Weed Control

Weeds can directly compete with trees for water, nutrients, and light. Weeds may also impact vine growth and fruit yield, indirectly, by serving as alternate hosts for insect pests and pathogens; providing habitat for rodents; altering the distribution of irrigation water; interfering with the deposition of pesticides; and impeding the movement of workers and equipment. Successful weed management can be achieved by employing a combination of strategies, which includes the use of herbicides to eliminate unwanted vegetation from vineyards. Herbicides, however, can act upon different weed species and in different ways; care must be taken to ensure that the selected products will be effective at controlling the weed species that are present in the system.

Herbicides are also capable of causing serious injury or death of trees and/or may have significant unintended consequences on neighboring ecosystems. Crop safety is paramount, however the safety of sensitive species and habitats outside of the treated area are also a significant concern. Growers must follow label instructions to reduce the potential of spray or volatility drift and damage to off-target organisms. Surface and groundwater advisories and buffer zone recommendations are also included on several herbicide labels; read and follow all labeled guidelines to prevent contamination of aquatic systems.

Herbicide Selection and Performance

The most appropriate choice of herbicide (or combination of herbicides) will be affected by numerous factors including:

• how weed control will be achieved,

• the kind of weeds to be controlled,

• the size and/or age of the weeds to be controlled,

• soil type and herbicide incorporation strategy,

• the quantity and quality of the spray water,

• the age and health of the trees. 

How the control will be achieved.

Herbicides differ with respect to how they are used. When describing herbicides, we often refer to them as being either ‘pre-emergence’ or ‘post-emergence’ applied. Pre-emergence herbicides are soil-applied products that act primarily on germinating weed seeds or young weed seedlings. They persist in the environment to provide extended weed control within or across seasons. Products classified as post-emergence herbicides must be applied directly to the foliage of emerged weeds to elicit control. Post-emergence herbicides can be further characterized as being either ‘contact’ or ‘systemic’ products. Contact generally refers to herbicides that only affect the tissues that are directly treated with the herbicide – these types of herbicides do not move (translocate) to untreated parts of the plant following application. Systemic or translocated herbicides can move from treated plant parts to untreated tissues via the xylem or phloem.  Although some herbicides may exhibit both pre- and post-emergence activity, a combination of both foliar and soil-applied products are typically used to successfully manage weed communities in orchards.

Not all herbicides are equally effective against all weed species. Often, herbicides are referred to as being either ‘selective’ or ‘non-selective’ (which is synonymous with 'broad-spectrum’). A selective herbicide is one that is effective at controlling some species but not others. Conversely, a non-selective herbicide is an herbicide that can control many different types of plant pests. The most well-known examples of herbicide selectivity are 1) the abilities of the ‘-fops’ and the ‘-dims’ (i.e. fluazifop and clethodim) to suppress grasses but not broadleaf species and 2) the control of broadleaved weeds, but not grasses, by synthetic auxins (i.e. 2,4-D).

The kind of weed to be controlled.

Weed species can also be characterized by the length of their life cycle. Annual weeds (both winter-and summer-germinating) emerge, grow, flower, and set seed all within the course of a year. Biennial weeds complete their life cycles over the course of two years whereas perennials can persist across multiple seasons. Not all herbicides are equally effective against all three types of weed species; for example, although the seedlings of perennial weeds may be controlled by pre-emergence herbicides, much like annual species, mature plants are unlikely to be impacted.

It is possible for weed communities in orchards to be comprised of species that are naturally sensitive to different herbicides or herbicide modes of action. They may also differ with respect to life history traits or emergence patterns. Herbicide labels list the weed species that can be suppressed or controlled by the active ingredient. The labels will also provide instructions on when to time applications to maximize herbicide efficacy and will also list appropriate tank-mix partners to help growers expand the spectrum of weed control. Always read labels to ensure that herbicide applications will be both effective and safe.

 The size and/or age of the weed to be controlled.

Weed control strategies may not always be 100% effective and escapes can occur for numerous reasons. One of those reasons is the size of plants at the time of application. The efficacy of post-emergence herbicides is often diminished when products are applied to large/mature plants. This can result from poor spray coverage and the ability of dense foliage to shield sensitive tissue from herbicide deposition. While plant size is mainly a concern with non-systemic, contact herbicides, the efficacy of translocated products can also be influenced. Perennial species are can be tolerant of many herbicides, including translocated products like glyphosate, because their root systems and nutrient reserves support regeneration/regrowth. There are several strategies that growers can take to maximize weed control with post-emergence herbicides, including timing applications to treat weeds while they are small/tender and applying herbicides at the labeled rates and volumes.

