Thursday, August 5, 2010

How Much Light Do Houseplants Need?

How much light do houseplants need in order to grow and thrive? Probably more than you're providing. The limited number of houseplants that can live in low level light (Cast Iron Plant, Chinese Evergreen, Dracaena, Peperomia, Pothos, Sanseveria and the ZZ plant) still need more light than what is found in a typical room: 50-250 foot candles. And those plants will thrive with 200-250 foot candles, but limp along with anything less than that. (Foot candles: a unit of measure of the intensity of light falling on a surface, equal to one lumen per square foot)

A few ways to measure the light in a room:
• Get an inexpensive multi-purpose plant meter that registers light, moisture and pH. Key word in that previous sentence: "inexpensive", as in "I hope it lasts a week" or "Please be somewhat accurate".
I took my $20 multi purpose plant meter to the various houseplants we have here. The shocking news: at noon, in a brightly lit room with a southern exposure, those plants were receiving about 25 foot candles of light.

• Measure the light with the chart contained in this link from the University of Missouri. Their report, "Houseplant Light", states that one way you can estimate the amount of light available is to calculate the number of watts available per square foot of plant area. Low-light plants should receive between 10 and 15 watts of fluorescent light per square foot of growing space. A single fluorescent tube, such as a 2-foot 20-watt tube or a 4-foot 40-watt tube without any other light provides only enough light for plants in this category.

If your houseplants are getting light assistance only from a nearby table lamp or a ceiling fixture, don't be surprised if that calculation shows less than 40 foot candles. 

• And this really simple, but scary light test from the Christian Science Monitor:
 "On a sunny day, get a piece of white printer paper and place it where you want to put a new houseplant. Then hold your hand 12 inches above the paper. Can you see an indistinct shadow? If so, that's low light.
If the shadow is a  bit fuzzy but mostly looks like a hand, that's medium light. A clear hand shadow indicates a high level of light."

I tried this. In many areas of the house, I saw NO shadow, or a dark, fuzzy image, at best. Oh, those poor houseplants.

Fortunately, my abode is populated with easy care houseplants, which I've talked about before here.

Bottom line: My houseplants are alive...but certainly not putting on any growth spurts. They are not going to get any more light. So, I won't stress them with fertilizer (as per this report from the University of North Carolina). Watering is limited to once a month, perhaps twice a month in the summer. Tough love.

If you want to coddle your houseplants, or expand the variety that will thrive in your house, here are a few tips from the University of Missouri:

• The amount of light necessary varies with each plant. In general, the light fixtures available for home plant lighting make it practically impossible to produce too much light for most plants.

• Plants that can adapt to interior settings usually are divided into three general categories: those suitable for low, medium and high light intensities. The categories generally indicate the minimum light required. Growth is often best at the higher end of these suggested light ranges.

• Plants referred to as low light intensity plants generally should receive between 50 and 250 foot-candles. 

Medium-light plants
 prefer 250 to 1,000 foot-candles. Best growth occurs above 750 foot-candles unless plants also receive extended periods of direct sunlight. Give them artificial light in the 500 to 1,000 foot-candle range or 15 or more watts per square foot of growing area. 

High-light plants generally are less satisfactory for growing under artificial lights in the home. However, if you want to try, use special high-intensity lamps. These plants need at least 1,000 foot-candles, or 20 watts per square foot of growing area, but should have higher intensities for best growth and flowering. Fixtures containing three to four fluorescent tubes are necessary for plants requiring high light.

• As a single light source for plants, incandescent light bulbs are not particularly good. They are a good source of red rays but a poor source of blue. They produce too much heat for most plants and, if used, must be located some distance from the plants, thus reducing the intensity of the light the plants receive. They are also about one-third as efficient as fluorescent tubes in converting electrical energy to light.

• Cool white fluorescent tubes provide one of the best artificial light sources available for plants in the home.

• Special fluorescent tubes also have been developed for growing plants. These have a higher output in the red range to balance the blue output. Many home gardeners have found that these tubes can be used in combination with cool-white tubes. Use one special plant-growing tube to each one or two cool-white tubes. This method is more economical than using all special tubes, since cool-white tubes cost less than the special plant-growing tubes. 

• Most plants should be located with the tips of the plants 6 to 12 inches from the light source. The intensity of light drops rapidly as the distance from the light bulbs or tubes increases.

• The brightest spot under a fluorescent fixture is directly beneath the center of the tubes.

• The light fixture's position should be adjustable so you can keep the distance between the light and the plant fairly constant.
• In most cases, plants receiving no outdoor light should be lit from 16 to 18 hours each day. If some additional light is received, 12 to 14 hours each day may be adequate.
• Space plants far enough apart to allow light between them. Arrange plants so they do not shade each other. Keep tubes clean and replace old tubes promptly.

There are a lot of houseplant lighting systems available. Choose one that is right for the requirements of your most finicky houseplants. Or not. Tough love.

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