From the UC Strawberries and Caneberries blog :: Oct. 11, 2013
A response to the question posed to us concerning how soon one should be applying water to Chateau (flumioxazin) sprayed in the furrows for weed control in strawberry. The label says that this herbicide can be applied at a minimum of 30 days before transplant of strawberries, but it doesn't seem to specify how soon to apply the water after application.
Probably the sooner you apply water to furrows the better, but in a study run by Oleg in Ventura County, Chateau was applied to furrows about a month before sprinklers were turned on and the herbicide was effective against several broadleaf weed species, including wind-dispersed weed seed that landed in furrows after application.
Chateau does not degrade and just remains on soil surface, provided furrows stay dry during that time. If the furrows get wetted enough to germinate weeds the herbicide will be sufficiently activated as well.
The residual efficacy of Chateau is diminished over time, but you can reapply it to furrows if needed—there is no co-distillation, so as long as there is no drift, strawberry plants should be safe. You can extend the control by reapplying it with shielded sprayer in-season as long as the strawberry plants do not have flowers and fruit (which is soon in Southern California but quite a bit later here in Watsonville and Salinas) and you do not intentionally overspray the beds (first photo below).
Also, thinking about traffic going through the field after application, specifically the wheels of tractors moving through the furrows, it does create some breaks in the barrier that the initial application of Chateau has formed. It might be interesting to do a little experiment with different levels of disturbance post-application to see how much efficacy really is affected. However, remember that Chateau can be reapplied later on in December and January in the furrows in order to get better control since it will be catching multiple flushes and cohorts of weeds from both the seed bank and wind dispersal.
UC IPM guidelines for Chateau in strawberry are available at:
The use of flumioxazin (Chateau) is extensively written about in this article. Before this or any other product, check with your local Agricultural Commissioner's Office and consult product labels for current status of product registration, restrictions, and use information.