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Prevent Light


International Year of Astronomy link

The National Academies Press: Status of Pollinators in North America (2007)

The National
Academies Press
Status of
Pollinators in
North America (2007)

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Light Pollution Harms Plants in the Environment

This page covers:

Light pollution does not just affects plants' cycles directly, as discussed below, it also affects them indirectly by interfering with the lifecycles of their pollinators or other animals that interact with them. Jump here to follow the effects of light pollution verses the plants' pollinators. This other article describes how artificial light at night affects bats that disperse the seeds of fruit trees.

Light pollution's impact on other species in the environment are found here.

A society grows great when old men plant trees whose shade they know they shall never sit in.
-- Greek Proverb


Many cactus species bloom only in the dark of night. They are pollinated by nocturnal insects or small animals, principally moths and bats. And those of Selenicereus grandiflorus (Queen of the Night) are fully open for only two hours at night. Increasing the lighting conditions around them may prohibit them from ever flowering and thus reproducing.

This not only affects the cactii, but also their pollinator species as well. In general, those plants that are strongly scented and have white flowers are typically pollinated at night. The flower's whiteness and aroma helps the night pollinators to find the flowers at night. Red flowered plants without smells are typically pollinated by birds, as most birds are attracted to red but only a few have a sense of smell. As the National Academies Press' booklet Resources on Pollinators states: one third of all human food requires a pollinator. Altering the lighting conditions at night can further unravel our web of life which ultimately disrupts our food sources.

Does Night Lighting Harm Trees?

Source: Purdue University, Forestry and Natural Resources, FAQ 17, 4 pages, Jun, 2002.

William R. Chaney

Department of Forestry and Natural Resources, Purdue University, West Lafayette, IN

Chaney points out that for normal growth and development, trees depend on light's quality (wavelength or color), its intensity (brightness), and the duration of its 24 hour light-dark period (photoperiod). It doesn't matter to a tree whether the light comes from the Sun or artificial sources as long as the required wavelength, intensity, and duration are provided. The two important photobiological processes in trees and their required color wavelengths are:

  1. Photosynthesis - requires visible blue (400-450 nm) and red (625-700 nm) - uses light to make sugars that the tree uses.
  2. Photoperiodism - requires visible red (625-760 nm) and infrared (760-850 nm) - controls vegetative growth and reproductive activities as governed by the lengths of the day and night.

Chaney said that relatively high light intensity of 1000 microeinsteins per square meter per second (E/m2/sec) is adequate for photosynthesis in most trees, while 200 E/m2/sec is what is needed for shade adapted trees. However, photoperiod responses may be induced with as little as 0.06 to 3 E/m2/sec, which is a mere tiny fraction of what they need for photosynthesis. For reference, indoor lighting sufficient for reading is about 4.6 and full Moon's light is about 0.004 E/m2/sec. A 100 watt incandescent bulb provides 5 E/m2/sec at 5 feet away, while a 150 watt fluorescent cool white bulb provides 17 E/m2/sec at the same distance.

It has been known since the 1940s that it is the duration of uninterrupted darkness during a 24 hour cycle that governs developmental processes in angiosperms (flowering) plants, such as trees. A plant's developmental processes are those such as dormancy, shoot growth, and flowering. A photoreversible pigment called phytochrome or cryptochrome reacts to the length of both the day and night periods depending on whether it absorbs red (625-760 nm) or infrared (760-850 nm) wavelengths of radiation. Even a momentary flash of light during the dark period is enough to trigger the condition induced by a short night or, conversely, a long day. (So called long day only because initial thinking was that it was the length of the daytime that was important.) Trees as well as other plants are classified as long-day, short-day, or day-neutral according to their response to day length.

  1. Long-day trees flower in early summer and continue vegetative growth until days shorten in the fall.
  2. Short-day trees flower and enter dormancy when day length shortens in late summer.
  3. Day-neutral trees are not affected by day length at all.

