All plants need light. They also need the right amount of light. Let's stay away from super vague words like "low light" and "moderate light" and opt for a better, clearer understanding together. In this blog post I invite you to think of your plant's relationship to light in terms of the intensity of light, duration of light, and what could be obstructing light. Ideally our houseplants would receive light emitted from open sky through a big, unobstructed window for at least 10-12 hours a day.
Why is light so important?
Light is the most important aspect of plant life. Without the right light, a plant is unable to synthesize the food it needs to live. This process is called photosynthesis- an amazing process in which the plant uses light energy to create food energy (glucose) from carbon dioxide and water. The plant then stores and uses this food energy for it's usual biological functions such as water transport, structure and stability, photosynthesis, nutrient uptake, resistance to disease/pests, and more.
Remember, fertilizer is not providing "food" to a plant, despite whatever clever marketing ploy a fertilizer company decides to use. All plants need light. Some plants are better at using very little of their stored energy and can withstand terrible light conditions for extended periods of time- but these plants are simply trying to not die. They are not thriving, and will slowly die. Please be nice to your plants. =)
What is the "right" light?
The "right" light is the light that keeps your plant healthy and growing. Light is hard to translate and describe, especially to a community of houseplant hobbyists and not engineers. I hate the words "low light" and "moderate light" that float around. They are incredibly non-specific and extremely subjective.
Sometimes I'll tell someone, "this plant receives a ton of light," but then I go on to say, "I keep it in a south-facing window where it receives direct sunlight for 5 hours in the winter, but in the summer I move it right up against a big west-facing window." The more we try to explain our houseplant care to others, the more specific we find ourselves getting in our descriptions.
I would encourage you to think of light in the following ways:
Intensity
Duration
Things that obstruct light or photosynthesis
Intensity
Light intensity is the amount of light energy (photons) hitting the plant. If it's too intense for too long, a plant cannot use it all to synthesize food, and the plant will either exhibit signs of stress to protect itself, or die. If it's not intense enough, the plant isn't able to synthesize enough food and it will begin to go dormant, or die.
The most intense light would be standing in the middle of a field at the equator, mid-day, with full sun, unobstructed by clouds, blasting your face, giving you a sunburn on contact. Ouch. This kind of intensity is far too great for 99.999% of plants and animals. On the other extreme, imagine standing in a completely dark cave 5 miles underground with absolutely zero daylight making it's way to you; critters in need of light surely cannot live there. Then there is everything else in between.
So, we want to find the happy balance of light intensity for our houseplants.
How far from the window should I place my plant?
The closer your plant is to the window, the greater the light intensity (this is a good thing). No matter where you place your plant, it should have a view from a window such that the open sky should be visible and unobstructed by trees and buildings as much as possible. Placing a plant in the darkest corner of a room or in the center of the house without window visibility is not ideal. Plants should be as close to the window as possible. Most of my houseplants are within a foot from a window.
What is direct vs. indirect vs. artificial light?
Direct light is sun rays touching the plant. Indirect light is the ambient light emitted from the open sky. Artificial light is some kind of bulb/fixture emitting specific wavelengths of light.
What is the intensity of different windows?
South-facing windows
In the Pacific Northwest, our most intense light comes from the south, usually accompanied by direct sunlight touching the leaves of plants exposed to this direction of light. In the summer, windows facing this direction are very intense and can be too much for some of our canopy-lovers like aroids, hoya, and jungle cacti. I keep most of my succulents and cacti in these windows.
North-facing windows
Our least intense light comes from the north, where plants sitting in north-facing windows receive all day ambient light from the sky, but no direct sunlight. I have found my orchids and hoyas do well here, but may need some supplemental light help in the winter.
East and west-facing windows
These windows are great to offer a few hours of direct sunlight to our plants either in the morning or evening, accompanied by all day ambient light from the sky. These are happy-medium windows and most of my plants do best in them. In the dead middle of summer, though, the direct sunlight from a west-facing window light may be too intense for some plants and they should be pulled further away from the window.
Can we scientifically measure intensity?
Ok, so this is a tougher one for me to write about. Light intensity can be measured using a light meter such as this one. We can measure in terms of footcandles (lumens per square foot) or lux (lumens per square meter).
The problem is, you need to measure at many times of the day and determine if the range is suitable for your plant. Be sure to hold the meter in the foliage facing the source of light, and watch how angling the sensor toward the darker places in your house drastically reduce the reading. Here is a small list to make some sense of the readings.
