In late November, an organic grower in Sutter County contacted UCCE about a weed problem in carrots grown for seed. The main weed issue was a grass weed (too small to ID from photo), but stinging nettle was also prominent from the picture the grower sent. Weed pressure can severely impact carrot because it is slow to germinate and cannot compete with the rapid growth of weeds, or shade them out once established.
Conventional management includes cultural and chemical control, though even this can be difficult because of the limited number of herbicides available, diversity of weeds, and plant back restrictions.
Flaming and solarizing can help but timing is important. Since carrots have a long germination period, flaming can only be done after weeds emerge but before carrots do. For solarization, a fallow period is needed in the summer for at least 4-6 weeks.
Cultivation and hand weeding may cause more crop injury than benefit, and hand weeding is expensive. Two weeks before planting carrots, pre-irrigating to germinate weeds and cultivating shallowly can help control some of the weeds early-on. The closer this is done to planting the crop, the better.
Avoid planting carrots in fields that were previously in cereals or with known perennial weed infestations.
Carrots can suffer severe yield loss from weed competition and fewer control options are available for organic growers. After following-up with the grower, they were able to get a successful stand establishment of their carrot seed field. They had crews hand weeding in-row to eliminate the grass weed and the stinging nettle.
References: UC IPM. Integrated Weed Management. Pest Management Guidelines: Carrot. http://ipm.ucanr.edu/PMG/r102700111.html