Photoperiodism can also influence leaf shape; surface hairiness (pubescence); pigment formation; autumn drop time; and root development, as well as onset and breaking of bud dormancy. Night lighting alters the natural photoperiod and so, upsets the plant's development.

Effect of Night Lighting on Trees

It is clear that most night lighting may not be enough to cause photosynthesis, but still can affect trees that are sensitive to day length. Artificial lighting, especially from a source that emits in the red to infrared range of the spectrum, extends the day length and can change flowering patterns, and most importantly, promote continued growth long after it is safe for the trees to do so, due to a coming winter.

A tree's protective dormancy should shed leaves before damaging winter storms occurs. The normal amount of ice and snow that builds up on the trees will not be enough to break their branches off and damage the tree. But if the tree has not shed its leaves in time, then the ice and snow build up CAN break its branches off and harm the tree. Young trees, because of greater vigor and tendency to grow longer naturally, are more subject than older mature trees to cold injury as a result of growth prolonged by artificial illumination. Continuous lighting, which is too common, is potentially even more damaging than lighting that is turned off late in the evening. The foliage of trees grown in continuous lighting may be larger in size and more susceptible to air pollution and water stress during the growing season because the stomatal pores in leaves remain open for longer periods. There is a good deal of variation in the susceptibility of woody plants to artificial lighting. Highly sensitive trees should be avoided in areas where high intensity lighting rich in red and infrared wavelengths are used. Chaney's FAQ paper lists the susceptibility of sixty-five different trees to artificial lights.
Cobrahead streetlight affects Amagansett, NY tree.
A tree in Amagansett, NY that holds its leaves longer in the same pattern as the semi cutoff fixture right next to it. It does not achieve full dormancy before winter weather sets in. This light can shorten the tree's life.

Spectra Produced by Different Light Sources and Their Effects on Trees

Different light sources have different emission spectra. Each type of lamp gives off different quantities of certain wavelengths (color) of light than another type of lamp. This next table sums up the lights sources, their spectra and their potential impact on trees.

Wavelengths emitted by different types of light sources and
their potential effects on photobiological processes in trees

Light sourceWavelengths
Potential effect
emitted on trees
FluorescentHigh blue, low red
IncandescentHigh red and infrared
Mercury vaporViolet to blue
Metal halideGreen to orange
High pressure sodiumHigh in red to infrared

In the early days of street lighting, the lamps used most commonly were either low-intensity whitish incandescent filaments or higher intensity, bluish fluorescent, mercury vapor, or metal halide lamps. While these sources highly attracted insects, they had little effect on plants because of their low levels of red or infrared light. In the mid-1960s, high pressure sodium (HPS) lamps were developed, which emit considerable high-intensity light in the red and infrared regions. Increased injury to woody plants has been reported since the widespread introduction of this type of artificial lighting.

If it has to been done, Chaney recommends using mercury vapor, metal halide, or fluorescent lamps that order of preference for artificial lights for trees. High-pressure sodium lamps should be avoided and even low-intensity incandescent are best excluded due to its high output of infrared and potential impact on some tree species. Light fixtures or luminaires should be shielded so that their light output is directed toward the ground. This not only keeps the light away from plants, preventing their potential harm and also reduces light pollution, but also better illuminates the pathways for pedestrians and vehicular traffic by avoiding blinding glare. In all cases, up-lighting and shining light over great horizontal distances should be avoided. Lights should be turned off or dimmed during off-peak hours to avoid continuous lighting of trees, which has the greatest potential for upsetting normal growth patterns. If you're determined to plant trees where supplemental night lighting already exists, select those with low sensitivity to light.

An observation for Florida: I was initially under the assumption that light pollution's effect on the trees and plants would NOT be much of a concern for us in South Florida, mostly because of the fact that it was of the incompatibility of the timing of the lights and the change in the weather from the seasons as Chaney pointed out in his report. Since freezing weather typically does not impact South Florida, there seemed to be little reason for concern for all of the palm trees that get uplighted all night for whatever vain reason an owner has, even if no one is ever around to see it. However, chance comments made me rethink this.

