Weed management systems in carrot are limited in part by a lack of fundamental understanding of crop–weed interactions. Irrigated field studies were conducted to quantify the effect of volunteer potato density and duration of interference on carrot yield and to determine relationships among weed density, duration of weed growth, and volunteer potato tuber production. A season-long volunteer potato density of 0.06 plants m⁻² produced from 150 to 230 g tubers m⁻² and resulted in an estimated 5% crop yield loss. At two volunteer potato plants m⁻², the same level of crop loss was estimated with a duration of interference of 430 growing degree days (GDD), a time at which the weed had already produced 130 g tubers m⁻². Volunteer potato height at the time of weed removal predicted carrot yield loss (R² = 0.77) and may be useful for timing of management strategies such as hand weeding. Functional relationships describing carrot–volunteer potato interactions provide simple information that is useful for developing weed management recommendations for carrot, a crop that relies on multiple tactics for managing weeds, and rotational crops that are negatively affected by persistence of volunteer potato.

Nomenclature: Volunteer potato, Solanum tuberosum L. ‘Russet Burbank’; carrot, Daucus carota L. ‘PS-104395’.