Colton D. Blankenship, Assistant Professor & Extension Specialist, Louisiana State University Agricultural Center, Hammond Research Station
Introduction
Management of weeds in blueberry production is critical for protecting crop yield and quality. Weed management costs represent a sizable portion of the expenditure necessary to produce a blueberry crop. An integrated weed management strategy that combines chemical and cultural practices is the best way to mitigate crop losses from weed competition and reduce overall weed pressure.
Chemical Management
Preplant
Weed management is an essential part of site preparation ahead of blueberry planting. Populations of perennial weeds such as greenbrier (Smilax spp.) and nutsedge species should be reduced as much as possible prior to planting to minimize weed pressure over subsequent years. Because perennial weeds persist over multiple years, any weeds present at the time of blueberry planting will compete with the crop during establishment and over time. Site preparation should ideally begin months ahead of blueberry planting to allow time for multiple chemical and cultural control actions; the more weeds that can be eliminated in this time, the less weed pressure there will be during crop production.
Non-selective herbicides such as glyphosate can be used for postemergence control of weeds prior to planting. Exercise care to observe all preplant intervals listed on herbicide labels when used ahead of blueberry planting. Sites should be scouted to determine which weed species are present; product selection is crucial to ensure good activity on weeds present at the site. Multiple herbicide applications may be necessary to control weed populations ahead of planting.
It is also key to ensure that living weeds are not present at time of planting. If emerged weeds are present at time of planting, consider delaying planting and applying a registered postemergence herbicide to eliminate weeds and prevent competition with new plants.
Young & Established Plantings
Preemergence herbicides should be applied to established plantings 2-4 times per year depending on product labels, soil types, environmental conditions, and weed pressure. After planting, most postemergence herbicides require directed applications. Directed applications are those that target weeds in row middles and avoid all contact with crop stems and foliage. Except for Herbicide Resistance Action Committee (HRAC) group 1 herbicides, also called grass herbicides, most postemergence herbicides will cause crop injury if they contact blueberry foliage. However, injury has been observed in Georgia after using clethodim on blueberries. Consider applying clethodim as a directed spray even though it is a group 1 herbicide.
When using herbicides in established plantings, be careful to observe all label restrictions including preharvest intervals (PHIs). The preharvest interval is the minimum amount of time legally allowed between herbicide application and crop harvest and can be found on the pesticide label.
Rotating between multiple herbicide modes of action is critical for mitigating development of herbicide resistance in weed populations. Avoid using the same herbicide sequentially and use herbicides with different modes of action when possible.
Cultural Management
Preplant
Cultivation is key for removing weeds and incorporating soil amendments ahead of planting. Mulch is a key component of integrated weed management programs as it acts as a physical barrier to weed germination and establishment. Mulch is also one of the most effective ways to manage weeds within the blueberry row. There are various types of mulch that can be used including plastic mulch, landscape fabric, pine bark, etc. When using mulch, remove all weeds before laying mulch, as living weeds may be able to survive and break through the physical barrier depending on the weed species and type of mulch used. Replace mulch as necessary over time; some types of mulch will persist longer than others.
Young & Established Plantings
Mechanical cultivation can be used to control weeds between blueberry rows, but use caution not to damage blueberry root systems. Hand removal is costly but often necessary to control emerged weeds within blueberry rows. Prevent weeds from producing seeds by all available means, including hand weeding. Management of the weed seedbank will reduce overall weed pressure and reduce the need for costly management actions. Mange weeds on field margins and at the ends of rows, as well as within rows. Even if weeds are not directly competing with blueberry plants, they may produce seeds which will increase weed pressure in the production area.
Another key component of an integrated weed management strategy is being proactive. Weeds are easiest to control when they are small. Regular, active scouting is critical for early detection of weed problems before they become established and difficult to eliminate. Scouting will help with deciding which chemical products to use as well as timing preemergence herbicide applications.
Always read and follow all pesticide label instructions and restrictions when applying pesticides, including herbicides. For information on specific herbicides and herbicide programs for blueberry production, refer to the Southern Region Small Fruit Consortium Southeast Regional Blueberry Integrated Management Guide, on the IPM/Production Guides page.