University of Vermont Extension
Department of Plant and Soil Science
FALL FOLIAGE—THE HOW AND WHY
Dr. Leonard Perry, Extension Professor
University of Vermont
Various parts of the country are known for their vibrant displays of fall
foliage colors, none more so that New England and, in particular,
Vermont. It seems odd that we get such joy from the annual death of
leaves from “deciduous” trees (those that lost their leaves in fall).
Why leaves change colors in fall, with some years more brilliant and
long-lasting than others, is based on some simple science and environmental
Fall leaf colors are due to plant pigments in leaves. The colored
pigments are the same that produce colors in flowers and fruits.
“Chlorophyll” is the pigment responsible for the green color in leaves, and
for producing carbohydrates (sugars) plants need for growth. It does
this through the process of “photosynthesis”, producing sugars and oxygen
from carbon dioxide and water. Chlorophyll does this through using
certain wavelengths of light—primarily blue and red. It is continually
being produced and replaced during the growing season.
“Carotenoids” are leaf pigments responsible for yellow and orange colors in
leaves, particularly in fall. Think of orange, as in carrots. They are
present during the season but masked by the green chlorophyll, except in
plants that may be stressed or with yellow leaves normally. One type
of these carotenoids is “xanthophyll”, coming from the Greek “xantho” for
yellow. Carotenoids are responsible for absorbing wavelengths of light
that chlorophyll doesn’t—mainly blue-green and green, as well as using
excess energy produced in leaves as in high light conditions. In fall,
with no chlorophyll around, they may act as a sunscreen. They’ve been around
in plants a while, perhaps 3 billion years. Some trees with mainly
yellow, or yellow and orange, fall leaves include sugar maples, hickories,
beeches, birches, and tulip poplars.
“Anthocyanins” are the pigments responsible for red and purple fall leaf
colors. These are only produced in the fall when sugars are trapped in
the leaves. They function similar to the carotenoids, and help the
leaf use up any remaining energy as chlorophyll disappears. Abundant
dry weather and sunlight lead to more sugars in leaves, which in turn leads
to brighter fall reds. More red in leaves, and earlier reds, can come about
too from plant stresses, low nutrition, and near but not freezing
temperatures. Freezing stops the process of making the red
pigments. Some trees usually with red leaves include scarlet oak,
sumac, some sugar maples, red maples, mountain ash, sweet gum, and sourwood.
Why sugars are trapped in fall leaves, and why chlorophyll disappears in
them, is due to something called the “abscission layer”. This is a
separation layer of cork-like cells that develops between branches and leaf
stems in response to shorter days (in reality it’s the longer nights) and
cooler temperatures. It seals off the flow of nutrients between leaf
and stems, and eventually roots, which causes the chlorophyll to not be
replaced. So it disappears during early to mid fall, leaving the
yellow and orange pigments to be seen, and red pigments to be produced.
With time, even these colored pigments break down, leaving brown ones called
“tannins”. What colors are produced by what plants, and in what amounts, and
when in relation to other plants, is due to the
programmed genetics of a particular plant.
Similar environmental cues as those causing red color in leaves also may
cause fall colors to come sooner, or later, or to last longer some years
than others. Drought during spring and early summer may signal the
plant to form the sealing barriers to leaves earlier, shutting down and
turning color sooner than usual.
So while moisture is good earlier in the season, too much later in the
season means more clouds, less sun, and more muted colors. Too much
rain in fall, and strong winds, may cause many leaves to fall prematurely.
Cooler temperatures in late summer (as in August 2014), and plenty of
sunshine, often leads to brighter colors sooner. This is particularly
true if the sealing layer on leaf stems has started forming. Cool is
good, but too cold (as freezing or below) can be bad, killing leaves
early. So ideal for fall color would be a moist growing season early,
dry late summer and early fall, with sunny warm days and cool nights during
When touring through areas with brilliant fall colors, as you enjoy them,
now you should have a better appreciation of what makes this natural yearly
art show happen. Although timing can vary from year to year, due to
conditions already noted, generally peak colors occur from late September in
northern New England and higher elevations, to late October in southern New
When thinking about plants with fall colors for your own landscape, check
out other articles on this topic (pss.uvm.edu/ppp/articleS.htm). Take
a virtual foliage tour, and get ideas on colorful landscape plants, at the
U.S. National Arboretum website under the Gardens Photo Gallery (www.usna.usda.gov/).
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