The appearance of robins in spring, flowering of crabapples and
lilacs, the flowering of the cherry trees in Washington, dates of
egg-laying of birds, and the dates of leaf coloring and drop in
autumn are all phenological events. They respond to a combination
of climate factors such as temperature, rainfall, and daylength.
Of course, these can be measured separately, but what I find
fascinating about watching plants is that they are programmed to
combine all such factors to determine when certain events such as
bud opening occur.
Phenology has been handed down for years in folklore, such as the
saying relating leaf appearance and rainfall. "If oak's before
ash, you're in for a splash. If ash before oak, you're in for a
soak." The term “phenology” comes from the Greek words for
“study” of “appearances.”
While the Belgian botanist Charles Morren is often credited with
the first use of this term, in 1849, the English naturalist Robert
Marsham is often considered the founding father of phenology. He
began recording 27 signs of spring in 1736, continuing for over 60
years. Succeeding generations of his family continued this into
the twentieth century. Marsham was friends with another
naturalist, Gilbert White, who observed (with naturalist William
Markwick) the seasonal events of more than 400 plant and animal
species between 1768 and 1793.
Watching dates of biological events each year preceded even these
naturalists, dating back centuries to pre-agricultural times. The
earliest written records were by the Chinese in 974 B.C. Perhaps
the longest record of flowering dates--1200 years-- shows that
cherry trees in Japan are blooming earlier over the last 100
years. Osaka researcher Yasuyuki Aono has compiled this data from
historical documents and records.
Another quite long record is that of the ripening of the Pinot
Noir grape in Burgundy, France. Grape harvest dates are closely
related to temperature, so provide a good indicator of yearly
changes, in this case between 1370 and 2003 (Nature, November
2004). The French researchers found that temperatures as high as
those in the 1990’s occurred several times since 1370, with the
summer of 2003 being extraordinarily high.
Looking at bloom dates in the northeastern U.S., plants at the
Arnold Arboretum in Boston are blooming about eight days earlier
than recorded there 100 years ago. Wildflowers in nearby Concord,
Massachusetts are blooming about three weeks earlier than in 1854
when Thoreau observed them (primacklab.blogspot.com). A
researcher at Longwood Gardens, near Philadelphia, has shown that
flowers in that area are blooming on average one and a half days
earlier per decade compared to 150 years ago. Similarly, in
Vermont, lilacs are blooming one and a half days earlier, with
leaves out 3 days earlier, per decade (alanbetts.com).
Much of the current phenology efforts can be traced back right
here to Vermont, to the research and network begun in 1965 by
Professor Hopp in the Department of Plant and Soil Sciences. He
was interested in relating bloom dates to insects, and so best
control times, and to helping farmers plant crops at the right
times. Back then, climate change and tracking it was not an
issue, nor even considered.
A more recent study, published by the Entomology program at the University of Kentucky (entomology.ca.uky.edu/entfacts) correlates the emergence of 33 important insect pests of landscape plants with the flowering of 34 commonly seen plants in landscapes. By knowing such relationships, it is easier to know which pests may be emerging, just by what is blooming, and to best schedule their control. For instance, they found that egg hatch of eastern tent caterpillars on average was March 16, just after first bloom of border forsythia and 95 percent bloom of cornelian cherry dogwood. Some gardeners look for this egg hatch when the buds of crabapples and wild plums open. Then later, they watch for grasshopper egg hatch when purple lilacs bloom and, in summer, watch for squash vine borers when chicory blooms.
Some gardeners also use phenology, or what is blooming, to
schedule their plantings and cultural practices. Examples would
be to prune roses and fertilize lawns when yellow forsythia and
crocus bloom; plant peas when daffodils bloom; plant lettuce when
lilacs have leafed out; plant cucumbers when lilac flowers have
faded; plant tomatoes when apple blossoms begin to fall; and plant
peppers when bearded iris are in bloom.
In the early 1980’s, I took over the phenology network begun by
Professor Hopp, at that time collecting specific bud and bloom
“phenophase” data on a couple of clonal selections of lilac and
honeysuckle from observers throughout Vermont and the region.
Clones or cultivars (cultivated varieties) were selected since
they would be identical at each observation site, the bud and
bloom timing just varying with the climate differences among
Since then, this network has been expanded under Dr. Mark
Schwartz at the University of Madison Milwaukee. The current
result is the National Phenology Network, which he helped to
co-found. There, you can learn more about this science and even
participate as a citizen scientist with your own observations (www.usanpn.org).
You can buy and observe the same lilac cultivar—Red
Rothamagensis—that Professor Hopp used, adding to the over 50
years of data already from hundreds of observers. Or, you can
observe many other plant and animal species—particular ones
depending on your state. For Vermont, for instance, 55 are listed
at the National Phenology Network site. On this site there are
also “campaigns”—observations relating to a plant, such as this
lilac, or group such as pollinators. All these, plus mobile apps,
a botany primer for observations, and more are part of this
network’s Nature’s Notebook—celebrating its tenth year in 2019.
There are many other phenology-related observation programs and
organizations listed on this site, some quite local and regional (www.usanpn.org/nn/connect/friends).
Dr. Schwartz summarizes well what phenology is, and its
importance. “Phenology is the study of periodic biological events
in the animal and plant world as influenced by the environment,
especially temperature changes driven by weather and climate. Wide
ranges of phenomena are included, from first openings of leaf and
flower buds, to insect hatchings and return of birds. Each one
gives a ready measure of the environment as viewed by the
associated organism. Thus, timings of phenological events are
ideal indicators of the impact of local and global changes in
weather and climate on the earth’s biosphere.”
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