The development of herbicide resistance is a significant concern for growers of perennial crops, including tree fruit. Weed species with resistance to glyphosate, glufosinate, and paraquat have been confirmed in California and Oregon as well as other Western states. Incomplete weed control can increase the chance of a herbicide resistant biotype reaching reproductive maturity, setting seed, and becoming established in a production system. Herbicide labels will often provide instructions to applicators regarding strategies for resistance management. Additional information can be found at the Weed Science Society of America’s (WSSA) web-page: http://wssa.net/wssa/weed/resistance/. 

 Soil type and herbicide incorporation strategy.

The length of time that pre-emergence herbicides may reside in a treated area will be influenced by multiple factors, including soil texture and organic matter content. Soils that are high in clay or organic matter can bind herbicides tightly to the soil matrix. Conversely, coarse soils can enhance leaching potential. Herbicide persistence is also a function of herbicide chemistry. Some herbicides bind very tightly to soil particles whereas others are significantly more mobile. The interactions between soil and herbicide chemistry can affect a product’s use rate within a given system. This, in turn, may influence how well a herbicide performs, as well as how long it remains in the treated zone.

To be effective, pre-emergence herbicides must be incorporated (usually 1-2” deep) into the soil profile. Incorporation is required because these products are mostly active against newly germinated weed seedlings. Additionally, incorporation is needed to reduce or prevent volatilization and photodegradation, which can result in reduced herbicide performance. The length of time an herbicide can remain on the soil surface varies dramatically among products. Even if an herbicide does not require immediate activation to prevent product loss, an unincorporated herbicide is unable to control emerging weeds. While many growers will time herbicide applications to take advantage of naturally occurring rainfall, uncooperative weather may require the use of irrigation for activation. Some products, due to their mobility and potential for crop injury, may be incompatible with sprinkler irrigation; this is of particular concern when tree fruits are grown on shallow, coarse, sandy, or gravelly soils.

Check pre-emergence herbicide labels, carefully, to ensure that the product is being applied at the proper time of year to effectively target the weed species of concern in an orchard. Apply and incorporate products as described to prevent crop injury; this includes following timing recommendations to avoid treating orchards when significant crop damage could occur.

 The quantity and quality of the spray water.

Water is the main carrier for crop protection products, including herbicides. Consequently, the quantity and quality of spray water can impact herbicide distribution and performance. In general, higher carrier volumes have been regularly shown to improve the efficacy of foliar-applied herbicides (glyphosate is a notable exception). Post-emergence herbicide labels will provide instructions regarding the recommended parameters (i.e. GPA, pressure, droplet size) to maximize weed control potential. Carrier volume can also affect the delivery of soil-applied products; always read the herbicide label to ensure that pre-emergence herbicides are used both effectively and safely.

Water quality factors can also affect herbicide performance and safety. For example, herbicides that are weak acids (such as glyphosate) can see their efficacy reduced under alkaline conditions (pH >7). Salts and soil particles in spray water can bind to herbicides, thereby affecting their dispersal, deposition, and uptake. Water quality recommendations exist for both pre- and post-emergence herbicides; read labels carefully.

The age and health of the trees.

Tree age can affect what herbicides are available for use in a production system. While competition from weeds is most severe in in newly planted orchards, not all herbicides are labeled 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 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. Unless specifically noted on the label, avoid herbicide spray or mist contact with leaves, green bark, roots, or fruit to reduce the potential for crop injury.

General precautions

1. Do not apply any herbicide to orchards unless there is a label registration for its use on tree fruits.

2. Check all herbicide or other pesticide uses with the processor or field representative before using.

3. Do not use a combination of herbicides or other chemicals with herbicides unless the combination has been thoroughly tested and confirmed to not cause phytotoxic effects.

4. Avoid herbicide applications to any part of the tree. Do not use weed sprayers to apply other pesticides to trees.

5. Do not use the same mode of action (MOA), repeatedly, throughout the season. Using different MOAs is part of a good herbicide resistant management program.