Light and why it matters
A plant never eats a meal. So where does its food come from?
No mouth and no fridge. Still, it grows bigger every day.
Think of each leaf as a tiny kitchen.
A kitchen needs heat to cook. A leaf needs light.
The leaf mixes light, water, and air to make sugar.
This cooking is called photosynthesisHow a leaf uses light, water, and air to make sugar.
See it in the glossary. The plant makes its own food.
- A · light
- B · water
- C · air (carbon dioxide)
- D · sugar
The sugar gives the plant energy and builds new leaves and roots.
The leaf also lets out oxygen, the gas you breathe in.
- A · light
- B · water
- C · air (carbon dioxide)
- D · sugar
- E · oxygen
Something green inside the leaves catches the light.
It is called chlorophyllThe green stuff in leaves that catches light.
See it in the glossary. It is what makes leaves green.
With too little light, a plant can’t cook enough sugar.
It stretches tall and thin to reach for light. Its leaves stay small.
Indoors, a window often can’t give enough light.
So growers hang a lamp made for plants, called a grow light. It is like a packed lunch of sunlight.
Your turn. Set the hours of light.
Most growers give plants light for much of the day, then some dark.
How many hours should the grow light stay on?
4 hours · Too little
Too little. The plant stretches tall and thin.
How many hours should the grow light stay on?
Hours of light: what happens in each zone.
- 0 to 11 hours · Too little. The plant stretches tall and thin.
- 12 to 18 hours · Just right. The plant cooks plenty of sugar and grows strong. Right Not quite
- 19 to 24 hours · Too long. Some plants, like tomatoes, get hurt without enough dark.
Think of a long summer day. There is still a night.
Check 1 of 3
What does a leaf make with the help of light?
Show the answer
Sugar. Right. The leaf uses light to cook sugar for the plant.
Check 2 of 3
A plant on a dark shelf grows tall and thin. What does it need?
Show the answer
More light. Right. It is stretching to reach more light.
Check 3 of 3
Which three things go into a leaf's kitchen?
Show the answer
Light, water, and air. Right. The leaf mixes them to make sugar.
You just learned how a plant uses light to make its own food.
Next up: Water as a delivery system.
Go deeper: the light recipeOpenClose
Photosynthesis uses light energy to turn carbon dioxide and water into sugar. Oxygen is given off as a by-product.
As a chemical equation: 6 CO₂ + 6 H₂O + light energy gives C₆H₁₂O₆ (a sugar called glucose) + 6 O₂.
Chlorophyll absorbs mostly red and blue light and reflects much of the green. That reflected green light is why leaves look green to us.
That is why many LED grow lights glow pink or purple: they are mostly red and blue. Other grow lights add green and other colors, so they look white.
Hours are only part of the story, because brightness matters too. Growers combine brightness and hours into one number, the daily light integral (DLI).
Guides give different numbers. Many suggest 12 to 16 hours a day for leafy greens and houseplants, and up to 18 for seedlings. The best number depends on the crop and on how bright the light is.
Plants also burn sugar for energy, day and night, through respiration, much as animals do. Some plants, such as tomatoes, get pale, damaged leaves when the light never turns off.
Day length also tells some plants when to flower. Lettuce, for example, tends to bolt (send up a flower stalk) when days are long and hot.
Sources
- Clark, M. A., M. Douglas, and J. Choi. "8.1 Overview of Photosynthesis." Biology 2e. OpenStax, 2018. Link
- Clark, M. A., M. Douglas, and J. Choi. "8.2 The Light-Dependent Reactions of Photosynthesis." Biology 2e. OpenStax, 2018. Link
- Moore, K. A., and L. K. Bradley, eds. "3. Botany." North Carolina Extension Gardener Handbook, 2nd ed. NC State Extension, AG-831, 2022. Link
- Weisenhorn, J., and N. Hoidal. Lighting for indoor plants and starting seeds. University of Minnesota Extension (reviewed 2026). Link
- Trinklein, D. H. Lighting Indoor Houseplants. G6515. University of Missouri Extension, revised 2017. Link
- Pennisi, B. V. Growing Indoor Plants with Success. Bulletin 1318. University of Georgia Cooperative Extension, 2022. Link
- Shaughnessy, D., and A. Pertuit. Indoor Plants: Cleaning, Fertilizing, Containers & Light Requirements. HGIC 1450. Clemson Cooperative Extension Home & Garden Information Center, 1999. Link
- Steil, A. Sources of Supplemental Light for Indoor Plants. Yard and Garden, Iowa State University Extension and Outreach (reviewed 2023). Link
- Meng, Q., and E. S. Runkle. Blue Photons from Broad-Spectrum LEDs Control Growth, Morphology, and Coloration of Indoor Hydroponic Red-Leaf Lettuce. Plants 12(5): 1127, 2023. Link
- Torres, A. P., and R. G. Lopez. Measuring Daily Light Integral in a Greenhouse. HO-238-W. Purdue Extension. Link
- Supplemental Lighting Run Time Worksheet. University of New Hampshire Extension. Link
- Lanoue, J., J. Zheng, C. Little, A. Thibodeau, B. Grodzinski, and X. Hao. Alternating Red and Blue Light-Emitting Diodes Allows for Injury-Free Tomato Production With Continuous Lighting. Frontiers in Plant Science 10: 1114, 2019. Link
- Matsuda, R., N. Ozawa, and K. Fujiwara. Effects of Continuous Lighting with or without a Diurnal Temperature Difference on Photosynthetic Characteristics of Tomato Leaves. Acta Horticulturae 956: 165-170, 2012. Link
- Schuh, M., and J. MacKenzie. Growing lettuce, endive and radicchio in home gardens. University of Minnesota Extension (reviewed 2024). Link