<25 footcandles is too low of light for your plant
25-100 footcandles is really darn low and will offer "not dying but not doing anything else" kind of light
100-999 footcandles is all of our "in-betweens" and suitable for houseplants but you'll have to navigate based on the specific plants you want to grow
1000+ footcandles is usually only possible outdoors and in direct sun
Using light meter readings can be helpful for differentiating the darkest and brightest places in your home, but to be honest, I wouldn't focus on the readings too much. It's much better to pay attention to visible sky from a window and the health of your plant, then adjust accordingly.
Duration
The number of hours your plant receives light is very important, especially as the seasons change. Many of our houseplants are equatorial tropical, receiving 12 hours of daylight daily, without much seasonal change. In the Pacific Northwest, however, the hours of light can reduce to a mere 6 hours of light a day during winter solstice, and slowly build back up to 14 hours during summer solstice.
This winter reduction is incredibly difficult for our plants. We should be adjusting the amount of water we give them to cope with the limited light, and for some of our more sensitive plants we may need to give them supplemental light to get them through winter.
Obstruction
Your plant's leaves are like a solar panel. Blocking the cells blocks the light and reduces their effectiveness.
Is anything blocking a view of the open sky?
The larger the window and more visible sky to your plant, the better. Be mindful of what could be blocking the light from the sky to your plant... tall buildings or trees outside, or furniture and walls and curtains inside. If your plant only receives light through a pinhole window, it will not thrive as the walls around the window are obstructing a good size view of the sky. Similarly, if you keep your blinds or curtains closed most of the day, those windows will not be appropriate for plants.
Are your plants dusty?
Dust is another light-blocker that we can easily take care of by rinsing our plant leaves or dusting with a soft cloth regularly.
Supplemental light
Unless you want to sent up a basement grow room with expensive light equipment, most of the time artificial light is not a suitable replacement for natural light. We can provide our plants some artificial light to help them get through less than ideal light situations, but the best you can do for your plant is give it a big, open, beautiful view of the sky. An artificial light should be seen as a supplement, such as helping us PNW'ers get our plant collections through our dark winters. My grow lights help maintain health but I do not expect active growth like I do in spring and summer with them.
How do you choose a grow light?
When selecting a grow light, I have learned it's best to look at the kelvins (basic unit of color temperature) in order to assess how "full spectrum" the light emitted is. The spectrum is of course referring to the light spectrum, where plants use various wavelengths based on different processes, such as utilizing reds for flower production, germination, and root development, and utilizing blues for chlorophyll production and foliage growth. I look for a light that has 6,000+ kelvins color rating- these emit an exceptionally bright white light.
Wattage is simply a measurement of power, and higher wattage bulbs tend to be more intense in their light output (but this does not change the light spectrum emitted). The higher the wattage, the further away the light can be from the plant itself. Most of my lights are under-cabinet 10w strip lights I keep 12" or less from the foliage.
There are so many lights on the market these days, and I doubt all of the manufacturers are doing research on the effectiveness of their products on plant growth and development (especially on the cheap products). A few are, though, and those are the ones you will want to focus on if you are looking for high-quality and guaranteed results. Plenty of marijuana farmers have been growing successfully in basements without natural light for years! But the equipment they use is pricey and not exactly aesthetically pleasing among your home decor.
How long should you keep the grow lights on?
Be sure to keep your grow lights on for at least 12 hours a day. 12-16 hours is ideal, followed by a dark cycle of at least 6 hours (yes, plants need darkness, too!). I keep mine on outlet timers to make life easy- they turn on at 3am and turn off at 5pm daily all through winter. In the spring I will likely keep them on, but for far less time, because I want to get good growth and the lights act as a lovely supplement.
What lights do you use?
Here is my list, straight from my Toolbox, or, you can jump straight to My Amazon storefront on lights .
Light summarized
All plants need light to live.
Consider what intensity of light your plant needs to thrive.
Increase intensity by keeping plants as close to a window as possible; giving plants some direct sunlight for a few hours a day through an east-west or south window; or supplementing with artificial lighting.
Consider how long the natural light lasts during the day (duration). If it drops below 8 hours you may want to supplement with grow lights.
Consider if anything is blocking (obstructing) light from reaching your plant through a window- curtains/blinds/frosted windows/walls indoors, or trees/buildings outdoors.
If you need to use grow lights, keep realistic expectations. They are meant to supplement and help maintain health until natural light conditions improve.
When shopping for a grow light, aim for "full spectrum" with 6,000+ kelvins color temperature and keep lights on for 12-16 hours daily. The lower the wattage the closer the plant needs to be to the light.
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