How Much Sunlight Do Indoor Plants Need?


Growing plants indoors can be a very rewarding experience. Whether you like the look of certain flowers, you want to be able to experience food you have grown yourself or you plan to be able to grow your own marijuana plants, the light they need to thrive can be an issue. While some plants need a lot of light to flower, other plants too much light can prevent flowering.

How much sunlight do indoor plants need? The short answer is none, but that is not the complete answer. While indoor plants do need anywhere from 2-16 hours of light to grow and thrive, they do not necessarily need the sun to get that light. You can use indoor grow lights, such as LEDs to accomplish this goal.

This is not to say that you can’t use sunlight to help them grow and for a long time this was the only way to be able to grow anything indoors. Time and technology, however, changes rapidly and can allow for more growing options than just hoping a window will provide the required amount of light needed. Now there are plenty of options for indoor plants available.

What is Sunlight?

Sunlight is obviously the light that comes from the sun, but it is so much more than that. It is also the heat that warms our summers and our planet. While it looks like white light to us, you have to take into account how colors work. Black is the lack of color, while white is the inclusion of all colors. This is why you can see a rainbow during a day shower or use a prism to see one.

The blue light sun is also seen in our sky every day. We see the green in the light when we look at things like grass and other vegetation. How we look at our world is dependant on the different colors in the light that comes from the sun, even though all we typically see is the white part. Without each part of the spectrum we would not see the colorfilled world we live in.

Sunlight contains all of the colors of the spectrum, which includes the white that we use to see with, but it also includes the red and blue light which is vital to plant growth. It is out of this blue and red light that a plant uses photosynthesis to create its own energy source or food, but it also uses it to create oxygen from carbon dioxide.

Why Does a Plant Need Light?


 Whether you are looking at a foliage plant such as a spider plant or an aloe vera plant or planning to grow veggies like brussel sprouts, all plants need light, but the amount of light needed is completely dependant on the type of plant you want to grow. Just as the food required for different kinds of animals varies, as does the light requirements for different kinds of plants.

Different types of plants have different needs and some plants, like brussel sprouts may only need a few hours of light each day where flowers such as asters require more than 12 hours a day. This all comes down to how and when the phytochromes in a plant react. Phytochrome in the leaves become active while absorbing red light whether from sunlight or artificial light.

Although light requirements do break down further, depending on the maturity level of the plant and what you want it to do. Do you want it to grow bigger or do you want it to flower? Though there are also plants that don’t seem to have a preference in how much darkness they need, darkness often plays as big a part of the cycle of the plant as light does.

Plants need light, and darkness, to be able to grow, make food and energy and to bloom, but darkness is just as essential as light is because it is when they rest that some plants can do their most important work which is to grow. Without darkness plants can’t develop flowers and bloom. They can’t grow into the wonderful fruits and vegetables that we love.

It is for this reason that a plants dark needs is just as important and its light needs are. Some plants require more than 12 hours of darkness to be able to do what they do best. This is also when a plant will convert the energy it made during the day into carbohydrates for storage. Some plants need these carbohydrates to be able to bloom or produce flowers or fruit.

Difference Between Plant Types

There are three basic types of plants when it comes to their lighting requirements. Each type defines how much light and darkness each type of plant requires. Some plants can’t handle as much light as others and can be kept from flowing if they don’t get the required darkness. These different types would be:

  • Long Day plants
  • Short Day plants
  • Day Neutral plants

Long Day Plants

Long day plants require more sunlight or artificial light to be able to be able to produce the energy they need. These are usually plants that don’t require energy stores but use up energy as they produce it. Seedlings of nearly any plant would fall into this category as seedlings us up energy quite quickly to be able to grow into a more mature plant.

Seedlings and less mature plants need phytochrome cells to be active to be able to grow. While this does not affect the blooming or flowering of a plant, it will affect its growth. So regardless if the plant, as an adult, is a long day or short day plant, it will still require extra light to be able to grow into an adult and begin to produce flowers and fruit.

In long day plants the phytochrome being active is what causes a plant to flower, but this requires the phytochrome to be active for more than 12 hours per day for several weeks. Just exposing a plant to 12 hours a day for a couple of days would not be sufficient. These are the types of plants that will only flower or fruit during the long days of summer. .

Short Day Plants

Short day plants are the kind that require more darkness than light. Any number of winter blooming plants are of the short day variety because they need the darkness to produce flowers and bloom. Often these are the plants that only bloom or fruit during the spring or fall, but also includes that rare plans that only bloom during the shortest days of winter.

In short day plants the phytochrome actually inhibits the plant from being able to flower. You see this in plants like poinsettias. Poinsettias are beautiful plants, but more people have issues with getting them to bloom a second holiday season and this is because they are actually getting too much light to be able to produce the lovely flowers we associate with the holiday season.

So short day plants need less than 12 hours of light per day, ideally getting only 8 – 10 hours per day when you want them to flower or produce fruit. Due to the phytochrome inhibiting the actions required to be able to flower, these types of plants do best when the phytochrome is limited in its actions so the plant is allowed to flower.

