The Ultimate Guide on Light Needs for Indoor Plants

Amongst all the care requirements, light is a vital life source for plants. In this guide, we will focus on the characteristics of light, how it applies to the broad light categories in gardening and the practical methods to ensuring your plant gets the right amount of light to thrive. Click through to understand more about how to provide optimal light to your plants.
Darren T'ngThu Jun 17 2021

The Ultimate Guide on Light Needs for Indoor Plants

Amongst all the care requirements, light is a vital life source for plants. For plants, light is food used in photosynthesis, a process where light energy is absorbed by chloroplasts to turn water and carbon dioxide into glucose sugars and oxygen. Similar to how animals require oxygen in the respiration process, plants require carbon dioxide, water and light energy for photosynthesis. These sugars are then used as food for the plant and fuels plant growth - this means that the more light the plant is exposed to, the more energy it can potentially absorb to create food and in turn, grow faster.

In this guide, we will focus on the characteristics of light, how it applies to the broad light categories in gardening and the practical methods to ensuring your plant gets the right amount of light to thrive.

  1. Understanding light quality and quantity for plants
  2. How do I measure light quantity or intensity?
  3. How do I find the optimal light levels for plants in Singapore?
  4. General rules of thumb for indoor plants in Singapore

Light Requirement Guide for Indoor Plants

1. Understanding Light Quality and Quantity for Plants

The quantity of light is based on the part of the amount of light that reaches the leaves - typically defined by the intensity or the brightness of light that hits your leaves. Receiving sufficient quantities of light is essential in your plant’s growth efficiency and flowering.

The quality of light based on the part of the light spectrum your plant is exposed to. In addition to the quantity of light, the quality of light is also an important factor in enabling optimal growth for your plant. While plants typically absorb lights across the broad spectrum, the peak of photosynthetic efficiency typically falls in the blue and red light spectrums around 400-700 nm respectively. This means that for advanced gardeners, using grow lights of specific wavelengths at specific times may accelerate growth rates, improve root development and plant nutrition amongst others. That said, sunlight covers the entire spectrum you require including ultraviolet, visible and infrared radiation.

For purposes of this article, we’ll only be focusing on the quantity of light which is typically affected by the distance of your plant from the light source and by objects that potentially refracts or reflects light from or toward your plant, including the material of your window.

2. How Do I Measure Light Quantity or Intensity?

Similar to how there are common units of measurements for distance, the same is true for light. The units used typically for measuring light includes foot candles (ftc), lux, candela and lumens. As a form of reference, light outdoors at noon can reach over 4000 ftc. While directly behind a north-facing window, you might barely even see 400 ftc. For reference, a typically well-lit office is at 40 ftc.

If you have understood the quantity of light and what affects it, you will naturally also realise that while the aforementioned are good rules of thumb, the quantity of light really depends on where your house or window is facing, where you are in the world and the types and number of interference mediums in between. This means that light quantity varies significantly from between houses and even between rooms in the house. To truly conclude the light quantity onto your leaves, it is important to use a light meter. Alternatively, you can use a lux light meter measurement application using your iPhone or Android device.

Pro-tip: It is important to measure light quantity multiple times across the day while the sun is not in view to get an understanding of the average quantity of light your plant receives. Should the plant be in direct exposure to the sun, it is important to ensure that the duration is less than what is tolerable by the plant. Scorching occurs otherwise.

3. How do I find the optimal light levels for plants in Singapore?

Light levels for indoor plants are significantly lower than those for outdoor plants that require full sunlight. This typically ranges between 100 ftc to 1000 ftc for maintenance and good growth respectively and direct sun tolerance levels at 3-4 hours depending on plant type. The House Plant Journal has collated a useful range of light ranges for growth and maintenance based on plant type.

3a. What is direct light?

Direct light typically refers to unfiltered outdoor sunlight. Some popular indoor settings that can provide direct light conditions include unshaded areas with large floor-to-ceiling clear glass windows. That said, plants that require direct light is best grown outdoors on an uncovered patio or in the soil unless you live in a glasshouse.

Footcandle ranges at mid-day for this category of light requirements should be over 1000 ftc on average with four or more hours of direct exposure to sun rays. It is unlikely that you would be able to get this amount of sunlight if you live in an HDB flat. In such a scenario, you might want to consider having grow lights to simulate a similar environment.

