How Does Light Affect Plant Growth?

Learning how light affects plant growth is the first step to becoming a better indoor or outdoor gardener. Kevin Espiritu explains the science behind lighting and what it means for gardeners.

How Does Light Affect Plant Growth?


Take a look at any plant care guide on a nursery label or online and you’ll find the plant’s light requirements. These are usually listed first, hinting at how important light is in plant growth.

Although we may understand its importance, many gardeners don’t know how light affects plant growth, and by extension, what they can do to provide the perfect lighting conditions for their plants. If you’re curious, you’ll find the answers to your questions here.

The Short Answer

Plants need light for photosynthesis, providing energy to break water and carbon dioxide into the components needed to fuel growth. Light-related factors like wavelength, duration, and intensity impact growth in different ways, good and bad.

Without light, plants cannot perform this essential process, stunting growth. Different plants have different light requirements based on conditions in their native habitats. If you can’t position them these conditions naturally, grow lights can provide everything your plants need for strong growth.

The Long Answer

You might recall the word photosynthesis from your old science classes when discussing light and plants. Perhaps it looked something like this: 6H2O + 6CO2 —-> C6H12O6 + 6O2.

That’s a lot of numbers and letters.

Perhaps this description is a little easier to follow if you have forgotten all that chemical equation stuff.

Plants use light, water, and carbon dioxide to make sugar, which is converted to ATP, or Adenosine triphosphate (the stuff that fuels all living things) by cellular respiration.

Chlorophyll absorbs the sun’s energy. Carbon dioxide enters the leaves through tiny pores. The roots draw up water from the soil. The energy from the light is what chops up the water molecules. The carbon dioxide befriends the abandoned hydrogen to make the plant’s fuel.

What Kind of Light Do Plants Need?

Close up of a small seedling sprouting up from the dirt with sunlight shinning on it.
Plants use different wavelengths for each phase of growth.

Most people are familiar with the breakdown of light into colors displayed by a rainbow after a storm. The spectrum includes these colors and many other wavelengths, like cosmic rays and gamma rays.

A new question may be forming in your mind now that we’re talking rainbows: does the color of light affect plant growth? It does indeed.

We use nanometers to measure wavelengths. Plants use different ranges of nanometers for different growth phases. The useful range for gardeners is known as Photosynthetically Active Radiation (PAR). Measured from 400 to 700 nanometers, this range encompasses all those rainbow colors we adore.

However, PAR is not used all at once. The purple and blue light wavelengths, 400 to 490 nanometers, stimulate the vegetative growth phase while the yellow-orange-red wavelengths are used for flowering and fruiting. The plant doesn’t use the green wavelength, which is why it reflects back to our eyes.

What Does This Mean For Gardeners?

Close up of a large green leaf with the bright sun behind it shinning through.
Every plant needs some light source in order to grow and thive.

Explaining the science gives you a bit of background knowledge, but it’s the application of this knowledge that matters to your plants.

If there is any takeaway, it’s that light is essential for plant growth. Without it, photosynthesis cannot occur. And as we know, without photosynthesis, your plant will not grow. Some plants need more than others, but all plants are fueled by light.

That means you can’t ignore lighting requirements for the plants you choose, or hope they will simply adapt wherever you plant them. Ignoring them will lead to a range of light-related problems, causing leggy or stunted growth, discolored leaves, and lack of flowers.

Seedling trays growing under a grow light indoors.
Grow lights are a good alternative if you’re in an area with unpredictable sunlight.

How well a plant grows depends on three factors: wavelength, duration, and intensity. If any of these factors does not match the conditions the plant prefers (in other words, what they experience in their native habitats), you’ll encounter a few growth problems.

Four potted plants inside on a window sill. Two of the plants are brown and wilted.
Certain wavelengths can cause irreversible damage.

We’ve already touched on the wavelengths that plants like. But there are also wavelengths plants don’t like. Different nanometers of ultraviolet rays may do nothing at all, but some ranges can be extremely detrimental.

Several small plants wilted and bent over in a garden bed.
It’s important to know how much sun each of your plants need.

How long a plant is in the sun will affect its growth. For example, Alaskan gardeners can grow gigantic pumpkins easily thanks to increased sunlight in summer. Other plants prefer shadier conditions and won’t tolerate being in the sun for a full day.

Woman's finger pointing at a small tomato plant with yellow, wilted leaves in a garden bed.
Too much sun can cause wilting whereas not enough can cause leggy growth.

Intensity refers to how strong the light is and goes hand-in-hand with duration. If the light is too strong, the plant will likely scorch. If it’s not strong enough, you’ll notice leggy growth and poor performance.

Grow Lights vs. Sunlight: Which Is Better?

Small square, plastic tray of tiny seedlings sitting in front of a sunny window.
Natural sunlight is ideal however grow lights can also be a good alternative.

If you can’t find a spot with the right amount of sunlight, grow lights are a great alternative.

The effect of indoor light on your little green babies depends on the type of bulbs you choose. While sunlight will always have a natural edge in quality, full-spectrum bulbs come close to providing everything your plants need to thrive.

Understanding lighting requirements will also help you adjust your grow lights for optimal growth. Many products have automatic settings, but others allow more control over wavelengths and intensity.

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Final Thoughts

Just like we have morning people and night owls, your plants have their own light needs. These needs must be met to fuel growth, ensuring your plants have all the resources they need to thrive.

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