Researchers reported on Mar. 16 that delaying the transplanting of processing tomatoes can significantly reduce the emergence of branched broomrape, a parasitic weed affecting tomato fields in Yolo, Solano, and San Joaquin counties. Field studies conducted near Woodland, California during 2024 and 2025 showed that transplanting tomatoes later in the season led to a dramatic decrease in broomrape parasitism.
Branched broomrape is an obligate root parasite whose seeds require specific soil moisture and temperature conditions to germinate. The research team found that when tomato transplants were delayed from early April to early May or June, broomrape emergence dropped by nearly 90 percent or was eliminated entirely. “Results indicate delaying the transplanting by a few weeks into the summer to early May reduced broomrape emergence by nearly 90% compared to the typical transplant timing of early April, and no broomrape was observed in June planting,” the researchers said.
A follow-up study in 2025 confirmed these findings with different tomato varieties and included tests with chemigated rimsulfuron herbicide. Both mid-May and June plantings had no observed broomrape emergence regardless of herbicide use. The researchers said these results suggest that adjusting transplant dates could be an effective strategy for managing this pest.
The University of California Agriculture and Natural Resources (UC ANR) supports such research through public contributions according to its official website. UC ANR promotes sustainable practices aimed at strengthening resilient ecosystems and economic vitality according to its official website. The organization operates nine research and extension centers across California’s diverse ecosystems according to its official website and is recognized for linking research with community needs according to its official website.
UC ANR is part of the University of California system, managing Cooperative Extension services statewide according to its official website. It conducts over 33,000 educational events annually with more than 18,000 volunteers involved according to its official website. Through workshops and partnerships in all 58 counties, UC ANR delivers science-based information supporting environmental stewardship and community development while mobilizing volunteers and research centers for state issues according to its official website.
Growers are advised by researchers to prioritize planting clean or low-risk fields at normal times but delay planting in high-risk fields by several weeks. They also recommend applying chemigated rimsulfuron according to label guidelines as a proactive measure against broomrape establishment.