by Leonard Perry
by Leonard Perry


In extension I serve as an advisor and consultant to the greenhouse and nursery industry, primarily in Vermont but throughout the region and beyond as well.

I give presentations on my research to the industry, and to home groups. In Research, my focus is "herbaceous perennial production systems".

His website is at  Leonards zone of gardening: home with my trials, generally USDA 4a. Campus in Burlington is 5.

September 2, 2019

Watching when buds and then flowers appear on specific plants from year to year now is being used to watch for changes in climate. Knowing these dates also can be related to the appearance of certain pests and diseases, so can tell you the best times for control. The study of such biological events is known as “phenology.” It is easy, can give you a better perspective on your own garden events and climate, and you can join networks of gardeners and naturalists all sharing this interest.

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 ( 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 (

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 ( 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 sites.

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 (

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 ( 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.” Recording phenology events is easy, doesn’t require much time (outside of what you probably already are doing in watching your plants in spring and summer), and through such observations over the years you will be learning more about your own climate. You can begin to relate certain events, such flowering and pest emergence. By then sharing your data as a “citizen scientist” in one of these networks, you’ll be helping scientists in their predictions, taking the “pulse of the planet.”

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