Photos of the Long Island Sound

Research & Monitoring

Status & Trends

LISS Ecosystem Targets and Supporting Indicators

Length of Growing Season

( Click labels in legend to hide data and adjust scale )

red Avg. Growing Season 1973-2015: 159 days
green Avg. Growing Season 1778-1865: 126 days

   Annual length of growing season (days) 
1973 181
1974 139
1975 153
1976 143
1977 167
1978 142
1985 162
1987 159
1988 165
1990 191
1991 154
1992 147
1993 156
1995 183
1996 142
1997 168
1998 173
1999 141
2004 129
2005 162
2006 166
2007 168
2008 159
2009 161
2010 150
2011 157
2012 166
2013 159
2014 162
2015 162

WHAT IS THE Length of the Growing Season indicator?

The length of the growing season is the variation between the last frost of spring and the first frost of fall. This indicator uses air temperatures measured at Tweed-New Haven Airport in New Haven since 1973, and compares it to frost measurements from an 18th and 19th century datasets collected and published in 1866 by Yale College professors Elias Loomis and Hubert Anson Newton. The datasets were discovered  by scientists Jennifer and James O’Donnell during their research for a Long Island Sound Sentinel Monitoring project on detecting climate change impacts (see Learn More). Loomis and Nixon  examined the temperature measurements that had been recorded by numerous presidents and professors of Yale College in New Haven between 1778 and 1865, and developed corrections to account for the time of day of the measurement. They then considered whether the annual, seasonal and monthly mean temperature had changed between the intervals 1778 to 1820, and 1820 to 1865. They also reported an analysis of the variation in the date of the last frost of spring and the first frost of fall. Since the records of direct observations of frost were unreliable, they used the occurrence of temperatures lower than 40°F (or 4.4°C) as a more convenient metric. They showed that this proxy was a reasonable predictor of frost when observations were available.


What does it indicate?

A longer frost-free season corresponds directly with a longer growing season. This may be good news for farmers and gardeners with the possibility of new crops or plant varieties to try, and is one of the few pieces of positive news with climate change for our region. However, the growing season is not increased for just beneficial plants such as, crops and forest trees. It is also increased for invasive plants and allergy triggering plants such as ragweed and grasses. Another consideration with a longer growing season is water availability. A longer growing season may increase demands on the water supply to water crops and gardens.  For 2016, much of the LISS study area remains in a severe drought, so careful consideration will be needed as gardens are increased and new plants grown.


The average length of the  growing season over the past 40 years (1973-2015) is  26 percent higher than the average length of the  growing season from 1788 to 1866, an increase from 126 days to 159 days. Over the past 11 years the length of the growing season has been equal to or exceeded the modern-era average every year, but two.

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