Lamps for the plants and SAD?
October 5, 2007 11:21 AM   Subscribe

Lamps for the plants and me?

Having recently moved to a high-rise, my trees (miniature bald cypress) are starting to show that they’re unhappy with the amount of light they receive. They’re facing south, but only get direct light for a fraction of the day thanks to some nearby buildings. My apartment isn’t bright enough as it is (moving from lower latitudes, the SAD is apparent), so here is what I’m looking for your recommendations on:
*light spectrum good for trees
*light spectrum attractive to people, helpful for the winter darkness/depression
*can be easily mounted to something mobile so that in the day they face the trees and at night can be bent to face the room
*energy efficiency always a plus

Bald cypress lose their needles in the winter, so during the worst months I won’t have to supplement them at all. There are only 4 bonzai-style trees, so the total quantity that I need won’t be too much hopefully. If you think that the requirements are mutually exclusive (eg, the best lighting for mood just won’t cut it for plants) tell me so and I’ll split it up!
posted by a robot made out of meat to Home & Garden (6 answers total) 2 users marked this as a favorite
good question: I would also like to hear answers
posted by francesca too at 11:58 AM on October 5, 2007

Grow lights work by mimicking the light spectrum of the sun. Full spectrum lights used to treat Seasonal Affective Disorder produce the same type of light. Thus, artificial lights beneficial for your plants will also be pleasant for you. Full spectrum bulbs can be had in incandescents, fluorescents and LEDs. We have the incandescents and fluorescents in our house. The big benefits of incandescents is that you can put them on a dimmer switch, though there are fluorescent screw in lamp bulbs that work as three way bulbs in the proper sockets. I'd say a floor or table lamp that can be directed to your plants would be your best bet.
posted by SteveInMaine at 1:48 PM on October 5, 2007

Choosing lighting for plants can be tricky—particularly if maximum growth is not your only concern.  If you also want the plants to look appealing to humans you will have to make a decision as to what is more important—plant growth, plant appearance, efficiency, or initial cost?  I am not aware of any lighting system which maximizes all four.

Plant Growth

As one might expect, if maximum plant growth is the goal then the ideal energy-efficient lighting has a very different spectrum than daylight.  Laboratory experiments have shown that the photosynthesis process that takes place in plants is least efficient in the region around 550nm—the same region to which the human eye is most sensitive.  Most of the light capable of inducing the photosynthesis reaction is either red or blue.  This is not terribly surprising since just by looking at plant leaves you can see that they mostly reflect green and yellow light while absorbing red and blue light. In addition, although plants require mostly red and blue light, enough light of any wavelength must be applied for the plants to open their stomata to permit respiration. Furthermore, some UV light is important to prevent certain kinds of plant diseases

The curve that results from plotting photosynthesis efficiency as a function of wavelength is the Photosynthesis Action Spectrum.  The curve is typically double-peaked or "saddle shaped" with maxima around 420nm (blue) and 670nm (red) and a valley around 550nm (green).  The curve drops sharply below 400nm and above 700nm.  Despite the valley in the green region around 550nm, there is still significant response in this region.  Many plant species can show specific action spectra that differ markedly from this curve.  In some extreme cases there is no response at all in one of either the red or blue regions.

Based on the Photosynthesis Action Spectrum, lighting manufacturers came up with fluorescent "plant bulbs."  These are designed to emit most of their light in the wavelengths that are more efficient for photosynthesis, namely the red and blue ends of the visible spectrum.  As expected, these lamps look dim to humans and consequently have poor lumen ratings.  They usually look purple or pink to the human eye.

So, if one calculates the number of photons per second emitted by a lamp in the wavelength range between 400nm and 700nm and weight this with an average Photosynthesis Action Spectrum, we can generate a figure akin to the lumen rating, but targeted towards plant use, not human use.  This figure is usually named "Photosynthetic Usable Radiation" or PUR in the literature.

Lamps can be ranked by their PUR/Watt ratio.  As one would expect fluorescent "plant bulbs" do extremely well on this scale.  Surprisingly, some of the more sophisticated fluorescent lamps also do very well on this scale despite having spectra designed to appeal to humans, in part due to their greater efficiency.  For example, the Philips Advantage 5000K fluorescent shows extraordinary performance, with a PUR efficiency similar to the best plant bulbs.


If your goal is lighting that is attractive for humans, you have a different set of concerns, particularly since color "correctness" for artificial light sources is a complicated subject. One way to rate artificial light sources is by comparing their spectrum to the spectrum of sunlight. However, by this comparison neither incandescent sources (like normally household lamps or halogen lamps) nor fluorescent sources are much like sunlight. The problem with the spectrum of most incandescent sources is that they are deficient in blue, while the problem with most fluorescent sources is that their spectrum is not very smooth (they tend to have strong spikes in blue, green, and yellow). To simplify the comparison of artificial lighting sources they are typically compared using two synthetic scales—measurements of their color appearance and color rendering. The color appearance of the light source is the Correlated Color Temperature (CCT). The ability of a light source to represent colors in objects is called its Color Rendering Index (CRI).

