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.

Your Houseplants are Screaming: "You Call This Food?!?"

This was going to be a rant on houseplant fertilizers. I was about to dump on the folly of the latest marketing innovation in houseplant food: the drip feeder. 

These inexpensive (about $1.50), hypodermic-like IV's for houseplants only offer miniscule amounts of nitrogen, phosphorus and potassium, and only for a short time, about two weeks. On the plus side when it comes to houseplants, these mini-meal drippers (which put your plants on a less than 1% N-P-K diet...wayyyyy less than 1%) are healthier that the big buffet offerings of a typical, water soluble plant food that may offer up to 20% N-P-K. 

Huge, salt-based meals like that for houseplants usually result in weak growth, root rot, leaf burn, salt deposits and the dropping of lower leaves.

The big problem with these one-ounce fertilizer drippers, which are intended to be used in an eight-inch pot? All the fertilizer is going to a very small area of the plant roots. One ounce of a liquid, dripping out at the rate of a few drops a day, is not going to percolate throughout the entire rootball. Yes, watering the soil before adding the dripper (as recommended on the label) will help spread the fertilizer. But not enough to nourish all the roots. Or even reach the bottom of the container. And the few roots the liquid does reach? Root burn could still occur, since it is dripping in a very small area. 

And seeing how the N-P-K content of these dripper products is measured in the hundredths and thousandths of one percent, you may be providing more nutrients by "feeding" your houseplants...tap water!

Texas A&M University advises to go easy on the fertilizer, following label directions. "When applying fertilizer in a solution, make sure that some runs out of the bottom of the pot. This prevents root burn and the buildup of soluble salts or excess fertilizer and reduces the chance of burning the plant."

An Ohio State University report concludes that the best way to fertilize houseplants is to... "fertilize at half the recommended strength, every 2 weeks from March to September. Do not fertilize most plants during winter months. Winter's reduced light and temperature result in little or no growth. Most houseplants are dormant during winter."

But, as always, the more I researched the topic, the more I realized there is a lot more to learn, including this surprising tidbit from the Horticulture Department of North Carolina State University: 

"Houseplants grown under less than 200 foot candles (a measurement of light intensity) benefit little from fertilization and may actually be harmed." 

Yes, there are good arguments for NOT fertilizing houseplants in low light situations. And your house, if typical, is probably well below that 200 foot-candle threshold. You may be slowly killing your houseplants with too much food and too little light.

Next time: how to measure the amount of light in your house, using ordinary household items. And, choosing houseplants that do best in low light conditions.