Where the Derwent River meets the Styx, amid dripping rows of vines, grows Australia’s single most important crop. Cultivated for well over a thousand years, it was originally used as an antimicrobial preservative. But ancient gourmands discovered an intriguing side effect of humulus lupus: it makes beer taste really, really good.

Grown in only two places in Australia, hops – as it’s more commonly known – gives beer its bitter flavour and its floral, herbaceous taste. Harvested in early autumn, vines are stripped of their papery, cone-shaped flowers. Each bud is filled with a sticky and hugely pungent pollen, the scent of which may remind some scamps of its more infamous cousin: marijuana.

Because the flowers only bloom in March (and the fact that many of us want beer year round), brewers tend to use dried hops. They love these little grey pellets because they’re predictable; all their volatile oils have already exploded and settled and mysterious odours have already made themselves known.

But Australia’s oldest brewer, Cascade, has been revelling in unpredictability. Its limited edition brew, First Harvest, uses green hops taken straight from the vine. Every year, head brewer Mike Unsworth chooses three new experimental varieties to brew a beer restricted to 5000 cases.

On a single morning in March, Unsworth and his team pour the fresh hops into an enormous kettle, combining the green flowers with the first malt of the season and water from the Derwent. First Harvest matures at Cascade’s sandstone beer palace until May, when it’s released to the world during Good Beer Week.

While the First Harvest project obviously capitalises on Australia’s growing thirst for unusual craft beer, it’s also part of an experimental program to develop new varieties of hops. At Bushy Park Estate, Australia’s oldest and largest hops farm, the growers at Hop Products Australia produce hundreds of new strains with an eye taking them to market. Working together, HPA and Cascade brew speculative beers to find out exactly what these experimental and uniquely Australian hops taste like.

“It’s really the most important part in the process,” says HPA managing director Tim Lord. “After six years of the work we do, let’s face it, if it doesn’t taste any good in beer, you go and find it and you pull it out.”

The 2013 First Harvest Because is still brewing, so we haven’t had the chance to sample it yet, but its forbearers displayed a dark, malty character with a distinctly fruity taste that one mightn’t immediately associate with Cascade.

“This year’s hop selection will create a brew that features prominent, leafy, herbal and fruity aromas overlaying a tropical fruit salad finish,” predicts Unsworth. “The green hop resin characters will be balanced by a rich and robust maltiness, derived from new season’s barley.”

Beer fans will be able to sample this year’s vintage as of May 13, with select pubs serving it up on tap, and four-packs available for $19.95.