Floral hemp production expanded rapidly after the 2018 federal farm bill, but practical production guidelines have lagged, especially in semi-arid regions where water is limited and irrigation costs are rising.

Many growers assumed that producing high flower yields for cannabinoid extraction required heavy irrigation, in part because hemp plants can grow large and have a “lush” appearance, and because early production models borrowed from greenhouse or high-input systems.

In Southern Oregon and other dry parts of the West, that assumption carries real risk. Overirrigation can waste scarce water, can increase chances of plant disease, and increase costs and complicate harvest and processing by producing extra stems and leaves that do not contribute to cannabinoid yield.

At the same time, underirrigation can reduce flower production and farm income. Growers and decision-makers need clear, field-based guidance on how much irrigation pays off.

Researchers at Oregon State University’s Southern Oregon Research and Extension Center (SOREC) in Central Point tested how direct-seeded floral hemp responds to moisture stress under open-field conditions in a semi-arid climate.

Over multiple years, the team compared both autoflower and full-season cultivars across a wide range of irrigation rates, from little or no supplemental irrigation to levels that met or exceeded the site’s seasonal potential evapotranspiration.

They measured floral and total biomass, cannabinoid concentration and total cannabinoid yield, and they separated primary flowers from secondary and tertiary flower production to see where irrigation mattered most. The team also evaluated adjacent non-irrigated plantings to understand what growers can expect when water is extremely limited.

More water did not always mean more CBD yield

The results delivered a clear message for growers and the partners who advise them: More water does not always mean more cannabinoid yield.

As irrigation increased from low levels to moderate levels, flower production and CBD yield increased substantially. But the gains generally leveled off for both autoflower and full-season cultivars once irrigation reached about 80% of the site’s seasonal potential evapotranspiration. Beyond that point, plants kept putting on more leaf and stem growth and often got taller, yet floral biomass and cannabinoid yield typically plateaued.

Hemp produced usable flowers under dry conditions

Just as important, the study, published in the journal Agrosystems, Geosciences & Environment, found that hemp can still grow, reach maturity and produce usable flowers and cannabinoids even under very dry conditions, including situations with little rainfall and minimal or no supplemental irrigation.

While yields are lower in those conditions, the work shows that low- to moderate-irrigation strategies — and in some cases dryland production — may be economically feasible depending on water costs, management choices and market value.

Practical decision support for growers and advisors

For growers, crop advisors and agricultural agencies, the research translates into practical decision support. It helps determine where irrigation dollars produce real returns, guides irrigation scheduling to avoid diminishing returns, and supports water-smart production strategies that better match the realities of semi-arid farming.

It also provides a research-backed basis for on-farm trials and Extension education that can reduce uncertainty for an industry still refining best practices.

Public value

Water is a shared resource in the West, and irrigation decisions affect farm viability, watershed resilience and the ability of communities to plan for drought. By identifying an irrigation “sweet spot” that maximizes cannabinoid yield without unnecessary water use, this research helps growers conserve water and manage costs while maintaining production goals.

These findings improve the return on investment of agricultural water and research dollars by helping producers avoid overirrigation that adds cost and water demand without increasing cannabinoid yield.

The result is more efficient water use, stronger farm decision-making and a more resilient approach to building stable markets for Oregon-grown agricultural products in water-limited regions.

This research was supported through a U.S. Department of Agriculture Agricultural Research Service non-assistance cooperative agreement (Project No. 2072-2100-054-00-D), along with state and federal funds appropriated to Oregon State University and the Oregon Agricultural Experiment Station.