Why do plants need sunlight? In the simplest terms, plants need sunlight to produce their food. All living things need food and water to survive; for plants, sunlight means food. Without sunlight, they won’t produce food for us, and when they don’t receive food, they won’t be able to grow. It sounds like playback, but it’s true.
The lack of light will not only stop their growth, but it will also jeopardize their mere existence. It’s not just the survival of plants at stake here; it’s also ours! Plants are the primary food source for humans and animals. Directly or indirectly, all living things are dependant on photosynthesis for their existence.
But What Is Photosynthesis?
You’ll probably recall the term from your seventh-grade science textbook. If you don’t, here’s a little reminder.
Unlike us, plants are autotrophs, which means they create their food. They use sunlight, water, and carbon dioxide to produce glucose. This process is called photosynthesis, and it’s a two-step process.
The First Stage – Light Dependant
The first stage is a light dependant process that occurs when light from the sun hits a plant’s leaf. Light is absorbed by chlorophyll that’s present within the chloroplasts of the plant’s leaves. Light absorbed by chlorophyll breaks down water into oxygen and hydrogen ions.
The Second Stage – Light-Independent
The second stage uses the energy produced in the first stage to produce glucose from carbon dioxide. It’s not a single reaction; instead, it’s a series of reactions where the end product is glucose.
Water is as essential to plants as sunlight since that’s also a vital input for photosynthesis. Want to know more? Read our post – Why Do Plants Need Water? A Simple Guide to This Wet Wonder to learn more about the importance of water to plants.
How Does The Plant Use Glucose?
So what’s the point of creating glucose? How does the plant use it? Glucose is the primary form of food that the plant makes for itself. It can be used in different ways. It can either convert it to cellulose or starch needed for building new plant cells or use it for respiration. Respiration, by the way, breaks down glucose to release energy for plant growth.
In the simplest terms, respiration in plants is the opposite of photosynthesis. While photosynthesis captures and stores energy from the sun in the form of glucose, respiration release the stored energy. So, respiration, and hence plant growth, are indirectly dependant on the energy from the sun. No light would mean no growth for the plants.
How Does Light Intensity Affect Plants?
If the plants aren’t getting enough light, the process of photosynthesis will slow down. Even when carbon dioxide and water are available in ample amounts, one of the critical ingredients, light, is limited. With a higher light intensity, the plant will be able to photosynthesize faster if other components are also available in desirable quantities.
Gardeners often use artificial lighting to replicate sunlight and help photosynthesis. Artificial light is an alternative to sunlight if you wish to grow indoors but don’t have a bright window where you can place your potted plants. Read our article – Growing Plants In Artificial Light: Can You Do That? – if you want to learn more about growing plants under artificial lights!
It’s not always too little sunlight that’s problematic for the plants. Too much sunlight can also be harmful to certain types of plants. Unsuitable light intensities can prevent your plants from flourishing to their true potential.
What Happens When Plants Don’t Get Enough Light?
Lack of ample sunlight can cause the plants to “focus their attention” on growing taller rather than fruitful. In an attempt to reach for sunlight, they’ll use all the little energy they have to their stem’s growth rather than in producing fruit.
Plants are usually able to withstand the temporary absence of sunlight. A plant that otherwise sits in a sunny spot wouldn’t mind a few days of shade. It will continue using its stored energy for growth during this period.
An extended period of no sun will ultimately kill the plant since there will be no energy source for them once the stored energy is all used up.
Can Plants Get Too Much Light?
Too much sunlight isn’t always an advantage, as already highlighted. A light intensity that’s higher than what’s desirable can even burn the leaves. Some plants are more sensitive to light intensity than others and need extra protection, especially from the bright afternoon sun.
Non-photochemical quenching, or NPQ, is a natural process in plants that protects them against excess sunlight. But, the protection comes at a price. During this process, and for some time afterward, the plant stops absorbing energy. As a result, you’ll see a stunted growth in plants exposed to too much sunlight.
How Much Sunlight Do My Plants Need?
“Enough with the science class! How do I know how much sunlight is good for my plants?”
If you have a garden, and the specific light requirements for your different plants are what you’ve been worrying about, the answer is relatively straightforward.
Different plant varieties have different light requirements. If you order a plant online or buy it from a gardening store, you may have noticed terms like “partial shade” or “full sun” in the specifications. These words tell you the ideal sun exposure for the plant.
Terms For Describing Light Requirements
Familiarize yourself with these standardized terms to understand the sunlight needs of different plant species:
- Full Sun – Plants that need “full sun” need more than 6 hours of direct sunlight per day.
- Partial Sun – Such plants ask for 3 to 6 hours of direct sunlight and are tolerant of the afternoon sun.
- Partial Shade – Plants that grow in “partial shade” need 3 to 6 hours of direct sunlight, with protection from the afternoon sun.
- Full Shade – These plants need less than 3 hours of direct sunlight. They’ll also survive and even thrive if they don’t get any direct sunlight.
