The few available herbicides for fresh market spinach do not provideadequate weed control, and there is need for additional herbicide tools.Phenmedipham is registered for use in processing spinach but not in freshspinach, because of potential injury and the short interval betweenapplication and spinach harvest. The objectives of this study were toevaluate the tolerance level of fresh spinach varieties to phenmedipham andevaluate the impact of light intensity on tolerance of spinach tophenmedipham. In the greenhouse, nine spinach varieties were treated withphenmedipham (0.55 kg ai ha−1). Spinach varieties exhibited awide range of tolerance, and dry weights of treated plants ranged from 40 to78% compared to the nontreated control. Based on the phenmedipham tolerancescreen, two varieties with low (Nordic) and high (Regal) tolerance tophenmedipham were treated, then exposed to half (shaded) and full(nonshaded) sunlight. Nonshaded Nordic treated with phenmedipham had 65%lower dry weight compared to similarly treated plants grown under shade,suggesting that spinach tolerance to phenmedipham was mainly affected bylight intensity. Measurements of electron transfer intensity in photosystemII also showed tolerance to phenmedipham that varied among spinach varietiesand light intensity. The maximum values of electron transfer in photosystemII of Regal treated with phenmedipham were higher than those of similarlytreated Nordic. In the field, phenmedipham was applied under varied lightand temperature conditions. The impact of light intensity on yield oftreated spinach was greater than the impact of temperature. Phenmediphamapplied under high light conditions was more injurious than when appliedunder low light conditions. Results from this study can contribute tosuccessful integration of phenmedipham into currently used fresh spinachweed management, which in turn can allow more efficient production of thiscrop.