Last summer eight trials were conducted to evaluate pre-emergence weed control materials and strategies on dry bulb and green onions. The cancellation of Dacthal in 1997 caused great concern for not only onion growers, but for producers of cole crops as well.
Because of the acute shortage of Dacthal in the 2000 season, several growers used no pre-emergence materials and this provided us with opportunities to evaluate pure postemergence strategies. Dacthal has been reregistered and was made available by Amvac Corp. for use in California as of March 2, 2001. Its use in conjunction with postemergence weed control materials will greatly facilitate weed control in onions.
Of the pre-emergence materials evaluated in the 200 trials Carfentrazone looked promising at the 0.075 to 0.10 active ingredient per acre rates. Valor did not provide selectivity for onions pre-emergence. Prowl looked safe for use pre-emergence on onions in two trials.
For postemergence, onions can be grown with no pre-emergence material to stop or slow weeds down, but typically the weeds develop rapidly and weed control sprays must be applied at the early second true leaf stage in order to catch the weeds before they get too big to be controlled.
A wide range of choices can be employed. Topical applications of fertilizers such as AN20 and 7-7-0-7 provide weed control, but care must be taken as both of these materials can cause significant stand loss under some conditions.