Once, a gentlemen pointed out to a group that I attended that he observed that since lights were put on some palm trees that he knew of, they no longer flower. This makes sense when you think of what Chaney wrote about. If the lights on the trees alter their photoperiodism, then, the tree acts like it is summer all the time, so the yearly progression of the variation to the length of the day halts and the tree never advances beyond this perpetually false summer. Similar to the actor Bill Murray's character in the movie Groundhog's Day, it is as if the tree is trapped in a particular time of year, doomed to forever repeat it and not be able to grow beyond it.

At another time, I brought my son to a birthday party for his classmate. While watching children play, the hosts brought out a wagon full of mangoes to share with everyone. Another mom then asked where they got them from as it was late November, well passed the season for mangoes. The hosts said that their tree always produces fruit late in the year.

To which I then asked Oh, is your tree next to a streetlight?

They said Yeah, how did you know?

I pressed on asking And did you collect the fruit from the side of the tree that is next to the streetlight?

They both looked at each other amazed as if to double check and then asked YEAH, how DID you know?!?!

Their tree's photoperiodism and its annual progression was altered by the streetlight and probably only on the side of the tree that was exposed to that light. The other side of the tree was shaded by lit side's leaves and acted with a more normal schedule.

An Argument Against Artificial Lights.

I would argue that it would be best not to use any lights at all. Even if you believe that they have to be used, I believe you should think again. For while it is better to use the bluer lights, mercury vapor, metal halide, or fluorescent lamps, for the sake of some trees, it is those very bluish lights, because of their impact on human melatonin, human vision and animals that should be avoid for our OWN sake. In the end, there is no winning in using any type of lamp. Some species of life will be impacted by whatever type of artificial light are used. And then we all suffer because of that weakening in the web of life. By their very name, artificial lights, should be seen as that, artificial. Their electromagnetic radiant energy output, which travels 299,792.458 kilometers in one second, is a pollutant that has negative impacts that are no different from any other unnatural chemical that is spilled across the land.

However, the Environmental Protection Agency does not consider artificial light to be a pollutant because it is not a "mass" that has been artificially entered into an environment. And by definition, light has no mass. If it did have mass, then, by Einstein's theory of Special Relativity, it could not travel at the speed of light. It should be very obvious to anyone that light has no mass to it. For the moment one turns on a switch, light blazes outwards instantly at the maximum speed limit of the universe. It does not increase or build up its speed until it reaches the maximum speed, it is instantly there. It has no inertia to overcome because it has no mass. That's the physics of it.

However, it would help all to understand just how damaging light can be if it was considered to be a massive" pollutant. So let's calculate what its Einsteinian mass equivalent would be. Einstein is famous for a little equation that even children in grade school seem to know which is:

E = mc2

this means that not only does a bit of mass can become an extraordinary amount of energy, but also by

m = E / c2

that an extraordinary amount of energy can become a bit of mass.

So, from the calculations performed on our Light Pollution vs. Economics page is a section that calculated the amount of total wasted outdoor lighting in the U.S. in the year 2012 alone. The wasted energy amount was found to come to over 35.4 billion kWh for the year. Thus, its mass equivalent would be:

35,429,671,271.12 kWh ×3,600,000 J
1 kWh
÷ (299792458 m/s)2 = 1.419149 kg of mass

Imagine trying to spread out the 1.419 kg mass (approximately 3 lbs 2 ozs in weight) over the U.S., tiny bits of it each night for the entire year! It is so small and yet has such far reaching, environmentally disrupting, species destroying, circadian disrupting human health impacts, that wasteful artificial light has to be the most toxic and damaging pollutant that humankind knows!

In the end, no species of life evolved under artificial lights. We should not be surprised that, in general, no species of life truly benefits from them in the long run.

Department of Physics
Florida Atlantic University
Boca Raton, Florida
E-mail: vandernoot at sci dot fau dot edu
Phone: 561 297 STAR (7827)

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