Like anyone else, I’m always somewhat confused about how to measure light levels in my home. I’ve spent a lot of time
researching, trying to put down in words how it all works. Although I’m not completely clueless anymore, I’m not
entirely enlightened either; no pun intended! The problem is that the human eye is not a good judge of light
intensity; it compensates for different light levels so as to give the impression that the light level is even
throughout the room.
Perhaps some basic information about light and its levels will help. The question is always how much light house
plants really need. First of all, when natural light enters a home, it has already decreased dramatically so your
light quality is already significantly lower.
As a houseplant lover, you already know that the most important environmental factor for growing healthy houseplants
is light. Most homes have inadequate light to begin with and if you don’t place plants in an area that has some
decent lighting you will make the indoor environment even more detrimental to proper growth.
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Light is necessary for all plants; no plant can do without it. They use this energy
source to photosynthesize. The intensity of light influences the manufacturing of plant food, leaf colour, flowering
and stem length. Light has three important aspects to it: intensity, quality and duration.
The intensity and quality of light received by an indoor plant depends upon how close to the source of light the
plant is and where the source is. There are other factors (some are obstacles) that influence all this. These are
things that can affect light levels: curtains, trees, buildings, weather, season, dirty panes, screens, location of
window (north, south, east, west).
A southern exposure has the most intense light. East and west locations receive about 60% of the intensity of
southern locations. Northern locations have the poorest levels of light: 20% of a southern exposure. Plants near
windows obviously receive far better/more light than plants placed several feet away even if the room is flooded
with light, and even if they’re in a room that has windows facing south. So imagine how poor the quality of light
is if you were to, for example, place a plant several feet away from a northern window, which already has very low
light levels.
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For a quick determination of the intensity of light anywhere in your home, place an object in a room and check the
shadow. The more defined the shadow, the more intense the light. The more intense the light, the brighter the
location, the better the quality, the happier the plant.
For yet another way to look at it there’s the foot candle explanation. Light is measured in units called foot
candles. As stated above, the amount of natural daylight that enters a home through windows is a small fraction
of the amount of light offered under an open sky. The ‘foot candles’ measurement doesn’t really mean much to most
but the definition is important: it’s the amount of light cast by a candle 1 foot away.
Consider this:
In the summer time, outdoors of course, the midday sun can produce as much as 10, 000 - 13, 000 foot candles of
light. As an example of intensity, imagine the amount of light falling on a 12-inch square of cardboard cast from
10, 000 burning candles 1 foot away.
That’s a lot of light.
So what happens inside our homes?
You would be surprised how low the levels of light inside a home can be. A sunny window, which provides the maximum
intensity of light, will usually produce no more than about half the amount of light as the outdoors, sometimes a
little higher, many times lower. It all depends on all the other factors mentioned above - curtains, trees,
buildings, weather, season, dirty panes, screens, etc.
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The light levels drop dramatically as you start to move away from the window. As little as three feet into the room
will drop light levels to as little as 3/4 of the amount of light available at the window. That’s a substantial
decline. That’s also an example of a south facing window. Now imagine the enormous deterioration of light intensity
if you move a plant further away from a north facing window, which has only 20% exposure of the amount of light
in a southern window, which has about 50% exposure of the amount of light in the open skies of the great outdoors.
That’s no longer a substantial decline; that’s a very detrimental one. If you do the math, you’ll get the picture of
each room’s light intensity as you move away from the windows.
There are a lot of tropical plants that will tolerate low light levels to a certain extent. But no plant will
tolerate extremely low levels for long periods. Without sufficient light, plants will draw on their own stored
resources until they have exhausted them. Plants in very poor locations will stop growing and start declining. A
plant without enough light will definitely deteriorate over time. Even if the light you offer your plants is enough
to keep them alive for extended periods, as if they’re attached to a life-support system, the question remains:
Do you want plants that thrive or cling to life?
Here’s another simple test:
Place a white piece of paper on the location you want to set your houseplant. Place your hand about a foot away from
the paper – your hand will be between the light source and the paper. If you see a well defined shadow, you have
bright light. If you see a shadow with blurry edges, you have medium light. If you barely see a shadow, you have
low light.
And if you don’t see a shadow at all – place an artificial plant there instead! 'Nuff said :)
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* I also recommend picking up a light meter; it's worth the investment *
