I keep hearing about how beneficial hemp is for the environment and what a shame it is that U.S. farmers aren’t allowed to grow it. Is hemp really that eco-friendly?

With the possible exception of soy, no plant has managed to spawn as many different products — and as much controversy — as hemp. You can buy hemp clothing, hemp paper, hemp milk, hemp oil . . . the list goes on. A Canadian company has even built an electric car out of hemp. Advocates talk about the leafy plant like it’s going to reverse global climate change. Opponents think it’s merely a Trojan horse packed with potheads hoping to get your kids stoned.

The legal problem for hemp is that it’s taxonomically identical to marijuana. Both are classified as Cannabis sativa L, and the only difference between them is the concentration of delta-9-tetrahydrocannabinol (THC), the psychoactive substance in pot. Marijuana contains at least 3 percent THC by weight, whereas hemp falls below that threshold.

Some hemp advocates think there are also visible differences. But Uncle Sam seems to believe that the only way to distinguish marijuana from hemp is by taking it to the lab (or, presumably, rolling it up and smoking it).

The plant is a Schedule 1 controlled substance in the United States, which means you need special permission to grow it regardless of THC content. Canada and several European countries allow their farmers to grow hemp with THC content below 0.3 percent — one-tenth as strong as the weakest marijuana. The United States has held off, largely because a government helicopter that flies over a farmer’s field can’t tell the difference between hemp and marijuana. For this reason, raw ingredients for all U.S.-manufactured hemp products must be imported.

(MICHAEL SLOAN FOR THE WASHINGTON POST)

Hemp is so versatile in part because it can be grown for either seed or fiber. The seeds yield milk, oil and other food products, and are particularly popular among vegans, who have trouble working omega-3s into their diets. The fiber is used for paper and clothing. Sailors have been using hemp rope and sails for centuries, and the crop’s abundance was valued during the Revolutionary War era.

So is hemp the answer to all our environmental problems, or is it just pot? It depends on what you want to use it for, and what you compare it to. For now, let’s focus on textiles, the most traditional and common use.

A 2005 report by the Stockholm Environment Institute compared the water, land and energy requirements of cotton, polyester and hemp textiles, among others. While the study is the most comprehensive investigation of this issue, the results were unsatisfyingly mixed.

Producing the raw ingredients for any textile consumes more energy than any other part of the process. Different production techniques, however, can significantly affect that consumption. Since pesticides and herbicides account for more than half of the energy used in growing hemp or cotton, organic methods for raising these two crops produce lower carbon-dioxide emissions. Organic cotton required less energy than organic hemp, but the difference was fairly small. Polyester, a petroleum-based synthetic fabric, was the clear loser — because it takes so much energy to extract the oil required to make it.

Cotton needs approximately twice as much territory as hemp per ton of finished textile. Further complicating matters is the inverse relationship between chemical use and land requirements. While organic farmers save on energy consumption, their yield per acre is lower than that of cotton and hemp growers who use synthetic pesticides and herbicides. Polyester does almost as well as hemp on land use. And apparently you can get more fabric from an oil field than a cotton field.

Cotton is again the big loser when it comes to water. The cotton plant needs around 50 percent more water per season than hemp, which can grow with little irrigation. More than half of the world’s cotton fields rely on irrigation, because it is raised in some relatively dry regions, including Egypt, China’s Xinjiang province, California and Texas.

When you add processing into the equation, cotton uses more than four times as much water as hemp. Polyester is difficult to compare, because it’s not an agricultural product. But some studies suggest it is the least water-intensive of the bunch, using just 1/1,000th as much water as cotton. (In fact, water is a byproduct of polyester processing.)

So where does all this leave us? Without a clear winner, unfortunately. There’s a case to be made for polyester, but the nonrenewability of synthetic textiles raises serious concerns. Overall, hemp appears to be slightly easier on the environment than cotton, superior on water and land requirements, and only slightly worse for energy use. But is the Drug Enforcement Administration responsible for all of our environmental woes? Hardly.

The Green Lantern is produced by the online magazine Slate and can be read at www.slate.com.