eGarden © 2003 Ed Mues

Using New Tools - Not from the Shed

As astute growers, we can measure and record much of the same environmental phenomena as our conventional calendars, but we don't need to rely on time as the measuring stick. "What's that?" you say. We use, instead, temperature. We measure and record "heat units" as they begin to accumulate near the end of winter and the beginning of spring. These numbers are called Growing Degree Days (GDDs for short). They are an algorithm extending over the length of the growing season.

Why does this matter, you might ask? If you enjoy the flora, it matters a great deal. When flower and leaf buds begin to expand in waking from dormancy, by design, plant feeding insects also wake up, and they're hungry. But, wait! Weed seeds are germinating, too. All of these newly emerging organisms are hungry for heat, light, moisture, space and nutrients. They are all prepared to compete.

If you plan to cultivate any of these organisms (I'll assume it's a fruit tree), you must control the competitors. An example: Conventional wisdom says that Eastern Tent Caterpillar (ETC) larvae hatch out and begin feeding on crabapple and other fruit trees in late April and continue well into May. But, what year was that? What was the spring like when that was the case? There is the shortcoming of the conventional calendar.

But if I stated categorically that research tells us that the juvenile caterpillar makes its appearance and begins feeding when the GDD equals between 90 and 190, you would know exactly when to begin looking for them

If, in addition, I could state with reliable accuracy, that the hatch takes place near the end of daffodil bloom while dandelions and trilliums are in flower, wouldn't that be even more refined information? The caterpillars are about finished feeding and are preparing to pupate when red clover begins to flower. This is another tool.

What we have above is the study of phenology, recurring weather-related biological phenomena. What's cited are specific plant phenological indicators (PPIs for short), plants whose bloom time is coincident with an early life stage of another species. It might be April 17, one year; it might be May 4, another. This old practice of observing repetitive natural events and how they are influenced by weather can also be a guide superior to the every day monthly calendar.

Every now and then, we experience an exceptionally early warm up where woody plants like flowering ornamentals are in peril of damage from sudden plunges in temperature. We cannot control the weather, but obviously the typical calendar is irrelevant. The thermal account or GDD number is most relevant.

Both plants and many insects (ectothermic) rely on an external heat source to maintain a minimum temperature favorable to growth and development. There is no more accurate a source for predicting events in your landscape than observing the PPIs, and the GDDs. If you were inclined to calculate GDD numbers for your specific homestead, and used the PPIs provided above, you could go on the hunt for the ETC with astounding accuracy and wipe them out without the need for chemicals or toxins.

Would it help if I stated that also in bloom are Amelanchier ( Serviceberry, shadblow), White spruce, Frasier fir, and Blue spruce? A few more PPIs. Fold these tools into the conventional timing reserved for needed response, and you will be way ahead of the curve.

Professional ornamental growers and food crop farmers have long known these things. It's kind of like folk wisdom, those bits of friendly advice that have been handed down from previous generations. Careful observations by interested naturalists has given us many wonderful flowers, fruits, vegetables, trees etc., as well as all this helpful information.

I give another example. Weeds are regarded as pests in many situations. Crabgrass is an annual pest in lawns. It does not overwinter, but sprouts from seeds produced the previous year. The seeds germinate when the soil temperature and moisture are just right. Some lawn guides recommend applying pre-emergent herbicides "...in the spring when crabgrass germinates." No other information about timing is given. Another says, "... about two weeks before the last expected frost". That means there is a potential for error of two weeks or more. Most pre- emergent herbicides are very short lived, with an effective life of only a couple of weeks. To prevent failure, correct timing is essential

A new and better gardening calendar is needed to insure success. The soil moisture and temperature are just right for crabgrass seed germination when the forsythia petals begin to fall and the lilacs have not yet bloomed. This is the window for most effective control. GDD between 50 and 72.

Consider the stages of plant growth that are all a product of seasonal climate change: Green-up, flowering, bud burst, leaf growth, plant senescence, and seed dispersal. The length of the growing season for every planted or emergent plant, and its corresponding stage of development are predictable because of the interaction of the photoperiod and the temperature value. GDDs are frequently used for sweet corn, field corn and sorghum crop practices.

I have put two simple and sophisticated science based guides on the table for you to look at. Is anyone interested in recording GDDs in 2004? I did it for eight years at CCE. A computer was at my service and John W., who furnished me with the needed data. All you need, however, is a min-max thermometer that can be reset daily, and a commitment to record the high and low temps over each twenty-four hour period. Starting March 1, record the average temperature and subtract 50. 50 degrees F is the threshold or base temperature for our algorithm. It is the minimum temperature at which there is tree and shrub growth.

Here's how to calculate Growing Degree Days: Max temp. F + Min temp. F. Divide the sum by 2.. Subtract 50F and enter the number. That is the daily GDD number. GDDs are cumulative. Add positive numbers only to the previous days totals.

You'll need a stand-in if you plan to be away, or you'll sacrifice some accuracy if you must average. The GDDs at my house and your house will almost always vary. Most continue until Sept. 30. This kind of record is extremely personal, as would be the recording of the dates of bloom times and durations of favorites on your property. The reward is not unlike that of knowing the name of a beautiful piece of music, the artist, the creator. The same applies to a pleasing garden plant or tree that you especially admire.

So, is anyone interested in recording a few PPIs in 2004? I'm proposing a non-physical way to be on more intimate terms with your landscape. If you want to go even further, weather proof labels for your favorites, true botanical names and nicknames, a little history (readily available), a personal written record for as little or as much effort as you choose. That's rewarding. No?

And, it's ready to be shared, if you wish. Or, handed down to the next caretaker of the property.

The true naturalist would include the first sound of the spring peepers, the first appearance of the red eft newt, the bluebird or red wing blackbird, butterflies, chipmunks, ooooh there is so much. But, if you're a plant lover, you have some starting points. And, hopefully, a new tool or two. They may never be used, but, then again, they may. And, besides, what a great legacy. name

From The Garden of Ed. Submitted for publication in The Towne Crier on November 5, 2003

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