Use of Pre-emergent Herbicides in Almonds

This is a good time to apply any pre-emergent herbicides. Orchard weeds need to be surveyed to determine any specific challenges. The pre-emergent product and burn-down partner selected should provide control for the whole spectrum of weeds within the orchard. Most pre-emergent products need to be "watered -in" with either an irrigation or rainfall to have efficacy. Usually a rainfall event over 0.5” (12.5mm) is sufficient, but several smaller rain events can provide a similar effect.Depending on weed pressure, a one- or two-spray residual program could be utilized. If weed pressure is minimal due to mature trees and a full canopy, a single spray may be sufficient to provide control into late spring or early summer. If trying to manage summer emerging grasses (i.e. goosegrass or junglerice), a second, late spring-timed pre-emergent application could be considered. This double-application strategy is useful when rainfall or sprinkler irrigation can be utilized to incorporate the material.Delays in pre-emergent herbicide application can create issues. Rain events knock down debris that can hinder the application or uniform incorporation of herbicides. If not able to re-clean the berms, avoid pre-emergent herbicides that are more tightly bound to organic matter (e.g. Alion®, Prowl H2O®). Consistent rains also limit field access which, in combination with warming temperatures in the spring, increases weed germination and rapid growth.  Delayed applications of pre-emergent materials will likely require tank mixing with burndown herbicides and possibly even a separate, post-emergent herbicide application. On very sandy soils, high rainfall events may also move some pre-emergent herbicides into the tree rootzone, which may impact plant performance if the tree is active. Reduce rates in these situations.In cases where orchard access was limited due to earlier than anticipated rain, a burndown application of herbicide may be needed before a residual material is applied. This will allow more herbicide to soil contact. Most winter emerging weeds are susceptible to glyphosate, but it is common for Malvasp. (common mallow, cheeseweed, etc). and Erodium sp. (Filarees) to escape. These weeds are often controlled by the inclusion of oxyfluorfen.If a burndown application is used in mid- to late winter, it is useful to consider applying a pre-emergent product in late January or February. Rainfall patterns become more erratic during these periods, so careful planning must occur to apply the material when rain is forecasted. Generally, any pre-emergent product can be considered, although there are uptake concerns with certain chemistries after pink-bud (e.g. penoxsulam).The use of pre-emergent products is not an all-or-nothing strategy. If rainfall is erratic, it may not be possible to apply pre-emergent products across the entire orchard. In these cases, there is value in applying pre-emergent materials across some of the orchard to reduce the operation’s overall weed pressure. This will reduce time spent applying burndown herbicides, allowing more time for other orchard activities.More Information: https://ipm.ucanr.edu/agriculture/almond/