Weeds can directly impact fruit trees via competition for water, nutrients, and light. Weeds may also affect tree growth and yield indirectly by serving as alternate hosts for insect pests and pathogens, and by providing habitat for rodents. Physically, weeds can interfere with crop management and harvest operations when they block irrigation emitters, inhibit the deposition of other pesticides, or impede the movement of workers and equipment. Successful weed management can best be achieved by employing a combination of strategies including: thoughtful site selection, proper weed identification, detailed record keeping, and by employing multiple strategies to control weeds.

One tactic is the use of dormant-season, soil-applied, pre-emergence (PRE) herbicide applications to provide residual weed control. A benefit of fall- or early spring-applied herbicide treatments is that growers can take advantage of naturally occurring rain events to incorporate/activate these products. Additionally, cooler soil temperatures may help to reduce chemical loss through dissipation and degradation, thereby preventing the potential for diminished herbicide efficacy. Weed suppression resulting from PRE herbicide applications may also reduce the need for foliar-applied, post-emergence (POST) treatments later in the season.

Several residual herbicides, representing multiple herbicide Sites of Action (SOA), are available for use in tree fruit systems. Specific active ingredients include: rimsulfuron (Weed Science Society of America (WSSA) group 2; acetolactate synthesis inhibitors); oryzalin, pendimethalin, and pronamide (WSSA 3; mitosis inhibitors); simazine (WSSA 5) and diuron (WSSA 7; photosystem II inhibitors); norflurazon (WSSA 12; carotenoid biosynthesis inhibitor); oxyfluorfen (WSSA 14; protoporphyrinogen oxidase inhibitors); dichlobenil (WSSA 20), isoxaben (WSSA 21 (non-bearing, only)), and indaziflam (WSSA 29; cellulose inhibitors); and penoxsulam (WSSA 2) plus oxyfluorfen (WSSA 14) (Table 1). A detailed description of how different herbicides specifically disrupt plant growth and development can be found in the Spring 2017 edition of the Viticulture and Enology Extension news.

Table 1. Residual herbicide SOAs available for use in tree fruit as detailed in the 2017 Pest Management Guide for Tree Fruit in Washington and the 2017 PNW Crop Protection Guide. Herbicide numerical classification is according to the Weed Science Society of America (WSSA).
Site of action Mode of action
2 Acetolactate synthase (ALS) inhibitors Amino acid biosynthesis
3 Microtubule inhibitors Microtubule polymerization
5 Photosystem II (PSII) inhibitors (D1 protein) Photosynthesis
7 Photosystem II (PSII) inhibitors (D1 protein) Photosynthesis
12 Phytoene desaturase (carotenoid biosynthesis) inhibitors Photosynthesis
14 Protoporphyrinogen oxidase (PPO) inhibitors Photosynthesis
20 Cellulose inhibitors Cell wall synthesis
21 Cellulose inhibitors Cell wall synthesis
29 Cellulose inhibitors Cell wall synthesis
Disclaimer: No endorsement is intended for products mentioned, nor is lack of endorsement meant for products not mentioned. The author and Washington State University assume no liability resulting from the use of pesticide applications detailed in this report. Application of a pesticide to a crop or site that is not on the label is a violation of pesticide law and may subject the applicator to civil penalties up to $7,500. In addition, such an application may also result in illegal residues that could subject the crop to seizure or embargo action by WSDA and/or the U.S. Food and Drug Administration. It is your responsibility to check the label before using the product to ensure lawful use and obtain all necessary permits in advance.

The utility/effectiveness of any residual herbicide will be impacted by multiple (and interacting) chemical, physical, and biological factors. To optimize product performance, growers should think about the following considerations before applying any soil-based treatments:

What crop is the herbicide being applied to?

Sensitivity to herbicide active ingredients can differ dramatically among crops both within and among fruit classes; as a consequence, not all herbicides are registered for use in all crops. For example, dichlobenil (Casoron) is labeled for use in cherries but not other stone fruits. Simazine is only labeled for use in apple, pear and tart cherry, and diuron is only labeled for use in apple, pear and peach. Always check the most recent herbicide label to ensure that all applications are safe and legal.

What weeds are present?

