Many of our winter annual weeds have started to green up and grow during the latter half of March. Before long, they will begin to dramatically increase their vegetative growth before reproducing later in the spring.  When the weather allows, Anhydrous ammonia and dry fertilizer applications will be on the priority list. However, let’s not forget the importance of timely herbicide applications, and the value that a spring burndown can bring when managing tough to control weeds.

With the fluctuating fall conditions across much of the territory, winter annual weeds have had a head start in some geographies, while in others, weed development may be much slower. While the harsh winter may have suppressed some of the growth of these weeds, it’s important to not let them get too far along in their growth cycle. If left unmanaged this spring, weeds can get out of hand quickly and management of weed escapes may warrant a second herbicide application. Earlier burndown applications will ensure that we get control of our winter annual weeds while they are small, before they get to a growth stage that is much more difficult to manage.  

When thinking about our herbicide options, make sure to use multiple effective sites of action.  Additionally, pay attention to the air temperature at the time of application when managing emerged weeds. Burndown herbicides are much more consistent if the nighttime temperatures stay well above freezing and the daytime temperatures reach at least 50 degrees. This ensures that the plant is actively growing and can translocate the herbicide to the site of action. Burndown applications that are combined with residual herbicides can help minimize the number of winter annual weeds that may emerge prior to planting.  This may be a good option for growers who have a seedbank that contains many winter annuals or large seeded broadleaves that are early emerging.  In addition, as we get closer to planting, a residual herbicide may be added to manage driver weed species, such as waterhemp.  The residual herbicide to manage waterhemp will be more effective the closer to planting it is applied. However, if we delay our burndown application for too long to ensure residual herbicide activity, winter annual weeds such as marestail may become too large to be controlled.  We need to find the best timing that ensures control of emerged weeds while being close enough to planting to get the best value from residual herbicides.

When using burndown herbicides containing auxins, be aware of the plant back restrictions on the label to avoid potential crop injury.  If the plant back restrictions of an auxin herbicide cannot be met, consider a different mode of action that doesn’t have a plant back restriction or has a shorter interval to planting. 

For more information on burndown and residual herbicide programs contact your local FS crop specialist.