Climate Change Indicators in the United States Length of Growing Season

Climate Change Indicators in the United States: Length of Growing Season
www.epa.gov/climate-indicators - Updated August 2016

Length of Growing Season

This indicator measures the length of the growing season in the contiguous 48 states.

Background

The length of the growing season in any given region refers to the number of days when plant growth
takes place. The growing season often determines which crops can be grown in an area, as some crops
require long growing seasons, while others mature rapidly. Growing season length is limited by many
different factors. Depending on the region and the climate, the growing season is influenced by air
temperatures, frost days, rainfall, or daylight hours.

Changes in the length of the growing season can have both positive and negative effects on the yield
and prices of particular crops. Overall, warming is expected to have negative effects on yields of major
crops, but crops in some individual locations may benefit.1 A longer growing season could allow farmers
to diversify crops or have multiple harvests from the same plot. However, it could also limit the types of
crops grown, encourage invasive species or weed growth, or increase demand for irrigation. A longer
growing season could also disrupt the function and structure of a region's ecosystems and could, for
example, alter the range and types of animal species in the area.

About the Indicator

This indicator looks at the impact of temperature on the length of the growing season in the contiguous
48 states, as well as trends in the timing of spring and fall frosts. For this indicator, the length of the
growing season is defined as the period of time between the last frost of spring and the first frost of fall,
when the air temperature drops below the freezing point of 32°F. This is referred to as the frost-free
season.

Changes in growing season have been calculated using temperature data from 750 weather stations
throughout the contiguous 48 states. These data were compiled by the National Oceanic and
Atmospheric Administration's National Centers for Environmental Information. Growing season length
and the timing of spring and fall frosts were averaged across the nation, then compared with long-term
average numbers (1895-2015) to determine how each year differed from the long-term average.

• The average length of the growing season in the contiguous 48 states has increased by nearly
two weeks since the beginning of the 20th century. A particularly large and steady increase
occurred over the last 30 years (see Figure 1).

• The length of the growing season has increased more rapidly in the West than in the East. In the
West, the length of the growing season has increased at an average rate of about 2.2 days per
decade since 1895, compared with a rate of nearly one day per decade in the East (see Figure 2).

Key Points

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Climate Change Indicators in the United States: Length of Growing Season

www.epa.gov/climate-indicators - Updated August 2016

• The length of the growing season has increased in almost every state. States in the Southwest
(e.g., Arizona and California) have seen the most dramatic increase In contrast, the growing
season has actually become shorter in a few southeastern states (Figure 3).

• In recent years, the final spring frost has been occurring earlier than at any point since 1895, and
the first fall frost has been arriving later. Since 1980, the last spring frost has occurred an
average of three days earlier than the long-term average, and the first fall frost has occurred
about three days later (see Figure 4).

• Patterns in the timing of spring and fall frost (Figures 5 and 6) largely mirror the overall patterns
in the length of growing season (Figure 3). States that saw an increased length of growing
season had comparable changes in frost-free days for both the spring and fall.

Figure 1, Length of Growing Season in the Contiguous 48 States, 1895-2015

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Climate Change Indicators in the United States: Length of Growing Season

www.epa.gov/climate-indicators - Updated August 2016

Figure 2. Length of Growing Season in the Contiguous 48 States, 1895-2015: West Versus East

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Climate Change Indicators in the United States: Length of Growing Season

www.epa.gov/climate-indicators- Updated August 2016

This map shows the total change in length of the growing season from 1895 to 2015 for each of the
contiguous 48 states.

Data source: Kunkei, 20164

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Climate Change Indicators in the United States: Length of Growing Season

www.epa.gov/climate-indicators - Updated August 2016

Figure 4. Timing of Last Spring Frost and First Fall Frost in the Contiguous 48 States, 1895-
2015

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Year

This figure shows the timing of the last spring frost and the first fall frost in the contiguous 48 states
compared with a long-term average. Positive values indicate that the frost occurred later in the year, and
negative values indicate that the frost occurred earlier in the year. The lines were smoothed using an 11-
year moving average. Choosing a different long-term average for comparison would not change the
shape of the data over time.

