Abstract
Mid–growing-season water limitations are becoming increasingly frequent in semiarid onion-producing regions, such as Oregon, posing a significant threat to onion productivity because onions are sensitive to water stress during bulb development, even though this region contributes more than one-tenth of total US onion production. A 2-year field experiment (2024–25) was conducted on an Owyhee silt loam soil at the Malheur Experiment Station in Ontario, OR, USA, to determine the effects of 3- or 4-day irrigation interruptions imposed either early (1–15 Jul) or late (16–31 Jul) during bulb development under drip irrigation with and without wheat-straw mulch. The treatments were arranged in a randomized complete block design (RCBD) with four replications. In both years, early-July irrigation stress between 1 and 15 July for 3 and 4 days reduced average marketable yield by 28% and decreased the number of larger-sized onions, such as Colossal and Super Colossal, compared with fully irrigated controls ( P < 0.001). In contrast, the irrigation stress applied later in July (16–31) resulted in comparatively smaller yield reductions, up to 21%, compared with fully irrigated controls. Mulched treatments generally moderate yield reductions under deficit irrigation. Soil biological indicators measured at harvest responded to the irrigation regime. Microbially active carbon, 24-hour CO 2 respiration, and Olsen phosphorus (Olsen-P) were significantly lower under early stress without mulch but were maintained under late-season deficit irrigation combined with mulch. Results suggest that early bulbing stages are highly sensitive to short-term water deficits, whereas once bulb structure is well established, late-July irrigation reduction allows the crop to tolerate short, controlled irrigation deficits without substantially compromising yield or soil activity. Full irrigation treatments with mulch resulted in improved soil health compared with stress irrigation without mulch. These findings indicate that irrigation in early July is crucial for achieving a significantly large bulb yield. However, strategic late-season deficit irrigation with mulch under the system may offer a viable water-conservation approach while maintaining yield and soil health in semiarid onion production.