It may not be as sexy as marijuana but hemp, a United States industrial staple for centuries until it was banned by anti-drug zealots, is making a comeback.

Last year a Colorado farmer harvested the first hemp crop since 1957. By then the hardy plant, celebrated by Founding Father Thomas Jefferson as "of the first necessity to the wealth and protection of the country"but criminalised in 1937 [it was briefly revived in the Second World War to help the war effort], had been eclipsed by synthetics, outlawed as marijuana's cousin and banished as "ditch weed".

Flash forward three-quarters of century and US farmers are chaffing at the bit to supply an estimated US$500 million-a-year domestic manufacturing industry, supported by hemp imports from Canada, China and Europe.

The plant provides seeds and oil used in food, cosmetics and body care products, plus fibre for the clothing, automobile and construction sectors.

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So, there were hosannas from the agricultural community last week, when President Barack Obama signed off on the 2014 Farm Bill, which includes a provision to grow hemp on a trial basis at universities and state agriculture departments, preparatory to re-establishing a viable domestic industry.

"I feel it is a great step forward, but certainly a lot more progress needs to be made before farmers in the US will be raising meaningful acreage," says David Monson, a farmer, hemp advocate, and six-term Republican representative in the North Dakota legislature, via email.

"All in all, I am very grateful for the inclusion of industrial hemp in the farm bill and feel it has put a light at the end of the long tunnel I have travelled through for the past 15 years."

Monson, who farms near the Canadian border and can see hemp fields across the line - with crops sold to the US - was locked in a legal dispute with the US Drug Enforcement Administration, which stymied his efforts to grow hemp.

A variety of Cannabis sativa, the plant species that includes marijuana, hemp was demonised by the DEA and farmers faced prosecution for growing the crop.

Ironically, given the military has been used to help fight the "war on drugs," the Pentagon is partly built on a former US Department of Agriculture hemp farm.

Both plants look similar but hemp, grown in some 30 countries, has only tiny amounts of THC, or delta-9 tetrahydrocannabinol, the psychoactive compound that makes marijuana a Schedule 1 Substance under federal law.

As US public opinion shifts on marijuana - with Colorado and Washington state legalising the drug - hemp supporters are focused on practicalities: how long will it take to acquire enough seeds to grow commercial crops?

"We'll see a year or two of research to ramp things up," says Eric Steenstra, president of lobby group Vote Hemp. "To be honest we've lost all the genetics we had. We haven't grown the crop for 50 years. So none of those varieties the USDA developed back in the day are left. They didn't preserve them. It will take time to come up with varieties for different states and figure out where it will grow well. But this is a great first step."

He says the industry is expanding at 15 to 20 per cent a year and expects "we'll definitely be headed towards US$1 billion in the next 10 years".

In addition to the Farm Bill amendment two congressional bills have been introduced to eliminate US restrictions on hemp cultivation.

The DEA fought hemp's rehabilitation down to the wire, arguing as one measure moved through the House of Representatives that hemp and cannabis were indistinguishable, a chestnut that had served the agency well for decades.

To unravel decades of restrictions under US drug laws, 13 states have passed or tabled industrial hemp bills, with others close behind. Steenstra says Kentucky will likely be the first state to plant hemp.

The key, either using imported seeds or home-developed varieties, is to create varieties that meet market demand. Historically, the US focused on fibre. Steenstra, who is adamant the burgeoning US market is opposed to GMO hemp, believes dual cropping - with seeds and fibre harvested by specialised combines - is the future, maximising sustainability and return on an investment.

The US decision to revive hemp has been a boon for Taranaki's Hemp Technologies. "We've been asked to help with an Oregon State University research study on growing hemp," says co-founder Greg Flavall, who will host American business visitors, keen to get into the US market, next week.

Growing hemp in New Zealand has been legal since 2006 - Taranaki had a flourishing industry until the 1920s - and Hemp Technologies will plant 100-120 hectares this year. Meanwhile, Dutch hemp fibre is imported and mixed with locally-sourced lime to create Hempcrete, used with a frame to build carbon neutral walls with high insulation values.

"The demand for building is growing," says Flavall. "We have permits for the first two hemp homes in New Zealand in the New Plymouth district and have 20-plus homes in the pipeline from Russell to Dunedin. We're ahead of the curve."

He intends to buy a decorticator [to separate fibre from hemp pulp] to meet local demand and says 1 per cent of agricultural land in New Zealand could supply enough low-cost Hempcrete to supply the nation's housing needs.

Advocates also tout an environmental upside, saying hemp gives a greater return from less acreage, has a deep root system to survive drought, and needs less fungicides, pesticides and insecticides. Far less thirsty than cotton - which also exhausts soils - hemp may be a standard bearer for a more sustainable future.

"The climate is changing and it's going to have an impact," Tom Vilsack, the USDA secretary, said last week.

"There are ramification today. If we are not proactive, we will find ourselves 5, 10 or 15 years down the road wishing we had done what we did today."

Hemp signals the shape of things to come. The DEA may object to fields of hemp but Jefferson would have approved.


Different strains
•Hemp
Grown for fibre or seeds, and used as animal feed or for rope, textiles, clothes, paper, building materials. Produces two to four times as much usable fibre as trees or cotton. China is the world's largest hemp producer.

•Marijuana
Cannabis has a far higher THC (drug) content. Used for for medicinal, recreational, and spiritual purposes.