How to Use Grow Lights for Seed-Starting

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.

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

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

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

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

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

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

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

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

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?

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.