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A two-year Oregon State University research project (2024 and 2025) examined what happens when onion irrigation is stretched in July, exactly the kind of squeeze growers can face when water gets limited mid-season. The research focused on July, when onions are actively forming bulbs and are highly sensitive to water shortages. Results showed that even a short break in irrigation during early July (July 1–15) can significantly reduce bulb size and overall marketable yield. In contrast, similar irrigation stress later in July had much less impact, suggesting that timing is more important than duration when managing limited water supplies.
Weather conditions during the study clearly explain why this mid-season period is so challenging. In eastern Oregon, both the 2024 and 2025 seasons had very low rainfall, often close to zero during peak summer months, combined with high temperatures and rising evapotranspiration (ET) rates. As temperatures peaked in July (average high of 96 degrees Fahrenheit, with a record of 105 degrees Fahrenheit on July 10), crop water demand increased sharply while soil moisture declined rapidly due to high ET and low relative humidity. These conditions create what growers commonly call a “July drought,” in which crops depend almost entirely on irrigation. Historical data also show that drought conditions frequently occur in July in Malheur County, Oregon, making irrigation management even more critical for sustaining yields.
The study also found that simple practices such as wheat straw mulching can help mitigate the effects of water stress by conserving soil moisture and supporting higher yields under deficit irrigation. Overall, the findings highlight a clear message for growers: maintaining adequate irrigation in early July is essential for producing large, marketable onion bulbs, while carefully managed reductions in irrigation later in the month may help conserve water without major yield losses. The practical message is simple: if you have to divert water, try not to do it in the first half of July.