Grow lights or supplemental lighting sources are useful for starting seedlings and giving houseplants a boost. This is especially true when natural light in your home isn’t very bright. But if your grow light is too far away from your plants, it won’t be doing much good. Here’s what you need to know to place grow lights at the right distance, depending on the type of light and the plants you’re growing.

Why Light Distance Matters

Like sound, light intensity dissipates or gets weaker farther from the source. And it does that rapidly. Light intensity decreases as a mathematical function called the inverse square law. You don’t need to know the equation, but you do want to know how it works. As the distance from the source rises, the light intensity drops (that’s the inverse part) but at a much faster rate.

It seems like moving the lamp farther from the plant’s leaves, for example, from 2 to 4 feet, isn’t a big deal. But doubling the distance means the light received by the plant is only one-quarter as strong. Triple the distance, like moving it 6 feet away, and the light is one-ninth the intensity it was at 2 feet, which is just over 10%. 

The bottom line to remember: As the distance increases, the light intensity drops significantly. And while industrial-strength lights can still provide enough light when high above plants, the typical homeowner-model grow light placed 4 feet from a plant really isn’t doing much good.

How Much Light Do Plants Need?

Our eyes developed to use light to see the natural world, not to turn energy from the sun into food. The light that seems bright to us, like a well-lit room, may be too dim for some plants to survive, let alone grow. To your eyes, the sun on a bright day can seem too harsh, but many plants need all those rays for several hours a day to fuel their growth.

Physical Signs of Too Little Light

Since our eyes are poor measures of light conditions for plants, we can use a couple of methods to determine how much light to provide. The old-school way is to let the plant tell us. Symptoms of low light, like when the grow light is too far away, include stretching leggy plants, small pale leaves, dropped leaves, and stunted or no growth. Flowering houseplants such as African violets that aren’t receiving enough light may also fail to bloom.

If your plants exhibit these physical signs of not getting enough light, it may be because your grow light is too far away or the duration of light exposure isn’t long enough. Try moving the grow light closer to the plant or relocating the plant closer to the light. If growth doesn’t improve in a week or two, increase light exposure as well.

Measure with a Meter

For a more quantifiable result, you can use a light meter. Light meters designed for plants are not the same as those for photography. They measure only light of the right frequency to be useful for photosynthesis. This light is called photosynthetically active radiation, or PAR. Light meters for plants provide measurements in a unit called a micromol, not in candlepower, foot candles, or lumens. 

A light meter held next to the leaves can tell you exactly how much light is getting to the plant at that moment in time. Light intensity will change throughout the day for a plant on a windowsill, but under a grow light, it should be constant.

Lighting Distance Guidelines

Most homes have only one of three types of lighting: incandescent, fluorescent, or LED. We’ll ignore incandescent bulbs, because they create too much heat (and too little light) to be effective plant lighting. They’re also not very available anymore.

Fluorescent Lights

Fluorescent lighting can be used as grow lighting, but it has limitations. Because fluorescent lights are not very intense or bright, they must be placed very close to the plants to overcome their limited power—we can’t afford to lose much intensity with the inverse square law. 

For seed-starting and growing young veggie and flower plants, fluorescent lights need to be 2-4 inches above the leaves. The light should be right on top of the leaves without touching them (the bulbs can get too warm).

Houseplants can be a bit farther since many of our favorite houseplants like dappled or weaker light, but within two feet is a good rule. As the plants grow taller, you’ll need to adjust the fluorescent light fixture to remain at the right height above the plants.

LED Lights

LED lighting is more powerful, even though it uses less electricity. The price of LED lighting has come down, and as a bonus, you won’t have to deal with the hassle of disposing of the spent lighting tubes when they burn out like you do with fluorescents.

For starting young plants, 8-12 inches above the leaves is a good distance. Houseplants can sometimes be 2-4 feet away, allowing you to group several plants under one light source. Of course, higher-powered LED fixtures can be farther away than low-powered lights. Industrial strength lights are available to purchase, although they are more expensive. 

Tips for Using Grow Lights

For all lighting fixtures, be careful when watering your indoor plants so splashes don’t get on the lights. And watch for symptoms of too much or too little light. Look for a mounting system that easily adjusts for height, allowing you to fine-tune the light for your plants. 

Putting the lights on a timer is the last key to providing the right light for your plants. A timer will remember to turn the lights off at night and will not forget to turn them on in the morning, even if you aren’t home or slept in on a day off.