About four years ago, a flurry of headlines declared that cannabis cultivation was “sucking California dry”. The stories appeared in several major news outlets, many of which made the assertion that a single cannabis plant guzzles about 22 litres of water each day.
“Reading those stories made me wonder just how big an issue this was,” says Van Butsic, an environmental scientist at the University of California, Berkeley. He found that the cannabis plant had also been described as being unusually thirsty by many scientists — dozens of peer-reviewed publications had cited the same 22-litre-per-plant figure. “We used that number in our earlier papers, too, because it’s the only one we could find,” Butsic says. “But we always wondered, where did it come from?”
As it turns out, the figure that researchers relied on was derived from an estimate in a cannabis growers’ manual from 1996. And in an April preprint1, Butsic and Ted Grantham, also an environmental scientist at Berkeley, and their colleagues presented data that suggest that the problem wasn’t how much water the plants were using but, rather, the source of that water.