Color temperature (CCT) is expressed in degrees Kelvin with 2700K being similar to incandescent (yellow or 'warm') and 5000K being similar to sunlight (blue or 'cool'). Color temperature preference is in part subjective, but people usually prefer warmer lighting in areas with lower overall illumination (especially for interior residential and hospitality spaces ), the intermediate temperatures for the illumination levels typical of most office spaces, and the higher temperatures for lighting levels approximating sunlight. At low light levels, as mentioned, most people find low color temperature sources most natural, but despite this the higher color temperatures sources, like moonlight, actually offer better visibility. The high color temperatures similar to daylight are usually too blue for typical indoor applications. I recommend a CCT of 2700K or 3000K for most residential uses, 3500K or 4100K for office lighting, and 5000K for high illumination day lighting. Standard household bulbs have a CCT of 2700K. Most halogen sources are 3000K. Fluorescent lamps are available with a CCT of 3000K, 3500K, 4100K and 5000K.

Another equally important part of light quality is color rendering, this is the ability of a light source to reveal the "true" relationship between colors. Color rendering is expressed numerically on the color rendering index (CRI), which is a scale from 0 to 100, with higher values being better. Most old style fluorescent bulbs have a CRI of 50 or 60 which makes people and surfaces look bad, colors are dull and people have gray or green complexions. The newer fluorescent bulbs can have a CRI starting at an acceptable 75 and can be as high as a superb 98. Incandescent sources like halogen lighting usually have a CRI of 98-100.


Luminous efficacy is the rate at which a lamp is able to convert electrical power (watts) into light visisble by humans (lumens), expressed in terms of lumens per watt. In terms of energy efficiency, no incandescent lighting source will be able to compete with more modern lighting sources like fluorescent or metal halide. In addition to the efficiency of the lamp, you must also consider the efficiency of the entire luminaire, including such factors as the reflector, shape of the bulb, etc. For instance, the 4' fluorescent tubes that are almost universally used for office lighting are the most cost-effective, energy saving, long-life performance lighting source available. However, the large, cumbersome shape of the 4' fluorescent lamps makes it difficult to use fluorescent lamps to illuminate a small, well-defined, area. Even the most efficient of lamps may not be the overall most efficient lighting source if you waste energy illuminating an area outside of your target. Metal halide lamps are not as efficient as fluorescent lamps, but they're small, very intense, and work well with parabolic reflectors. These lamps are frequently used for retail lighting because they're not only an excellent choice for illumining specific targets, but because their high-intensity means that you can use fewer lamps (reduced lamp changes are often important when you have a high ceiling).


I'd recommend you consider using a luminaire that incorporates either metal halide lamps or 4' fluorescent "tube-style" lamps. There are a number advantages and disadvantages to them both.

Metal Halide lamps are commonly used for indoor plant lighting, particularly when space is at a premium—their energy efficiency, intensity, and compactness being all but ideal. On the other hand metal halide is more expensive in terms of initial costs than fluorescent. Furthermore, they do get quite hot, so one does have to be careful that they aren't placed so close as to burn foliage. In addition, metal halide makes poor ambient lighting, unless one has high ceilings or you can position them to "bounce" their light off a wall or ceiling.

Fluorescent "tube-style" lamps are not only very energy efficient, but they also have a number of advantages when used for both plant and human lighting. Because of their large size and energy efficiency they have a low operating temperature, so they can be placed closer to plants without causing heat damage to foliage. Unfortunately, the fluorescent fixtures one is likely to find in consumer retail outlets are usually designed for ambient lighting, so due to their size and poor light control you'll end up wasting some of this energy efficiency as ambient lighting for the rest of the room. In addition, since you mentioned that you had a concern about mobility, a fluorescent fixture will definitely be less mobile then a metal halide fixture. If you choose to go with fluorescent lighting, use a fixture designed to use the more modern, thinner, energy-efficient, T8 fluorescent lamps, not the older T12 fluorescent lamps used in fixtures frequently sold as "shop" lights. For the lamps themselves, use T8 lamps with a CRI of 85 or better. For best plant growth, use lamps with a CCT of 5000K, but this may be a bit too blue for attractive indoor lighting unless you install enough bulbs to achieve a relatively intense level of illumination.
posted by RichardP at 2:06 PM on October 5, 2007 [5 favorites]

Wow, RichardP said a lot. I'm a plant scientist, so I could say a lot about lighting for the plants, but since RichardP covered everything so thoroughly I'll keep it short and simple. I'm actually about to move and looking into this exact same thing myself! You'll find lots of "full spectrum" light bulbs for cheap that actually skew to one side of the spectrum, and will make plants unhappy over time. I'd say go with a true grow light like this compact fluorescent and put it in a regular floor lamp fixture that you can direct at the plants.

In case you don't know, the prescription for SAD and full-spectrum lights goes like this: sit underneath the light reading or doing some other activity for at least half an hour first thing every morning. The blue-light receptors in your eyes that have been implicated in affecting SAD are in the bottom of your retina, so it is important to position the light so that it is hitting your face at a 45 degree angle. If you put the light in an adjustable fixture, you can aim it at you in the morning, and then re-aim it at the plants for the rest of the day.
posted by emyd at 2:12 PM on October 5, 2007

Thanks! Wow, I think based on your recommendations that I will have to combine some warm CFs (for general lighting) and a cool tube for my special purposes. I have a repositionable tube base in the closet that could probably do this.
posted by a robot made out of meat at 3:43 PM on October 5, 2007

Oh, by my early best-answering I don't want to shut anyone down. For example, anyone with specific product recommendations would be great.
posted by a robot made out of meat at 5:06 PM on October 5, 2007

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