The plants were destroyed by law enforcement officers under the auspices of the DEA’s cannabis eradication program. This program allocates money from the Department of Justice’s Asset Forfeiture Fund to 128 state and local law enforcement agencies. The agencies use this money to locate and destroy indoor and outdoor marijuana grow sites.

Marijuana eradication teams, who criss-cross the countryside in helicopters looking for grow sites, occasionally become overzealous in carrying out their missions. The Georgia Governor’s Task Force for Drug Suppression earned notoriety last year when they raided a retiree’s garden after misidentifying okra as marijuana. Last month, a member of DEA’s marijuana eradication team in Utah warned lawmakers there that a medical marijuana bill would lead inevitably to stoned rabbits in the state’s forests.

For fiscal year 2014, the DEA estimated that the asset forfeiture program provided $18 million dollars to fund the cannabis eradication program. At 4.3 million marijuana plants destroyed, that works out to a cost of about $4.19 per plant. For simplicity’s sake, let’s round that number to the nearest dime and call it $4.20.

But in recent years funds available for cannabis eradication have been drying up. The great recession forced some states to curtail various drug enforcement efforts — and flying those helicopters gets expensive. Public opinion toward marijuana has shifted rapidly over the same period. Together, these factors have caused state and local governments to prioritize more serious and deadly drug threats, like meth and heroin.

With 2.7 million plants destroyed, California alone contributed 63 percent of the total haul last year. But California’s numbers have fallen sharply during the Obama administration, taking the national numbers down with them. “Coinciding largely with the downsizing of, and then ultimately the disbanding of, the state’s nearly 30-year-old Campaign Against Marijuana Planting (CAMP) program, DEA-assisted annual marijuana seizures in California have fallen over 60 percent percent since 2010,” said Paul Armentano, deputy director of NORML, in an email.

There’s evidence that as the cannabis eradication program grows leaner, it’s growing smarter too. For instance, the total number of grow sites destroyed has fallen nearly tenfold from a peak of 73,000 sites in 1997 to 9,000 sites in 2014. But the number of plants found at each site has risen more than tenfold over the same period, from an average of 43 in 1998 to 473 in 2014. This suggests that enforcement efforts are doing a better job of targeting truly large-scale commercial operations — rather than, say, college kids who grow a couple plants in their dormroom closets.

The cannabis eradication programs face some serious challenges going forward. For starters, the asset forfeiture program funding these efforts was recently curtailed sharply by the Justice Department. The trend of state-level legalization will continue, with a number of states looking to do so by 2016.

But even in a world with legal marijuana needs marijuana enforcement efforts. Just as you can’t currently distill whiskey without getting the proper permits, you wouldn’t expect to be able to grow 100 pot plants on your land outside of the regulatory structure either.