Question
QUESTION: Hi Will,

I hope this question is within the scope of acceptable questions even though it doesn't pertain to a particular plant. It seems the most crucial aspect of growing houseplants successfully is the amount of light they receive. So I've been reading up on light, lux and lumens. All very interesting from a theoretical perspective but I don't know how to apply that knowledge. Is there an easy way to determine how much light enters a window and how it disperses to different areas of a room or is a light meter required and, if so, can you recommend one suitable for a homeowner? Thanks.

ANSWER: Hi Mark,

Your question is an excellent one and addresses an issue that I wish more houseplant owners were familiar with. Many years ago I did similar research and managed to get myself totally confused about the various methods and units of measuring different characteristics of light. Most of those scientific details turn out to irrelevant to the average owner of plants.

The two aspects you do need to understand about light for plants is intensity and quality or color. Intensity is nothing more than how bright the light is. Light color refers to the portion of the light spectrum emitted by the light source. Natural sunlight is full spectrum, but artificial lights include only limited portions of the spectrum. The bottom line is that incandescent lights, which most people still use in their homes, do not provide light in the spectral range appropriate for healthy plant growth. However, fluorescent lights do provide excellent light for plant growth. Of course, nothing beats natural light.

There are various measures of light intensity, but foot-candles is most commonly used for indoor plants. Indoor plant professionals often designate a range of acceptable foot-candles for particular plant species that is useful for them to make comparisons. There are excellent digital meters that measure light intensity in foot-candles, but they are expensive, costing $75 or more. I have one, but like most professionals I almost never use it anymore. After a little initial experimentation with it in different locations, you soon become adept at gauging light levels without the use of any devices.

When I go to a new location, here are some factors that I consider n determining light intensity. How large is the window and which direction does it face? Are there any tall trees or buildings that block some of the light? Are there any window treatments that filter out some of the light? Is teh glass tinted? Most importantly, how far away from the light source will the plant be located?

For your own home, observe carefully how deep into the room the sun shines directly onto the floor on a sunny day. I the northern hemisphere, light from a north window will never shine directly into the room, whereas light from a south-facing window will penetrate 6-8 feet into the room during the winter when the sun is low in the southern sky.

How much the sun penetrates directly into the room is important, because the difference between direct and indirect sunlight is very important. Many common houseplant species will not tolerate direct sunlight at all or for more than a couple of hours per day. Other high light species require direct sunlight for all or most of the day. High plants plants must be located directly in front of a window and never more than a few feet away. Lower light species can be placed off to the side of the window or deeper into the room outside the range of direct sunlight. However, because light intensity drops off dramatically with each foot of distance from the window, even low light plants will not survive long-term if they are more than 4-5 feet from a north window or more than 10 feet from a south window.

All plants have a range of light in which they can survive. At the lowest end of the range, a plant will grow almost imperceptibly and use little water and nutrients. That same plant at the upper end of its light intensity range will flourish, grow rapidly and demand frequent watering and fertilizing. A plant just below the minimum light range will very slowly deteriorate, perhaps over the course of several years or more. So, it is light that controls growth rate and everything else.

This is probably far more than you wanted to know, but light evaluation is a critical determinant of plant success and should be the most important criterion used in selecting plants for your home. I hope this will help you without your having to understanding lumens and lux or buying light meters.

I have written articles on evaluating light and on low and high light plant species that I will email to anyone who sends a request to my email address below.

Please let me know if any of this is unclear or if you have any additional questions.

If this information has been helpful, please click the Rate Volunteer bar below and enter a rating and nomination for me. I am a volunteer on this site so Ratings are the only compensation I receive for answering plant questions.
         
Need more information? Visit my website at www.HorticulturalHelp.com
or email me at [email protected]
or call me at 917-887-8601 (EDT)
  
Regards,
Will Creed, Interior Landscaper
Horticultural Help, NYC


---------- FOLLOW-UP ----------

QUESTION: Thanks, Will, for such a thorough answer. You provided great general guidelines to evaluate light. My guess, though, is that light intensity is not equal in different parts of the country (e.g. winter light in Florida has a higher intensity than that of Washington.)

Am I correct in assuming that if I want to know light levels accurately in different parts of a room I'll need a light meter? Can you please recommend a reasonably priced, yet accurate light meter?

Thanks,
Mark  

Answer
Hi Mark,

If you can afford to spend $100 or more on a light meter, I am sure you will find it to be instructive. But you can also make yourself crazy with a light meter as soon as you discover that light intensity in the same location changes hourly, seasonally and with the weather. So you will have multiple different readings for any one location. Ultimately, you have to make a decision as to whether those multiple readings are collectively suitable for your designated plant. I know this from experience because I once had charts and data for a dozen locations in my home which just increasingly confused me. So go for it but don't get carried away!

The digital light meter that I have is no longer available. Here is a link to a site that has light meters: http://www.reliabilitydirectstore.com/Light-Meter-s/32.htm?gclid=CMnkmb3AkqUCFQY

I suggest the ones for $109 or $129. Perhaps you can find a comparable one for a bit under $100, but don't go too low or you will lose quality and accuracy.

The primary difference between the light in FL versus Washington is the number of hours of daylight in winter. The FL light is not necessarily a lot more intense, but there are more hours of it. Of course, the reverse is true in the summer. As professionals, we want to know how intense (direct or indirect) the light is and how many hours per day that light is available.

Please let me know if any of this is unclear or if you have any additional questions.

If this information has been helpful, please click the Rate Volunteer bar below and enter a rating and nomination for me. I am a volunteer on this site so Ratings are the only compensation I receive for answering plant questions.
         
Need more information? Visit my website at www.HorticulturalHelp.com
or email me at [email protected]
or call me at 917-887-8601 (EDT)
  
Regards,
Will Creed, Interior Landscaper
Horticultural Help, NYC