Light is a form of energy in transit. When we say something gives off light, what we are really saying is that the device is releasing or broadcasting energy. The light we see, and that used by plants, falls in a small portion of the entire electromagnetic spectrum. Thinking about how a plant uses light, this makes sense. 

Plants use light as their energy source for conducting the photosynthesis process. If light weren’t energy, that chemical reaction wouldn’t work. Outdoors, light is provided daily by the sun, even on a cloudy day. But for indoor plants, much of that is up to us. 

We gather light with our eyes, and, through some biological processes, it gets turned into signals in our brains. Over time, we learn to decode those signals subconsciously to gain information about the world around us. But that’s not what plants are using light for. The amount of light our eyes perceive as bright, or even too much, may not be enough for a plant to fill its daily energy needs. Our human eyes are poor judges of how much usable light actually reaches a leaf.

Think of a plant’s daily lighting needs as a bucket to be filled. If you were filling a bucket from a hose, the level would rise faster if you used a larger hose. Alternatively, the bucket could still be filled with a smaller hose, but it would take longer. With some reasonable limits, that same idea applies to plants and their light needs. 

If you took calculus, you might remember an integral is a tool for measuring the area under a curve on a graph. If you graphed how much light was falling on a particular spot every few minutes throughout the day, you’d get a curve, with less light in the morning, more light per unit time falling at midday, and less again in the evening. The area under that curve would be a light integral for the day, or the daily light integral (DLI). It represents the light intensity and the light duration.

The value of DLI has some fancy units (mols/m2/d), but we can omit them from here on and just use the numbers.  DLI is the amount of plant usable light that hits an area in a 24-hour period. In other words, how much light fell on the leaf.

We also use the term DLI to describe the amount of light a given plant needs for optimal growth and health. You can look up DLI values for various houseplants to get a rough idea. For example, Phalaenopsis orchids do well with a DLI of 4-6 (remember we’re ignoring those pesky units) and Monstera deliciosa does better with 8-10. That pothos may survive with a DLI of only 1-2, but will grow better with a DLI of four or more. Growing basil for the kitchen needs much more light to do well, with a DLI in the teens or higher. 

Light Intensity Matters

If you’ve had plants sitting in a windowsill that seemed to get leggy and spindly, those plants wanted more light. It’s a common problem in winter in the northern hemisphere. Even if there were 10 hours of daylight, the intensity, due to the sun’s lower path through the sky, is lower. What looks like a bright sunny window in winter to us may not be enough energy for a plant. Remember, our eyes are terrible judges of how energy lands on a leaf. It’s not what they do. 

Light from the sun varies hugely by season. For example, the daily light integral for a location might be 60 in summer, but only 20 in the middle of winter. To us, both are bright sunny days, but from a plant perspective, there is a threefold difference.

A low light intensity means our light bucket (DLI) takes too long to fill up. Remember, DLI is intensity multiplied by duration. Plants need an appropriate light intensity to satisfy their energy needs in a  reasonable timeframe. 

Intensity of light is controlled by not only the power of the source, but also the distance from the source to the leaf. As the distance from the light fixture increases, intensity drops quickly, as described by the inverse-square law: doubling the distance can reduce intensity to roughly a quarter. It’s why lower-powered fluorescent lights must be located quite close to the plant, while stronger LED lighting can be farther away. But how does time factor into the mix? 

Duration of Light

Thinking of our bucket analogy or the DLI, we might assume the plant would be fine with a super-bright light source for a few hours. After all, the bucket should be filled, right? However, there is a maximum amount of light per unit time that a plant can effectively use. Extra light beyond that level doesn’t help to fill our bucket. Similar to how you can’t (or at least shouldn’t) eat all the food for a day in one sitting, a plant needs that energy spread out throughout the day. 

Light duration, when talking about artificial light for indoor plants, can be shorter with more powerful lighting or longer for less intense sources. If your plant needs a DLI of 20, you can achieve it with several different combinations of intensity and duration. However, there are limits. Most plants don’t perform well without a daily light/dark cycle.

Light Spectrum

Plants need specific wavelengths of light, or portions of the electromagnetic spectrum, to conduct photosynthesis. Not every wavelength is optimal. The light wavelengths usable by the plant are called photosynthetically active radiation (PAR), with wavelengths roughly 400–700 nanometers. 

In general, light which we can see is also PAR, but within that broad category are various wavelengths which we perceive as red, blue, and other colors. The idea that red and blue wavelengths are better for plants is why you sometimes see purple-looking fixtures labeled as grow lights. However, broad-spectrum white ‘daylight’ LEDs can be just as effective and are usually more pleasant to live with.

When choosing a light for indoor plants, look for lights labeled as full spectrum, or those that mimic daylight. These lights imitate sunlight and will provide balanced light across the PAR band. Lights labeled “warm” or “cool” lean toward one end of the spectrum or the other. 

How To Use This Information

For indoor plants, whether houseplants, starting seedlings, or growing herbs and veggies, lighting is probably the biggest issue, or tied with overwatering as a source of problems. And it’s harder to solve. We can feel the soil with our finger and decide if it’s time to water, but how do we know if the plants are getting enough light? There are two main methods: listen to the plants, or buy a light meter.

Observe your plants

Listening to the plants is just a figure of speech for observing their growth and health. Although we call fertilizer plant food, it isn’t. Plants make their own food using light. If they aren’t getting enough light, they aren’t getting enough food, and their growth, color, and form will suffer.

Plants that don't get enough light will stretch, with more space between their nodes. We call this getting leggy. It means they are reaching for the light, looking for more, and prioritizing growing taller over getting bushy, branching out, and taking up space. Other symptoms of insufficient light include slow or no growth, pale or smaller leaves, lack of flowering, and loss of variegation in variegated plants. If you observe these phenomena, increase the light intensity or duration, wait a few weeks, and reobserve.

Use a light meter

If you’d like to be more precise, a light meter can help you really dial in your plant lighting. Not just any light meter will do. Those made for photography aren’t the right choice for measuring the light plants use. 

Look for a PAR meter, which measures only the photosynthetically active portion of the light spectrum. It will provide a snapshot, or instant reading, of the light hitting the leaf at that exact moment in time (called PPFD), which you can then use to calculate DLI. Don’t fret if you don’t want to do the extra math. Search online for a DLI calculator; enter the meter reading and the duration (how long you have the light on), and it will convert to DLI for you. If your calculated DLI is too low for the plant, increase the intensity with a more powerful light or by moving the light closer, or extend the duration by leaving the light on longer.