Why is Phenology of Plants Important?

How to Keep a Phenology Calendar

Why is Phenology of Plants Important?

By Gail Damerow – I started studying the phenology of plants when I missed out on my fresh asparagus harvest one year. Next to my goat barn is an old pear tree. Behind the pear tree is a patch of wild asparagus, from which I harvest delicious tender spears each spring. One spring I missed the asparagus harvest and didn’t remember until the young shoots had grown into visible feathery fronds several feet high. How did I miss it?

After long thought I realized that most years I spy fresh asparagus spears while visiting the pear tree to see what kind of pollinators are exploring the pear blossoms. The spring that I missed the asparagus harvest, the weather was too rainy for pollinator watching and indeed for pollinators themselves. I realized then that I need to watch for young asparagus spears, come rain or shine, when the pear tree blooms.

Grandma Was Right

That’s phenology — a study of the recurrence of biological events, as influenced by environmental conditions such as temperature, rainfall, and available sunlight. In contrast to seasonal events — such as the much anticipated last spring frost or much dreaded first fall frost, which occur independently of living organisms — phenology measures the timing of biological events in the life cycles of living organisms. The word phenology (pronounced “fee‐NOL‐o‐gee” and derived from Greek words phaino, meaning to show, and logia, meaning science) may not be familiar, but most of us already use the concept.

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Native Americans, for instance, planted corn when oak leaf buds were the size of a mouse’s ear. In my area, foragers look for morel mushrooms when dandelions go to seed. My gardening grandmother used to say that when lilacs bloom, the last hard freeze has past. Indeed, one of the first extensive phenology of plants studies organized in the United States involved tracking the blooming of lilacs across the continent.

Phenological events vary in their timing from year to year, but in a given locale nearly always occur in the same order. In the woodlot surrounding our farm, for example, among the earliest trees to leaf out are the red maples and the earliest to bloom are the red buds, while the sweetgum is the last to leaf out. In contrast to tracking seasons on a calendar and basing our gardening and other biological activities on the area’s average dates for last spring frost and first fall frost, phenology is much more accurate.

The Chinese long ago observed that “Spring is sooner recognized by plants than by men.” Modern plant biologists often refer to plants as phytometers — from the Greek words phyton, meaning plant, and metron, meaning measure — because observing the development and reproduction of plants provides excellent information about environmental conditions.

Accordingly, the asparagus incident made me realize that if I pay more attention to the natural life cycles of native plants and wildlife on our farm, I would have better success in timing my gardening activities.

Artwork by Bethany Caskey

I’ve discovered that growing beets, chard, onion, pea, and potatoes do best when redwing blackbird males return, redbuds begin to bloom, pear trees bloom, lilac is in first leaf, dandelions bloom. Phenology in action!

Artwork by Bethany Caskey

Plant warmth loving vegetables — like beans, corn, cucumbers, squash, and melons — when daylillies start to bloom, irises bloom, lily-of-the-valley is in full bloom, dogwood in full bloom, new growth appears on grape vines, lilacs begin to fade.

Keeping Track

I began looking for a simple way to keep track of seasonal biological events so I could find linking relationships, such as that between pear blossoms and the emergence of asparagus spears. I thought about using a computer database, but in the end decided on a simple paper and pencil scheme.

All I needed was a place to list seasonal events and jot them down as they occur from year to year. I decided to use an old 7-by-10-inch monthly planner. The year doesn’t matter, since I’m tracking only the month and date (but not days of the week). In this planner, each month covers two facing pages, and each day has a nice big square in which to write, with six lines in each square, allowing me to use the same planner for six full years. Down one edge is a column provided for my phenology of plants notes. Perfect!

In the notes column for each month I created a numbered list of cyclical biological events I observe during that month. The April column, for instance, says: 1. apple leaf, 2. apple bloom, 3. lilac bloom, 4. oak leaf, etc. These events don’t occur on precisely the same date each year, but they generally occur in April. This list is not in chronological order, because each year I add new things as I notice them.

