Urban heat and growing seasons

This year I wanted to put more effort into my garden planning than I usually do. In past years, I have not been the best at starting my vegetables from seed. I would start my seeds too late or not give them enough light. Ultimately, making me resort to buying seedlings at my favorite local garden center, Metropolitan Plant and Flower Exchange in Fort Lee, NJ.

The first step to a successful gardening season is to find out your area’s frost dates. Frost dates help you figure out when to start your seeds and how long your growing season is. The Old Farmer’s Almanac website gives more information about frost dates as well as a tool for looking them up.

nearest climate station: new york central park tower. altitude 131 feet. last spring frost april 5th. first fall frost november 12th. growing season 220 days.

The closest climate station to me is located within Central Park. It turns out that this climate station is very old and has a lot of history. I also wanted to help my mom plan her garden in Central Jersey so I looked up the frost dates for her area as well.

nearest climate station: somerville 4 nw, nj. altitude 134 feet. last spring frost may5th. first fall frost october 8th. growing season 155days.
nearest climate station: somerville 4 nw, nj. altitude 134 feet. last spring frost may5th. first fall frost october 8th. growing season 155days.

The distance between the NYC and Somerville NJ climate stations is about 40 miles so I did not expect to see much of a difference in growing seasons. However, the difference is a little over two months! Two months can make a significant increase in the amount of yield you are able to get out of your garden in a growing season. I began to wonder what would cause such a big difference in the length of growing seasons. The most obvious factor I could think of was that NYC and Somerville are in different plant hardiness zones, 7b and 6b respectively. This means that the two locations have about a 10℉ (5℃) difference in average annual minimum winter temperatures. NYC has a more similar growing season length to Lubbock Texas, which is also in zone 7b, than Somerville. Still, the difference between the growing season length in Lubbock and a nearby zone 6b city such as Wichita Kansas is only 12 days.

NYC is much more urbanized than Somerville. Urban heat is a major issue that can cause serious health issues. Maybe urban heat is also causing larger growing seasons as well? If urbanization impacts growing season length there should be an upward trend of growing season length throughout time. Fortunately, there is a lot of historical weather data for the NYC climate station because of how old it is. I was able to obtain the historical weather data for the NYC climate station from the NOAA climate data tool which gave me the minimum recorded temperatures for the NYC climate station every day since 1870. With this data, I was able to figure out the growing season length in days for all these years (code on GitHub).

A graph of the length of growing season in days for every year from 1870 to 2020. Includes a trendline which shows a steady increase in growing season within this time range
A graph of the length of growing season in days for every year from 1870 to 2020. Includes a trendline which shows a steady increase in growing season within this time range

As you can see from the trend line in the graph, the growing season is steadily increasing. However, urbanization is not the only factor that can impact growing season length. Global warming most definitely also contributes to longer growing seasons. It turns out that the Third National Climate Assessment covered the effects of global warming on growing seasons, as well as urban heat. Most likely global warming has more of an impact on longer growing seasons than urban heat. However, I believe it is safe to say that urban heat also contributes.

At face value, it may seem that extended growing seasons would be a good thing for agriculture. However, there are so many more ecological factors that are impacted by warmer climates that could hinder agriculture (See the NCA link above for specifics). As a quote from the NCA3 summary puts it:

“Some aspects of our economy may see slight near-term improvements in a modestly warmer world. However, the continued warming that is projected to occur without substantial and sustained reductions in global greenhouse gas emissions is expected to cause substantial net damage to the U.S. economy throughout this century, especially in the absence of increased adaptation efforts.”

software engineer, naturalist