Mother Nature's Calendar
Indicator Plants and GDDs or When the oak leaves are the size of a mouse's ear, it's time to plant the corn.
by Ruth S. Foster

Time was when we watched the groundhog in February, the winds of March, the rains of April and tried to imagine when was the right time to plant the peas, for each year is different.

Last year was slow. Unceasing snow and rain kept the ground cold and wet. This year we have a drought and an early, warm spring. So how are we to know when to plant the peas for the traditional July 4th salmon and fresh peas supper

While every bug can read Mother Nature's calendar, we, like the Druids, have always relied on the unfolding Rites of Spring. And sometimes even on the Old Farmer's Almanac. In old Ireland, when one could sit naked on the bare ground, it was warm enough to plant the peas.

Experienced farmers have long recognized that when certain indicator plants grow, it's time for something else to also happen. My favorite is "When the oak leaves are the size of a mouse's ear, it's time to plant the corn".

Indicator Plants, by sprouting or blooming at a certain time (which is triggered by heat) also indicate which other plants or insects will come out at the same time. But the actual dates vary from year to year. For example, over a 24 year period at Harvard's Arnold Arboretum, one particular willow tree first leafed out between March 27 and May 1, a difference of 33 days.

Growing Degree Days Fortunately, we now have something a bit more modern than just indicator plants. It's a system that relies on a Growing Degree Days number which coordinates with the indicator plants. And it makes Mother Nature's calendar more readable to us mere mortals.

The Growing Degree Days number, called
GDD, is a measurement of the amount of cumulative heat that has been received by the plant that year. (The word "Days" is confusing so ignore it.) The Growing Degree Days numbers are known now for most plants because they've been monitored for several years. (And most state agriculture agencies have compiled GDDs for their regions.)

For example, when Shadblow (Amalanchier) blooms, which is approximately 90 GGD, gypsy moth eggs begin to hatch. The old farmer's wisdom said, "When the shadbush blossoms blow, the river shad (which are really herring) are running in Massachusetts streams."

Or consider Magnolia soulangiana, which blooms at approximately 105 GDD. Just then European pine sawfly eggs also begin to hatch, so watch for damage on mugho, scotch and red pine shoots. (The overwintering eggs are yellow bands in groups of 6 to 12 on the needles.) I wish I had known this before I threw out my mugho pines because I got tired of always spraying the sawflies after they ate all the new shoots.

The real problem with all this new knowledge is the realization that with each lovely flower there also appears a noxious bug! A few of the Rites of Spring follow:

Some Common Indicator Plants and Their Growing Degree Days
GDD l Starts in March. Carpenter ants emerge from winter hibernation
GDD 14 Red maples, willow and aspens bloom
GDD 7 - 120 Apply dormant oil when temperature is above 40 o.
Avoid freezing and wet weather
GDD 28 Early Daffodils begin. Squirrels eat the crocus
GDD 48 Japanese andromeda and flowering cherry bloom.
Tent caterpillar hatches
GDD 86 Dandelions start to bloom
Tick season starts
GDD 90 Shadblow in full bloom
Gypsy moths hatch
GDD l00 Forsythia, PJM rhododendron and violets open.
Crabgrass seeds begin to germinate
GDD 105 Magnolia blooms
Pine sawflies hatch
GDD 112 Dogwood, bridal wreath and spirea flower. Poison ivy leafs out.
Hemlock wooly adelgids hatch. Beetle grubs eat grass roots
GDD 146 Apple blossom time. Sow hardy vegetable seeds
GDD 203 About mid-May, lilacs and carolina rhododendron in full bloom
Noticeable gypsy moth holes appear in oak leaves. Last date for BT
GDD 286 Catawba rhododendron and enkianthus are in full bloom.
Sow corn, melon, cucumber and squash
GDD 577 Fireflies appear about mid-June
GDD 720 Littleleaf linden trees bloom
Adult Japanese beetles emerge