Chapter 7 Understanding Light

How much light, and what type of light, do trees really need to be their best? There is quite a lot to know about light beyond just “full sun, part sun, shade.” We can use the basic physical properties of light to make better decisions about how to provide appropriate conditions for our trees.

Horticultural textbooks talk about both the quantity and quality of light. Intensity describes the quantity of light falling in a given area. Usually light intensity is measured in power units such as foot-candles (ft-c), watts (W), lumens or lux.

Light intensity is the amount of light falling in a given area. In this illustration it is described in foot-candles. Link to original image.

All plants need a certain intensity of light for a certain length of time to maintain photosynthesis. Each tree species has an ideal, minimum, and maximum intensity of light and length of time that it will tolerate.

The ideal intensity and duration is what you will see recommended on plant tags and in books. At this level all leaves can capture sufficient light to maintain photosynthesis.

The minimum intensity of light needed by a particular species depends on its natural habitat. Below this the leaves do not capture enough energy to offset what they expend to stay alive. If a plant does not get enough light it begins to drop leaves in an attempt to reduce its energy demands. Some species like azaleas and yews are understory specialists, meaning they require less light than species adapted to full sun like junipers and pines. Yet even shade-lovers need at least some sun to avoid dropping leaves or branches.

As light intensity increases during the day, photosynthesis increases up to a certain light intensity and then levels off. This maximum light level is the light saturation point. Beyond this point any further increase in light intensity does not result in an increase in photosynthesis.

Light saturation points can vary widely depending on plant species. Above the saturation point a tree’s leaves can absorb too much light and burn, much like overexposed skin can burn. Some trees which supposedly do not like full sun are actually intolerant of the sun’s heat, not the light. If the roots and leaves are kept cool, the tree leaves will tolerate more light. Mountain laurels are a good example of a shade-lover that is actually a “cool-roots lover.”

Some species have wider tolerance ranges than others. For example, Japanese holly is happy in anything from full sun to part shade, and will tolerate full shade. In contrast, pines like full bright sun, and suffer if they are shaded for more than a couple hours daily.

7.1 How Much Light is Enough?

These are the generally accepted descriptions of light intensity for indoor and outdoor locations.

7.1.1 Outdoors

Full sun. At least 6 full hours of direct sunlight. Full, unobstructed sunlight in summer has an intensity of approximately 10,000 foot-candles while the light of an overcast day be around 100 foot-candles.

Partial sun/partial shade. These 2 terms are often used interchangeably to mean 3 - 6 hours of sun each day, preferably in the morning and early afternoon. Partial sun means getting the minimum required sun will be more important. Partial shade usually means having afternoon shade is a priority.

Dappled shade. Dappled sunlight is similar to partial shade, but some sun makes its way through tree branches to reach the ground. Woodland plants and trees prefer this type of sunlight over even the limited direct exposure they would get from continuous partial shade.

Potted trees that do well in partial sun or even full sun in spring and fall may benefit from being moved into dappled shade during the hottest summer months.

Full shade. Less than 3 hours of direct sun each day, with shade during the rest of the day.

7.1.2 Indoors

Indoor lighting varies a great deal. Handheld light meters are available from Amazon or lighting stores for about $100. They have a round sensor on a cord that can be placed directly below lights right where you need to take measurements. For comparison, indoor residential lighting will be 5-40 ft-c for general spaces and 70-90 ft-c in kitchens and bathrooms. Offices will be less than 200 ft-c. 

Direct light/full sun. Gets 8-9 hours a day of direct sun through a window. This is close (but not identical) to the conditions in a greenhouse. Light intensity is 800-1000 foot-candles (ft-c).

Partial sun. Gets 4-5 hours of direct light, but not in the hottest part of the day. This is the preferred light level for almost all tropical trees when they come indoors. Light intensity still averages 800-1000 ft-c.

Indirect light. Does not get direct sunlight but still is in brightly lit area for most of the day. Intensity is 100-300 ft-c.

Low light. Anything farther than eight feet from a large window, and no artificial light. Light intensity drops to 50-100 ft-c.

7.2 Quality of Light Matters Too

For plants the quality of the light is just as important as intensity and length of time.

Light travels as wave-like packets of energy called photons. Photons of light travel at different wavelengths, measured in nanometers (nm). Our eyes see photons traveling at different wavelengths as different colors. White light is made up of 7 colors we see in a rainbow combined together: violet, indigo, blue, green, yellow, orange, and red.

How white light breaks down into different colors on a spectrum. Link to original image.

To describe light quality more precisely we refer to the specific wavelength of the photons in the light rather than the color. For example, a standard traffic signal emits red light of 625 nm wavelength, yellow light of 590 nm, and green light of 505 nm.

