The phenology maps clearly reflect the influence of latitude, altitude, proximity to bodies of water, and yes—urban areas. 

I am attaching two maps. The first shows the Appalachian mountains as a cooler later area and the cities of Balt/Wash, Philly and NYC as hot spots. The second shows how much warmer Manhattan is to areas not that far away. 

In my own neck of the woods the maps clearly show the difference between where I live in the rural countryside and the urban area. Unfortunately the difference is confounded by the difference in elevation (we're higher) and the proximity to the large lake (they're closer).  

So you can't really mark the difference down to urban heat production, though I certainly don't doubt there is an effect. There are certainly pockets where plants survive and even thrive that wouldn't in more exposed locations. Some of these are urban, some are geographic.

PLB

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