As a result of extensive urban development coupled with warming temperatures, urban heat islands (UHI) have become an important factor affecting energy consumption and human health in cities. Prior research has shown that evapotranspiration (ET) from urban vegetation can have a significant cooling effect, but there are relatively few direct measurements from urban vegetable gardens. We compared hourly temperature measurements during two summers (2017 and 2018) in a 750 m2 research garden at the University of St. Thomas (Saint Paul, Minnesota, USA) to hourly temperatures at the nearby Minneapolis-Saint Paul (MSP) International Airport, located 6 km to the south. We also quantified seasonal ET (June-October) in 132 garden plots and 5 reference turfgrass plots during summers of 2017 and 2018, based on the difference between water inputs (rainfall plus supplemental watering) and leachate recovered in lysimeters installed beneath each plot. ET was also compared among various soil amendment treatments applied to garden plots (high or low levels of manure compost and municipal compost, synthetic fertilizer, or no inputs) and among crop varieties (collards, peppers, bush beans, and carrots).
Results/Conclusions
For both years, an increase in temperature of 1.00°C at the MSP airport resulted in an average increase of 0.55°C in the research garden. At temperatures greater than 22°C, the garden was cooler on average compared to MSP airport. ET in the garden plots was significantly higher than in the grass reference plots both years, with means of 46 cm for garden plots compared to 19 cm for grass plots in 2017, and 51 cm for garden plots compared to 33 cm for grass plots in 2018. These results are consistent with other research showing potentially large benefits of cooling through ET from urban gardens that are primarily aimed at crop production. Among garden plots, high inputs of manure compost resulted in lower ET (more leachate), and high inputs of municipal compost resulted in higher ET (less leachate), relative to other treatments. ET varied among crops as well, with collard greens having highest rates of ET, and peppers and carrots having the lowest rates of ET. These data indicate that outdoor urban agriculture can be used strategically to mitigate UHI effects, and potentially also to reduce urban runoff.