The longest growing season – Updated

UPDATE: A sudden cold front ended the growing season (at least in Kentucky) on the morning of November 11. This story has been updated to reflect the changed growing season length. We came close, by two days, from setting an all-time record. This year is the second-longest growing season on record.

In much of the eastern US, gardens are still growing, late fall flowers are still blooming, and trees are only reluctantly and slowly changing colors. It is still the growing season. There are various ways of defining the growing season, but for plants, a temperature of 28 F for several hours is considered a killing frost. Plants that have not prepared for winter by entering dormancy will be damaged at this temperature.

For Lexington, Kentucky, the US Weather Service provides the following dates from long-term records, using the 28 F measure.

Latest first fall hard freeze (28°):  December 4, 1899
Longest growing season:  242 days in 1922

For 2017, the numbers are:

First fall hard freeze (28 F):  November 11
Growing Season Length as of November 11:  241 days

The last hard freeze was March 16, when the temperature reahed a low of 18 F. There has not been a hard freeze since, although a few nights flirted with 32 F and there was patchy frost. The current short term weather forecast shows little chance of a hard frost in the next two weeks. The current long-term weather forecast tells us that we are unlikely to see below-freezing temperatures before December 12 (though long-range forecasts are not very reliable). As it happens, the long-range forecast is still for warm weather, but the trend was abruptly ended by a hard freeze in the early morning of November 11.

While this may be good for gardeners and farmers, there are negative consequences. Many natural processes, such as flowering and plant growth, hibernation and emergence of animals, are tied to careful measurement of temperatures. A very short and warm winter may prevent some plants from beginning growth at the right time in spring. Bird migration is profoundly affected, because migration north is cued by daylength, at least in some birds. In a warm and early spring, emergence of caterpillars and other insects that provide food for foraging birds may occur before the birds are on the move, leaving the migrants with too little food.

Growing seasons throughout the US have been getting longer since 1980, a clear result of a warming world. Throughout the US, growing seasons are now an average of 15 days longer than they were prior to 1980, with more northerly states experiencing up to 30 days’ longer growing season.

Frost free seasons in the continental United States.

Sassafras in autumn

Kentucky Horse Park