Day Neutral Plants

Day neutral plants are ones that are not restricted either way as far as the light or darkness part of the cycle. While yes they will need darkness to rest, they would not need more than a few hours of rest to be able to bloom and produce fruit. This also means that they do not require more than a few hours of light to be able to grow and produce fruit.

This does not mean that they can grow in darkness nor does this mean that they would grow best in constant light, it just means that they are more flexible on their blooming schedule. Once mature, as long as they are healthy and getting the proper amount of water, they can be grown with either long day or short day plants.

Normally you would want to make sure they get a certain amount of time of light each day. This could easily be done with a light timer, which many home growers and horticulturists use. Day neutral plants like tomatoes, however, can be grown with either poinsettias or with aloe vera plants and would still bloom and produce fruit.

Day neutral plants such as corn, potatoes or cotton do not have a requirement of at least 12 hours of light needed to bloom like a long day plant. They don’t have the 12 hour darkness requirement like the short day plants either. Some may flower as soon as they reach maturity while others may require things like a temperature change before they begin to flower.

Differences Between Light Types

If you have ever read the back of a seed packet then you have probably also come across the following terms (though they may differ a bit by company):

  • Full Sun
  • Partial Sun
  • Partial Shade
  • Full Shade

When I first got into gardening and especially planning the flowers for the front of my home, I thought it would be a great idea to just buy some seeds and grow my own. I never realized how hard it could be or how much I had to learn. I was ready to quit right there, but I am glad I didn’t cause it wasn’t really hard to decipher the language used on the packets, with a little searching.

Worse yet was that I wanted to grow some of them in my home and that made things even harder for me. I knew that to grow plants in my home I would need things like soil and plant food. I even knew I would need lights and I knew exactly what kind of lights I wanted to get. It was when I began to shop for the plants that I had so many questions:

  • If a package says full sun, will my front window be enough?
  • How many hours does full sun mean?
  • Does full sun mean all day?
  • What about packages that say “Full sun to partial shade”?

And that’s just what had me going on the first packaging. Imagine how confused I was after an hour of looking. That was when I decided I needed to know the meaning of each term, but the problem was there was no official definition, but I did manage to figure out approximately what each company meant.

Full Sun

This meant that a plant needed, minimal, 6 hours of light a day, but would do better with more light. 6 hours was what it needed just to survive. To grow and thrive more would be better. Typically these were long day plants needed that extra light to be able to bloom and produce fruit. When it comes to artificial lights, this would mean at least 12 hour days.

Partial Sun

Partial sun means that the plant must receive at least three hours of sun or light a day, but would do better with closer to six hours. Less than three hours a day will kill this type of plant. In terms of artificial lights, this would still fall among the long day plants, but will do well with less than 12 hours, unless you are trying to get them to bloom.

Partial Shade

This type of plant is more susceptible to heat from the sun when outdoors, but indoors could still do well with a full day’s worth of light. They are more heat sensitive, so need lights that won’t dry them out or produce much heat.

Full Shade

This type of plant would need less than three hours of direct sun and would typically be more of a short day plant. They would have more issues with direct sunlight and heat than most plants. These types of plants may be more likely to need at least 12 hours of darkness to flower.

Multiple Terms for One Plant

This is the one that got me the most. The package that says things like “Full sun to partial shade” or “Partial sun to full shade”. It took me a little bit of research to discover that this meant a plant could survive with less light, but would do better in more, or vice versa. Plants like this, I discovered, usually required more personalized research.

Oftentimes this type of description was found on packaging for day neutral plants. Ones that didn’t have a light requirement to be able to flower. These were the types that are usually the easiest for a home gardener to be able to grow and produce flowers or food.

Plant Life Cycle

Depending on where a plant is in the life cycle can determine what it’s light requirements are as well. From seeds to mature plants, the light requirements can and often do change.

Seedlings

A seed or seedling is the exception to the rule in that seeds or seedlings require more light as they grow from a seed to maturity. While they will require a rest period of at least eight hours every night, they can easily tolerate 12 – 16 hours a day, at least until they get close to becoming more than just the tiny beginning of a plant.

With the fact that seedlings are just the beginning of life for a plant, they do require more specialized care, at least until they are matured a bit. As they begin to grow leaves and are able to produce food more readily from light, they usually do have a higher light requirement than the adults of the same species.

Young Plant

A young plant will still have a higher light requirement than a mature plant, but not nearly as much as a seedling does. At this stage you could reduce the light from 16 hours to 14 hours if need be to give the young plant more of a chance to rest in the darkness. This will allow it to be able to start converting some of the energy made to carbohydrates.

Mature Plant

When we talk about light requirements, this is the stage that the requirements refer to. A mature plant will be using the light to convert carbon dioxide to energy and produces oxygen as a by product. This is also when the concern for flowers and fruit becomes an issue as well. Knowing whether the plant is a long day or short day plant is useful as well at this stage.

Knowing how much light a plant needs a day is useful, but also knowing how much darkness is needed is useful as well. While some plants may thrive in the light, those same plants may never flower if they are not given enough darkness to be able to begin the flowering process.

Recent Content