Richard Smith is a University of California Cooperative Extension Farm Advisor in Monterey County.
Leeks are in the onion family and are increasingly important crop in Monterey County. In 2017, there were 475 acres produced worth $8.6 million. They are mostly established by transplanting and days to maturity is generally 120+ days, but varies widely depending on the time of year and market demand. Given the long crop cycle and the lack of a dense crop canopy, leeks are susceptible to successive waves of weed emergence. Growers complain that leeks “blow up” weed populations by allowing a significant number of weeds to mature and set seeds.
The preemergent herbicides DCPA and pendimethalin are registered for use and provide good weed control. However, post emergent herbicides such as bromoxynil and oxyfluorfen which provide excellent control of escaped weeds in onion production are not registered on leeks. In Canada prometryn is registered for use on leeks. In 2016, we began studying this material for use on leeks to bringing it into the US market. This project is now supported by the IR4 program and they are evaluating crop safety and developing a tolerance to eventually bring it to market.
The potential for prometryn as a preemergent herbicide for use on leeks is quite exciting. It has the potential to provide a level of weed control in this crop that we have not seen. We are exploring its use on other allium crops as well and recently observed good safety on onions in one trial when it was applied at the 5th true leaf stage.