Reduced herbicide efficacy and crop injury from post emergence only application. Photo: Washington State University

Not all herbicides are equally effective against all weed species. For example, the dinitroanilines are useful for controlling annual grasses and some small-seeded broadleaf weed species (such as pigweeds), but are ineffective against broadleaf species with larger seeds (such as puncturevine/goatheads). Given that PRE herbicides may have very different spectrums of activity, knowledge about the makeup of the existing weed community is a necessity prior to application. If a variety of species are likely to occur, tank-mixtures may be necessary to ensure that the entire weed spectrum is effectively targeted. Refer to the current tree fruit management guide and herbicide labels for more information. Remember, although the seedlings of perennial weed species may be controlled by PRE herbicides, mature plants are unlikely to be impacted.

When will these products be incorporated?

PRE herbicides must be incorporated (usually 1-2” deep) into the soil profile; incorporation is required because these herbicides are only active against newly germinated weed seedlings. Incorporation is also necessary to minimize or prevent volatilization and photodegradation, which can reduce herbicide performance. The length of time a herbicide can remain on the soil surface varies dramatically among products. For example, trifluralin (WSSA Group 3 herbicide) must be physically incorporated within 24 hours whereas pendimethalin (also a WSSA Group 3 herbicide) may remain stable for several days to weeks. Even if a herbicide does not require immediate activation to prevent product loss, an unincorporated herbicide is unable to control emerging weeds. How herbicides are activated can also affect product performance.

What are the soil conditions like? How mobile is the herbicide?

Soil-applied herbicides are often referred to as ‘residual’ herbicides because they persist in the soil following application to provide extended control of germinating weed seedlings. The length of time that herbicides may reside in a treated area will be influenced by soil texture and organic matter content. Soils that are high in clay and/or organic matter allow for more of the herbicide to be bound up in the soil matrix, requiring higher application rates. Conversely, coarse soils are less adsorptive than fine soils, which enhances leaching potential, thereby necessitating lower herbicide rates to prevent crop injury and environmental contamination. Herbicide retention is also a function of herbicide chemistry. Some herbicides bind very tightly to soil particles (e.g. trifluralin) whereas others are more mobile (e.g. simazine) and prone to leaching.

How much litter is on the soil surface?

Standing vegetation (such as winter weeds) and crop residue (such as leaf litter) on the soil surface can interfere with the deposition and incorporation of PRE herbicides. To improve soil-herbicide contact, existing weeds should be controlled using POST herbicide treatments or some form of physical weed removal. Smaller weeds are easier to control than larger ones; younger plants have less surface area that requires treatment and are generally more succulent than older plants. Always follow herbicide label recommendations regarding rates, spray volumes, and adjuvant partners to maximize weed control efficacy. A blower can be employed to clear trash off berms prior to herbicide applications.

How old are the trees?

Tree age can affect what herbicides are available for use in a production system. While competition from weeds is most severe in newly planted orchards, not all PRE herbicides are appropriate for use around young trees. Growers should be aware of re-plants in mature orchards before making soil-applied treatments. Regardless of tree age, residual herbicides should only be applied to soil that is settled and free of cracks to minimize the potential for crop injury. Avoid using herbicide-treated soil to backfill planting holes.

Dormant-season, soil-applied herbicides can be an effective tool for managing weeds in tree fruit. However, orchard managers must be aware of the fact that multiple and interacting factors can affect their utility and efficacy. Always read the most recent versions of herbicide labels thoroughly prior to application to ensure that the right product is being applied at the right time for the control of the weeds that are present. When factoring in costs, remember that PRE herbicide programs may be economical if the number of in-season weed control activities can be reduced or if weeds with resistance to commonly-applied POST herbicides are present. With respect to herbicide resistance, the use of multiple weed management strategies, including soil-applied products, can help to prevent the development and spread of resistant biotypes. Diversifying herbicides is, however, only one component of a weed/herbicide resistance management program. When/if possible: cultivate, hand-weed, mulch, or inter-crop; prevent weeds from going to seed; and prevent weed seed from being dispersed on farm equipment. Evaluate weed populations both BEFORE and AFTER weed control strategies are employed; this will allow you to detect potentially resistant populations early and manage them most effectively.

Lynn Sosnoskie, Washington State University weed scientist

Source: Washington State University