As a space saving device, in the date square I list the year and the item number for an event I observe. A notation in the square for April 25th says “2016-13,14.” Checking the notes column for the events correlating with 13 and 14, I see that on April 25, 2016, the first iris blossoms appeared (#13) and I harvested the first strawberries from our garden (#14).

Sometimes I miss noting the date of an event. For example, I might notice in February that the periwinkles are in bloom, but I failed to notice when they started blooming. So in the notes column for February, I list “periwinkle bloom” as a reminder to watch for the first blossoms in future years. For an explanation of key phenological events to watch for — such as first leaf or first bloom — see “Phenophases” on page XX.

I also record the days on which we have frost, indicated by an F in place of a number: 2016-F. And I keep track of precipitation, which strongly influences cyclical activities. For instance, there’s no point in searching for morel mushrooms unless we’ve had ample rain. So whenever it rains, I jot down the year in the date box and make a circle around it (indicated here with parenthesis). My notation in the square for April 7th says “(2016)-3” signifying that it rained on April 7, 2016, and the lilacs started to bloom.

For gardening purposes, the plants I most like to observe are those that reseed themselves (“volunteers,” in gardening parlance). One of particular interest is Mexican Midget tomato, which unlike other tomato varieties, doesn’t do at all well when started indoors, but reseeds freely in the garden. Furthermore, the seedlings are already sprouting in the garden at about the time I usually start growing seedlings indoors in pots. By noting when the first Mexican Midget seedlings appear in relation to other biological events, I hope to better predict ideal planting dates from year to year, as well as to better anticipate plant pest activity and seasonal weed growth so I can take early evasive action.

Scientists use much more detailed methods of tracking phenological events, which allows them to measure such things as climate change and the long-term sustainability of a given population of plants or animals. While I keep track only of an event’s first occurrence, according to The Phenology Handbook, a phenologist may record such details as the dates that a plant opens its first and last flowers (from which the duration of flowering may be calculated), and the number of flowers that open each day or week during the flowering period. Further, scientists describe and measure phenological patterns at multiple levels, such as within and among individuals; within and among plant and animal populations; within a community of coexisting species; and across large geographic regions such as states, continents, hemispheres, and even the entire planet.

Artwork by Bethany Caskey

A monthly planner makes a good start for keeping track of local phenological events and noting those that tend to occur simultaneously. The planner’s year is irrelevant, since only dates are tracked, not days of the week.

Why Bother?

Lots of information about phenology of plants is available online, so why go through the trouble of keeping track of events occurring in your own yard? Well, for one thing, your immediate environment may not include the traditional biological indicators, such as blooming lilacs and forsythia, so you will need to identify your own indicators.

Further, biological events are not necessarily the same from one microclimate to another, but are influenced by such things as temperature, precipitation, day length, genetics, shade, and proximity to buildings and other structures. The traditional phenological adages therefore don’t necessarily apply to every situation. Because so many environmental conditions influence the life cycles of plants and animals, the only good indicators of your local conditions are found in your local environment.

After a year or two of keeping records you will begin to notice a pattern in how apparently unrelated seasonal events occur in relation to one another. You will also notice that these events don’t occur on the same date each year, but may be earlier or later than the year before, depending on weather conditions, and especially temperature.

Phenology of Plants Benefits

Phenology of plants offers many benefits for farmers and gardeners. It is a helpful tool for such things as:

  •  successful planting
  •  managing insect pests
  • controlling crop diseases
  • designing a flower bed for sequential bloom
  • planting an orchard for timed fruit maturity
  • predicting berry or fruit harvest
  • planning refuge gardens for beneficial insects
  • predicting when to plant the best plants for bees and other pollinators
  • preparing for allergy season
  • tracking trends in climate change

Online Resources for Phenology of Plants

Lots of helpful information on phenology is available online, along with several websites through which amateur and professional volunteers contribute data that helps expand scientific knowledge of phenological events. Here are a few good online resources:

Gail Damerow studies phenology of plants on her farm in Tennessee’s Upper Cumberland.

In your climate, do you have dramatic changes in plant activity from year to year? Do you keep a monthly planner and practice phenology of plants? We’d love to hear from you!

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