There's an old myth that one-fifth of the state of New Jersey is covered in people's lawns.
It certainly seems to be true if you drive around New Jersey for a while -- and I grew up there, so I feel like I can comment on this -- but then you start to break it down. The state of New Jersey has 7,425 square miles of land -- or 4,752,000 acres -- and has 8,414,350 people (as of the 2000 census). That means there's 0.56 acres of land for each person, or two and a quarter acres for the typical family of four. I would guess that most families that live on lots that large don't have the entire lot as a lawn, because who wants to do all that mowing?
And there's a map of how much of the U.S. is covered in lawns, from NASA. If you're at all familiar with how population is layed out in the United States you will not be surprised. The darkest green areas are, not surprisingly, the suburbs.
Lawn density seems to correlate pretty strongly with population density when population density is low, but when population density gets above a certain threshold lawn density starts to drop off; see, for example, this Wikipedia map of the population density of New Jersey. This makes sense -- people living at, say, 100 a square mile don't have lawns which are ten times as big as people living at 1,000 a square mile, because they don't want to maintain them. But people living at 10,000 a square mile have less lawn acreage per square mile even though there are ten times as many of them, because 10,000 a square mile is a city like Philadelphia or Boston where there's just no room for lawns.
This map also doesn't look all that different from this picture of the United States at night. Conclusion: the people who light up the skies are the ones with lawns.
Each different color on the map symbolizes a different density of lawn. If I knew how, I'd convert those colors back to the densities and average them out over the entire state of New Jersey and put this question to rest once and for all. (For all I know, someone at NASA has already done that.)
If you told me Long Island was one-fifth lawns, I'd believe you -- the whole island is mostly suburban.
But the state of New Jersey? I'm not sure. Lawns are a suburban phenomenon. Most of the Philadelphia suburbs are in Pennsylvania, not New Jersey. And a lot of the New York suburbs are just too dense to support a density of lawns much above one-fourth or so.
What's really shocking is that, say, the Phoenix or Las Vegas areas don't look all that different, in this map, from any other urban area. People, grass was not meant to grow in the desert. The reason that we have lawns is because in England it is not so hard to grow them, because their weather is for the most part wet and cool. But we don't live in England. (To my readers from England: well, I don't live in England. More importantly, the people of the desert Southwest don't live in England.)
(From a comment to this entry at strange maps.)