Data source: Kunkel, 20165

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Climate Change Indicators in the United States: Length of Growing Season

www.epa.gov/climate-indicators- Updated August 2016

Figure 5. Change in Timing of Last Spring Frost by State, 1895-2015

Change in timing of last spring frost date (days):

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This map shows the total change in last spring frost date from 1895 to 2015 for each of the contiguous
48 states.

Data source: Kunkel, 2016s

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Climate Change Indicators in the United States: Length of Growing Season

www.epa.gov/climate-indicators - Updated August 2016

Figure 6. Change in Timing of First Fall Frost by State, 1895-2015

Change in timing of first fall frost date (days):

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This map shows the total change in first fall frost date from 1895 to 2015 for each of the contiguous 48
states.

Data source: Kunkel, 20167

Indicator Notes

Changes in measurement techniques and instruments overtime can affect trends. This indicator
includes only data from weather stations with a consistent record of data points for the time period.
Methods for producing state, regional, and national values by year were designed to provide spatial
representativeness regardless of station density.

&EPA

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Climate Change Indicators in the United States: Length of Growing Season
www.epa.gov/climate-indicators - Updated August 2016

Data Sources

All six figures are based on temperature data compiled by the National Oceanic and Atmospheric
Administration's National Centers for Environmental Information, and these data are available online at:
www.ncei.noaa.gov. Analysis of frost timing and growing season length was provided by Kunkel (2016).8

1 IPCC (Intergovernmental Panel on Climate Change). 2014. Climate change 2014: Impacts, adaptation, and
vulnerability. Working Group II contribution to the IPCC Fifth Assessment Report. Cambridge, United Kingdom:
Cambridge University Press, www.ipcc.ch/report/ar5/wg2.

2 Kunkel, K.E. 2016 update to data originally published in: Kunkel, K.E., D.R. Easterling, K. Hubbard, and K.
Redmond. 2004. Temporal variations in frost-free season in the United States: 1895-2000. Geophys. Res. Lett.
31:103201.

3 Kunkel, K.E. 2016 update to data originally published in: Kunkel, K.E., D.R. Easterling, K. Hubbard, and K.
Redmond. 2004. Temporal variations in frost-free season in the United States: 1895-2000. Geophys. Res. Lett.
31:103201.

4 Kunkel, K.E. 2016 expanded analysis of data originally published in: Kunkel, K.E., D.R. Easterling, K. Hubbard, and
K. Redmond. 2004. Temporal variations in frost-free season in the United States: 1895-2000. Geophys. Res.

Lett. 31:103201.

5 Kunkel, K.E. 2016 update to data originally published in: Kunkel, K.E., D.R. Easterling, K. Hubbard, and K.
Redmond. 2004. Temporal variations in frost-free season in the United States: 1895-2000. Geophys. Res. Lett.
31:103201.

6 Kunkel, K.E. 2016 expanded analysis of data originally published in: Kunkel, K.E., D.R. Easterling, K. Hubbard, and
K. Redmond. 2004. Temporal variations in frost-free season in the United States: 1895-2000. Geophys. Res.

Lett. 31:103201.

7 Kunkel, K.E. 2016 expanded analysis of data originally published in: Kunkel, K.E., D.R. Easterling, K. Hubbard, and
K. Redmond. 2004. Temporal variations in frost-free season in the United States: 1895-2000. Geophys. Res.

Lett. 31:103201.

8 Kunkel, K.E. 2016 update to data originally published in: Kunkel, K.E., D.R. Easterling, K. Hubbard, and K.
Redmond. 2004. Temporal variations in frost-free season in the United States: 1895-2000. Geophys. Res. Lett.
31:103201.

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