Indoor light for houseplants: an engineer’s perspective - gardenstead Skip to content

Indoor light for houseplants: an engineer’s perspective

As an engineer, I love to measure things – and measuring light is my absolute favorite activity when it comes to houseplant care. It is the only way to truly know if your plant is working – as in, doing enough photosynthesis to stay alive. And only a light meter (measuring in foot-candles or lux) can give you an accurate sense of light – don’t just trust your eyes! Your eyes adjust to varying levels of light so you won’t *feel* the brightness differences as you move farther from a window. A light meter will give you an objective assessment of light.

Just for context, here are some examples of light intensities and the physical situations that produce them:

Have you ever wondered: ‘what does it mean to have perfect light?’ It occurred to me that a plant nursery would be the place where plants got perfect light. Why? They have a monetary incentive for growing plants as fast and as perfectly as possible. “In the wild” would, in fact, not be where you’d find the ideal conditions – by definition, the wild is only where the fittest would survive. And survival doesn’t always look pretty. Therefore, it is the controlled conditions of the nursery where we would create the perfect conditions for plant growth.

Through doing some research online, I found a treasure trove of datasheets that specifically defined the ideal NURSERY conditions/practices needed for producing many of our favorite house plants.

What do they say about light? Here are some findings on common house plants:

  • “Spathiphyllum (peace lily) will not tolerate high light intensities without reduction in quality and should be grown under 1,500 to 2,500 foot-candles.” (FC) 1
  • “Boston ferns have a wide tolerance to changing light levels, but grow well when receiving 1,500 to 3,000 ft-c, with best quality usually produced near 2,000 FC.” 2
  • “(Pothos) Stocks plant are best grown at 5,000 FC. Number of cuttings and weight of cuttings are larger than those from stock plants grown at 2,000 FC. Pothos cuttings will have rapid shoot and root growth if light is 3,000 FC” 3

Great! So how do they achieve these light levels?

The roof of the greenhouse is ***COVERED*** by several layers of shade cloth so the sun’s light is evenly diffused and distributed.

Therefore, the ideal light intensity for growing most tropical foliage plants is achieved by blocking direct sun, weakening it to the prescribed intensity. At the same time, the open sky reflects light down onto the plants from all angles from above. The light intensity rises and falls with the sunrise and sunset.

The Dark Truth

When you start analyzing light with measurements, you will come to the realization that the light situation for plants indoors is not ideal – with windows, walls, and ceilings – there is both too little indirect LIGHT and possibly too much direct SUN. When the sun is not in the direct line of sight, the overall view of the sky is constricted by walls and the ceiling – this is “low light” – the smaller the window, the lower the light because the view of the open sky is smaller. When the sun does shine directly through the window and onto the plant, it’s not that direct sun itself kills the plant, it’s the rapid water loss that can lead to permanent wilting if you’re unable to keep the plant hydrated in a timely manner.

The truth is: how well your plants grow is most influenced by the physical size of your windows. Think of anyone who seems to have the proverbial “greenthumb” – they likely have very large, unobstructed windows (or they know how to use grow lights). Now, I’m not saying there’s some conspiracy in horticulture – it’s just that truly understanding light might discourage people from buying certain plants. Since indoor plants are often seen as decor, to say that light should determine where they should be placed would go against where a designer might think it would look best.

Don’t be discouraged that you don’t have the light conditions like a commercial greenhouse – remember that you don’t need to grow the plants for business! What you do need to understand, however, is that there will be a maximum potential for light in your home – some locations that will be “as good as you can possibly provide.”

For any plant that requires “bright indirect light”, you want to give this plant as wide a view of the open sky as possible. The larger the window, the better. The fewer outside obstructions, the better. The more bright objects that reflect the sun into your view, the better. If the sun happens to come into direct line of sight with this area, you’ll want to take note of the number of hours that occurs. Most plants that enjoy “bright indirect light” can tolerate 2-3 hours of direct sun, sometimes more for larger plants. You must understand that when the sun shines directly on a plant (especially the leafy tropical kind), although photosynthesis is maximized, the rate of transpiration is also very high, causing rapid water loss from the leaves and subsequently, the soil.

Once you understand the details of your indoor light situation and accept the fact that not EVERY plant will grow well in your space, you will be happier with the plants that ARE suitable. Combine this with a healthy dose of realistic expectations: that your plants will grow and change – and won’t stay nursery-perfect forever – and you’ll be able to enjoy this hobby for a lifetime!

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Light Meter –

yellow petaled flower by elias sorey unsplash

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