light.

Adding a grow light to your seed-starting setup allows you to produce healthy transplants. And starting with healthy transplants increases the odds that you’ll end up with healthy plants!

How to Choose a Grow Light

If you walk into a gardening store or browse through an online marketplace, you’ll quickly see hundreds, if not thousands, of grow lights available. With so many options, how do you choose?

While it’s easy to become overwhelmed by the array of available products, remember your end goal is to choose a system that provides your seedlings with the necessary light. Consider the following aspects when making your decision.

Spectrum

Close-up of tomato seedlings in biodegradable pots under LEDs indoors. Tomato seedlings are characterized by their slender stems and distinct, serrated leaves that emerge alternately along the stem. The leaves are medium green in color and exhibit a slightly fuzzy texture.
Full spectrum lights mimic natural sunshine, ensuring optimal photosynthesis.

The light spectrum is one of the most important factors to consider when deciding whether or not a light will work well as a grow light. Scientists use the term spectrum to refer to the wavelength a light produces and use nanometers to measure wavelength. Visible light, or the type of light that humans can see, falls between 300-700 nm.

The exact wavelength determines the light color. For example, a wavelength of 450 nm appears blue to humans, and a wavelength of 650 nm appears red.

Plants use light in the range of 400-700 nm to complete photosynthesis. Therefore, scientists refer to this range as photosynthetic active radiation (PAR). While some people think that plants only use red and blue light to complete photosynthesis (which is why you see grow lights with only red and blue lights), plants use all wavelengths during photosynthesis.

So, what does all that mean for grow lights? Full spectrum lights work best for seed-starting! They contain the entire spectrum of PAR light. They mimic natural light and ensure your tiny plants can access every wavelength of light they need.

Temperature

Close-up of starting trays with various seeds with white and yellow sticky notes with the inscriptions of the sown seeds. The trays have transparent plastic domes with droplets of condensation.
Choose grow lights with 5,000-7,000 K for seedlings.

All full-spectrum lights appear white, but the exact color varies depending on the temperature. People measure light temperature in Kelvins (K). More Kelvins correlate with warmer light, while fewer Kelvins correspond with cooler light.

For example, people say a light with 2,500K is warm white, while a light with 7,000K is cool white. When choosing grow lights for seedlings, look for light ranging from 5,000 to 7,000 K since cool light encourages vegetative growth. Opt for a warmer temperature range when growing mature flowering plants like citrus trees.

Electricity Use

Close-up of young seedlings of strawberries, parsley, peppermint under an LED lamp. Strawberry seedlings are characterized by their small, compact stature with clusters of trifoliate emerging leaves from a central crown. The leaves are bright green and serrated, composed of three leaflets arranged symmetrically.
Choose energy-efficient LEDs for seedlings to save on electricity bills.

While your seedlings won’t care how much electricity your lights use, your pockets will! Seedlings require at least 12 hours of light each day, so differences in energy efficiency can noticeably impact your electricity bill.

LEDs are the most energy-efficient option, and their widespread availability makes them a popular choice for home growers. I recommend using them over metal halide, high-pressure sodium, and fluorescents.

Shape

Close-up of fresh tomato seedlings under LED lamps near a bright window. Tomato seedlings have small, delicate, bright green leaves with a slightly fuzzy texture. The leaves are compound, pinnate, and consist of oval, jagged leaflets of bright green color.
Ensure the grow light size matches seedling trays for optimal illumination.

Along with choosing the correct type of grow light, you also have to make sure the light can reach your plants! Since you should set grow lights a few inches to a foot above your seedlings, the size of the light should be similar to the size of your seedling trays.

Although you can start seeds in many different containers, 10×20 trays are a popular seed-starting vessel and the standard in commercial operations. I always start seeds in some variation of a 10×20 tray, whether I’m tucking six-cell packs into a bottom tray or planting spinach seeds in a 128-cell tray. If you plan on using some variation of a 10×20 tray, choose a grow light to match!

If you’re using multiple lights to illuminate your seedlings, make sure the output of each overlaps. Say you’ve hung shop lights six inches above a 20-inch wide seedling tray. If each one illuminates six inches of the tray, you should use four of them to ensure all areas of the seedling tray receive bright light.

It’s okay if your grow light is larger than your cell trays, but all of the trays should fit underneath it. If the outer few inches of a tray are outside of the light’s range, they’ll curve inward as they search for light.

How to Set Up Your Grow Light

Close-up of young basil seedlings in a black starter tray under white LED lights. Basil seedlings have tender, light green leaves that are oval-shaped and slightly serrated along the edges. The stems are delicate and slender, with a pale green coloration.
Position grow lights 4-8 inches above seedlings to avoid leggy growth.

Even if you have the best grow light on the market, an improper setup can render the lights practically useless! Placement in the proper position ensures your plants receive all the light they need to grow.

One of the most important aspects to keep in mind is the distance between the grow light and the plants. Your grow light should be four to eight inches above the tops of your seedlings to ensure your plants receive adequate light. The further it is from the plants, the less light the plants will receive.

If your plants are too far away, they’ll become weak and elongated as they stretch towards the light. When plants develop this stretched-out form, gardeners say they’re “leggy.” Move your grow light closer to your plants if they become leggy.

While you want your lights close to your plants, you don’t want them too close. Modern LEDs emit very little heat, but they can still burn any foliage they touch. Fluorescents emit more heat than LEDs, making them even more likely to burn plant leaves. 

Since it’s the distance between the light and the top of the plant that matters, you’ll likely have to raise your light as your plants grow. This is especially true if you start seedlings in short trays and later pot them up into taller pots.

How Long Should I Leave Grow Lights On?

Close-up of young sprouts in starting trays under pink LED lights. These young sprouts have short stems with oval, slightly elongated cotyledons.
Automate grow light schedule with outlet timer for 14-16 hours.

When you’re growing seedlings, leave your grow lights on for 14-16 hours each day. For example, I like to turn mine on in the morning at 6 AM and off at 10 PM. However, I don’t do this by hand!

Attaching your grow lights to a standard outlet timer allows you to easily automate this process.

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Final Thoughts

While grow lights can seem intimidating, they’re not so scary after you learn a bit about them! Choosing a good grow light will allow you to grow healthy seedlings regardless of the time of year. Remember to look for a full-spectrum light, position it close to your seedlings, and leave it on for 14-16 hours each day.

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