Bulb onions are poor competitors and volunteer potato, commonly observed in western USA onion fields, is difficult to manage. To improve the understanding of onion and weed interactions, relationships were quantified among volunteer potato density, onion yield, and volunteer potato tuber production using hyperbolic or linear models. Onion yield losses because of volunteer potato interference occur at densities commonly observed in the field. A volunteer potato density as low as 0.067 plants m⁻² resulted in a 10% reduction in crop yield. Asymptotic yield loss (A parameter) was 100% and achieved with 4 volunteer potato plants m⁻². Volunteer potato competition limits onion bulb size, resulting in a lower quality and thus a less-valuable crop. Volunteer potato tuber density and biomass increased linearly with initial weed density as high as 8 volunteer potato m⁻². Onion yield loss from volunteer potato competition occurs to a greater extent and at a lower weed density than demonstrated in previous research on small-seeded annual weed species.

Nomenclature: Volunteer potato, Solanum tuberosum L. ‘Russet Burbank’ and ‘Ranger Russet’; onion, Allium cepa L. ‘Pinnacle’ and ‘Vaquero’.