Most light sources emit light that is a mixture of many wavelengths. Natural sunlight is made up of approximately equal amounts of blue, yellow, and red wavelengths. A standard incandescent light bulb emits less at 400 nm (blue light) and more in the range of 550 to 800 nm (yellow to red). Fluorescent bulbs emit more 420 nm (blue) and 590 nm (green) light, which is why photos taken in office lighting have a sickly blue-green cast.

Plants and trees need specific wavelengths of light.

• Chlorophyll in leaves captures photons in red and blue light, and uses their energy to power photosynthesis. This pigment reflects yellow and green light, which is why leaves are green.
• Phototropins are pigments that control leader and branch growth. If you have ever seen a main leader bending towards the brightest light, you have seen phototropins at work. Phototropins are sensitive to blue light; the more blue wavelengths there are coming from a light source, the more a branch or stem will grow towards it. Ironically, high levels of blue light also may prevent trees from stretching too much indoors. When plants were grown in a greenhouse where blue light was filtered out, they grew 10-100% taller than plants exposed to normal sunlight containing blue wavelengths. *Phytochromes are pigments that react to red light of 660 nm, and far red light of 730 nm. Sunlight has about equal amounts of both red and far red light. Leaves in full sun will absorb 90% of the red light, but almost none of the far red. Leaves in shadier areas receive mostly far red light, activating phytochromes that stimulate node elongation of the branches in shade, making them stretch towards brighter light. Other phytochromes regulate flowering and fruiting.

7.3 Putting All This Information to Use

So could we grow outdoor bonsai species indoors under lights? Theoretically yes, but practically, no.

Imagine an outdoor tree that is adapted to partial sun. It will need 10,000 ft-c of light for 3-6 hours per day. I checked several sources, and found out that one standard 4-foot fluorescent tube puts out an average of 5000 candles of light. Two tubes should be plenty for keeping outdoor trees happy, right?

Nope. The inverse square rule states that light intensity falls off as a square of the distance from a light source. If a 3-foot tall tree is placed 1 foot below 2 fluorescent tubes, leaves at the apex are receiving (2 x 5000 candles x 1 foot =) 10,000 foot-candles of light. Leaves on branches that are 2 feet away from the bulbs receive 1/4th as much light. Leaves on branches 3 feet away from the bulbs get 1/9th as much light. Leaves near the base will be getting just (2 x 5000 candles)/(4 feet)^2 = 10,000/16 = 625 ft-c.

Demonstration of inverse square law. As an object moves away from a light source, the light reaching the surface of the object drops with the square of the distance. S=light source. Link to original image.

There are ways to provide sufficient light indoors, but they require banks of fluorescent lights anchored parallel to the trunk of each tree. The other alternative is to use a brighter source. HID (high intensity discharge) lamps with metal halide bulbs can generate 36,000 candles at the source. When mounted 6 feet above the trees, each lamp provides adequate light for a 3’ x 3’ square. The main downside is that the lights with housing are about $200 a piece. They also get very hot, and can burn the apex of trees. Also these lights are very popular with indoor marijuana growers, so if you purchase more than 1 or 2 at a time, expect a state DEA agent to show up and ask to see how you are using them.

Now let’s consider a tropical species like a ficus that likes 800-1000 ft-c for at least 4-5 hours per day, but not during the hottest part of the day. Two, 4-foot fluorescent tubes hanging 3 feet above the lowest branches will deliver 10,000 x 1/9 = 1,100 foot-candles of light, and will not overheat the leaves.

Based on light intensity alone, fluorescent lights can do the job, but what about the quality of the light? When I looked at graphs showing the wavelengths of light emitted by several different types of lights, this is what I found:

Source	Wavelengths, Comments
Incandescent	Low intensity with high heat. Produce considerably more far red than red light.
Cool white fluorescent	Standard office lights. Ample blue and red light, almost no far red light.
Warm white fluorescent	Common in under-the-counter kitchen lights. More red than blue light, but still no far red.
Plant gro-bulb (fluorescent)	Almost entirely red and blue. Produce some far red. Lower total light, higher price.
HID: metal halide	Similar wavelengths as cool white, but less green. No far red.

Far red light would be important for flowering plants, but is not essential for foliage trees. I can forego both special gro-bulbs and supplemental incandescent lights. In this case, plain cool white fluorescent lights would provide enough light to keep the ficus healthy. They produce the needed red and blue light, without the far red light that encourages shaded branches to stretch.

7.4 How Long Should the Lights Stay On?

Leaving indoor lights on too long is a common mistake that many people (including me in the past) make hoping to compensate for lower light intensity. The horticultural references I consulted said the optimum light regimen for indoor growth of most plants is 18 hours light: 6 hours dark. The short dark period is necessary to prevent disrupting the normal light-dark cycle.