Once you know these terms, you’ll figure out the light requirements of a specific plant variety only by looking at the specifications. If the specs aren’t available, type in the plant’s name in any search engine and tons of sources will appear to tell you how much light your plant needs.
But Are My Plants Getting The Right Amount Of Sunlight?
So even if you follow the instructions for the plant and try to make sure they get the desired number of hours of sunlight, how can you tell if your plants are happy? Are there any signs to know if the plant is getting too much sun, too little sun, or the right amount of sunlight? Yes. Look for the symptoms, and you’ll know for sure if your plants are getting only the light they need.
If you have a healthy-looking plant with bright leaves in dark green color, you have nothing to worry about. Your plant is getting all the sunlight it needs. The dark green color comes from chlorophyll in the leaves. With the right amount of sun, the leaves will have ample chlorophyll in them for photosynthesis and thus give the plant that lush green look we’re all so fond of.
Here are the signs you can look out to see if the plant is getting more light than it would like:
- Drooped stems
- Dried flower petals
- Faded flower colors
- Scorched leaf edges
- Bleached spots on leaves
Here are the signs that tell your plant is getting less light than it needs:
- Slow growth
- Spindly stems
- Fewer flowers
- Thin, small leaves
- Leaning towards the light source
- Increased spacing between leaves
- Yellowing leaves, or loss of dark green color
Figuring the right amount of sunlight for your plants doesn’t come entirely from the specifications sheet or the internet sources. It’s a lot about hit and trial too. If you plant a variety in a suitable location, and the plant still looks unhappy, you’ll need to move it to a more suitable spot and see how it responds to the change.
Light Requirements Of Some Popular Plants
There are many plants out there, each one with its own needs. But, we have put together some of the most popular plants that gardeners like to grow and their light requirements.
Lettuce – Partial Shade
For warmer planting zones, growing them in partial shade, where they get protected from the heat, is ideal, especially if you plant in late summers. If you’re growing them in a cooler region during the spring or fall, the plant would love to grow in full sun.
Cilantro – Full Sunlight
Cilantro thrives in full sunlight in most regions. But, the partial shade will be preferable for southern areas since the plant quickly bolts if it’s too hot.
Ginger – Full Shade
The ginger plant prefers a shaded spot with 2 to 3 hours of filtered sunlight per day.
Radish – Partial Sun / Full Shade
Like most other root vegetables, radishes prefer a shady spot. Direct sunlight can make radishes woody.
Beans – Partial Sun
Beans need a few hours of sunlight to grow flowers and pods; too much sun can fade them. A cool, shady spot works best for beans.
Kale – Full Sun
Kale grows best in full sun, with about 8 hours of sunlight every day, but it also tolerates partial shade.
Marigold – Full Sun
Plant them at a sunny location with at least 6 hours of direct sunlight for heavy blooming.
Sunflower – Full Sun
Give them at least 6 hours of direct sunlight for best results. Sunflowers that get plenty of sunlight produce more flowers that turn throughout the day to face the sun.
Parsley – Full Sun
Parsley grows best in full sun outdoors but will also do well when planted in partial shade.
Tomato – Full Sun
Tomatoes need a lot of unfiltered sunlight to produce fruit. 6 hours of sun is the minimum requirement, but more is better.
Carrots – Full Sun
Carrots ask for at least 6 to 8 hours of direct sunlight to produce a good yield. But, they’ll tolerate partial shade as well.
Broccoli – Partial Shade
Brocollis prefer to grow in partial shade, particularly in hot climates. In the cooler climates, they’ll like full sunlight.
Do My Houseplants Need Sunlight, And How Much?
Houseplants may live indoors, but they’re still plants. They exhibit the same characteristics and have the exact basic requirements. Light is a fundamental requirement of a plant, whether it’s growing indoors or outdoors. You already have the answer to why plants need sunlight.
Now you know that houseplants need sunlight, just like every other plant. The next question will most probably be how much sunlight will be ideal for them? Different indoor plants have different light requirements, but many don’t need full sunlight to grow. That’s precisely why they’re suitable for growing indoors.
Understand the light conditions in your home and select the suitable varieties that will grow well indoors.
How To Make Sure That They’re Getting Enough?
Depending on the type of plant you’re growing, different locations may be ideal for them. Bright-light houseplants such as aloe vera and palms will be happier near a south-facing window.
Medium-light indoor plants, such as most ferns, prefer to get filtered light. Placing them near a west-facing window or placing curtains to diffuse direct sunlight should do the trick.
Low-light indoor plants are adapted to growing in the shade. They won’t stand any direct sunlight. Please place them in a shady corner or away from a window.
Yellowing leaves and slow growth are some of the signs that your houseplants aren’t getting enough sunlight. Either move it closer to the window or move it to a sunnier one.
So now you have the answer to why do plants need sunlight. All plants need sunlight to thrive and give plenty of flowers and fruits for us. It is a little tricky to figure out specific plants’ exact light requirements, but a little research and some hit and trial will make you a pro!