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Over the Fence

Where gardeners come to find out what's happening out in the yard.

Cool Weather and Plant Development

Posted by Richard Hentschel -

Down the Garden Path

Richard Hentschel, Extension Educator

Plants in the garden (and insects too) develop based on something called "Growing Degree Days" or GDD for short. This is an accumulation of heat units using a base of 50 degrees. For every degree above fifty goes towards the growing degree-days and plant development. Most of us do not follow GDD, but rely on the catalog or seed packet information on how long it takes to go from seed to harvest. You will see a variety listed as a short or long season plant. Those that have grown sweet corn are very familiar with these terms. The same can be said for tomatoes.

It has been an interesting spring and early summer watching the recorded numbers and comparing them to the historical averages. The sample below is indicating that the St. Charles reporting station has shown us slightly ahead of the historical average. I can only guess that given our cooler weather the predicted GDD for the next two weeks will be changing downward.

Modified Growing Degree Days (Base 50°F,March 1 through July 10)

Station Location

Actual Total

Historical Average (11 year)

One- Week Projection

Two-Week Projection

Freeport

1499

1249

1659

1820

St. Charles

1299

1183

1451

1605

DeKalb

1296

1313

1451

1608

Monmouth

1448

1382

1610

1776

Peoria

1511

1459

1685

1862

Champaign

1604

1506

1782

1964

Springfield

1762

1617

1950

2139

You would expect t that as you move from north to south in the state, GDD would be accumulating at higher levels than northern Illinois. This matches up well with the typical planting dates throughout the state that gardeners use to begin their vegetable growing season.

If our weather pattern continues on the cooler side, those warm loving vegetables will be delayed in their development, which will delay any fruit set. The opposite has been seen on those vegetables and flowers that enjoy cooler temperatures; they are still growing very well.

This continues to be a "wait and see what happens" kind of summer.



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