3b. What is partial direct light?

Partial direct light or partial sunlight typically refers to plants that do well with less intense unfiltered outdoor sunlight. While seemingly confusing at first glance, some popular indoor settings that can provide partial direct light includes an unshaded area that only experiences east or west-facing window. This is most houses in Singapore as long as their windows have not received any shade treatments or any front porches with shade. It is also worth noting that east-facing windows typically experience more gentle sunlight, meaning lower light quantity.

Footcandle ranges at mid-day for this category of light requirements should be between 500-1000 ftc with an average of fewer than four hours of direct exposure to sun rays. You would likely be able to get this amount of sunlight in the corridor of your HDB flat.

3c. What is indirect bright light?

Indirect light as a broad category is sunlight that is exposed to your plant only after interacting with an interference medium, which is defined as an object that reflects light or refracts light before reaching the plant. This ranges from the commonly heard bright indirect light of east or west-facing windows in Singapore to fainter indirect light of north or south-facing windows. If a plant is tagged to excel in “partial shade” or “indirect bright light” it will likely do well in indirect light indoors.

Footcandle ranges at mid-day for this category of light requirements should be between 150-500 ftc with no direct exposure to sun rays. You would likely be able to get this amount of sunlight in your HDB flat, condominium or apartment behind even a treated window in a shaded area.

3d. What is low light?

Low light as a broad category is similar to indirect light. The only difference is that low light is at a significantly lower light intensity than indirect light. You can achieve this by placing the plant further away from the light source or window. This means in your living area a few metres away or even in a bathroom with little ventilation. Be aware though that all plants require some sunlight. For plants in the bathroom to thrive, they typically need to get some level of sunlight or supplement light with grow lights that provide the right light levels you require.

Footcandle ranges at mid-day for this category of light requirements should be between 25-150 ftc with no direct exposure to sun rays. You would likely be able to get this amount of sunlight in your HDB flat, condominium or apartment a distance behind even a treated window in a shaded area. That said, please note that light intensity reduces exponentially as a function of distance so in the spirit of this article, please measure your light levels before leaving even your low-light plant at the desired spot.

4. General rules of thumb for indoor plants in Singapore

On the off chance that you are not looking to measure the light intensity as described above (please measure - it’s way more straightforward that way), there are some general guidelines you can follow to quickly hypothesise if the area is optimal for your houseplant.

4a. If you see the sun and the sky above your indoor plant, this ain't it, chief

Houseplants or indoor plants are labelled that way for a reason. Put yourself in the position of your plant - if it requires bright indirect light, then it probably does not want to see the sun and sky above it. That said, unless you have specifically designed your house to be that way with shade treated windows on the ceiling to achieve the right light intensity levels, it is likely best to move your indoor plant away from areas with the sun and the sky above it.

4b. The bigger and clearer the window, the more intense the light

In general, the more sky you see in front of the plant, not above, the more light your plant receives. This is the same with having more windows around it. On that same vein, in Singapore, because we’re relatively near to the equator, north or south-facing windows do not vary in light levels much. In addition, east-facing windows typically provide more gentle sunlight than west-facing ones.

4c. The further from the window, the less intense the light

In general, the further you are from the window, which typically is the light source, the less intense the light. This is because photons of light spread across a wider area over time. This is known as the inverse-square law. You can test this out by shining a light onto a surface and gradually stepping back - you should see that the light projected on the surface starts increasing in size and at some point in time, loses brightness and definition visibly. This is the same for the sun shining its light onto your plant and since the sun is significantly further away, even a seemingly small change in distance from an interference medium such as a window can result in a large change in light intensity.

4d. The less of the sky you see, the less intense the light

In our urban environment, it is typical to be living around and between other high-rise buildings. These high rise buildings also act as obstructions - thus causing you to see less of the sky especially if your buildings are in a darker shade or are reflective. In general, if you see more trees, buildings and objects in front of you, likely, the light you or your plant is being exposed to on that spot is less intense.

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Darren T'ngDarren is the Founder of Porchbuds, a technology-enabled plants and gardening marketplace aiming to make the world a greener place.See More Articles By Darren T'ng