The active ingredients listed below are designated as restricted-use by the U.S. Environmental Protection Agency (EPA); buying them requires a certified pesticide applicator’s license.

Herbicides are listed by the active ingredient. Some or all uses of these active ingredients are considered restricted-use. Before purchasing any pesticide, consult the label for usage category.

acetochlor
acrolein
alachlor
atrazine
methyl bromide
paraquat
picloram
pronamide

Additional Restricted-use Herbicides in PNW States

Some or all uses of the following herbicide active ingredients are classified as restricted-use in one or more of the three PNW states in addition to the federal restricted-use pesticides listed above. Before purchasing any pesticide, consult the label for usage category and site.

• All aquatic uses of herbicides in Washington
• All sulfonylurea herbicides in Washington
• 2,4-D
• 2,4-DB
• benefin
• bromacil
• bromoxynil
• carfentrazone
• chlorsulfuron
• clopyralid
• copper sulfate
• DCPA
• dicamba
• dichlobenil
• dichlorpropene
• diflufenzopyr
• diquat
• diuron
• ethametsulfuron
• fenoxaprop
• fluazifop
• flufenacet
• fluroxypyr
• glyphosate
• hexazinone
• iodosulfuron
• imazamox
• imazapyr
• MCPA
• MCPB
• MCPP
• mesosulfuron
• metam-sodium
• metribuzin
• metsulfuron
• MSMA
• nicosulfuron
• primisulfuron
• prometon
• prosulfuron
• rimsulfuron
• simazine
• S-metolachlor
• sodium chlorate
• sodium metaborate
• sulfometuron-methyl
• sulfosulfuron
• tebuthiuron
• thifensulfuron
• triasulfuron
• tribenuron
• triclopyr
• trifluralin
• triflusulfuron

* With permission from H. J. Hopen, Horticulture Facts, VC-15-81, Cooperative Extension Service, University of Illinois, Urbana-Champaign, IL 61801.

Residues of triazine herbicides (such as atrazine and simazine) or of substituted urea herbicides (such as linuron and diuron) may persist in the soil for long periods. Attempts have been made to correlate residue persistence with rainfall, temperature, soil characteristics, cultivation practices, method and time of herbicide application, and rate of application. Thus far, however, predicting the extent of residue damage to sensitive crops the year following residual herbicide applications has been only partially successful.

This method is not adequate for, and does not pertain to, testing for sulfonylurea herbicides such as those in Glean, Finesse, and Ally. Consult manufacturer’s label for recommended bioassay method.

Testing for Herbicide Residues

Chemical analyses for herbicide residues are slow and quite complicated. Such tests can be done in only a few specialized laboratories and usually are expensive. Biological assays are more feasible, since they can be done with simple equipment found in most homes or offices. Although the biological assay outlined here does not provide an exact measure of the amount of residue in the soil, the assay will indicate whether the residue is enough to harm sensitive crops. The bioassay for atrazine (using oats as a test species) was published in the December 1967 issue of Crops and Soils. That method can also be adapted to test for other residues.

1. Secure a representative soil sample from the field you suspect has atrazine residue. Sample from several locations, as when collecting soil samples to determine fertilizer requirements. Atrazine residue may be in patches of a field. Sample enough areas to avoid missing ones with a high residue. Headlands and knolls often show the greatest residue injury. Take separate samples from areas where residues may be excessive. Always sample to the full depth of the plow slice, whether or not the field is plowed. Remember that the assay is only as reliable or representative as the samples. Each sample to be assayed requires about 10 lb of soil.
2. Assay samples within a week or two after they are collected from the field. If the samples cannot be assayed immediately, store the soil in a cold place—in a freezer if possible. When samples are stored indoors under warm conditions, the atrazine residue may be lost.
3. If the soil is wet, spread it out and allow it to dry so it can be worked readily. If the soil is cloddy, crush clods to the size of peas or wheat seed, but do not pulverize the soil.
4. Adding about 50% by volume of coarse sand will improve the physical condition of silt and clay soils. If sand is added, mix with the soil properly.
5. Add about 0.5 g of activated carbon to half (5 lb) of the soil or of soil–sand mixture. Mix carbon and soil thoroughly. Carbon deactivates atrazine or other residue. For purposes of comparison, soil treated this way provides the equivalent of soil without residue.
6. Partially fill two containers with soil that does not contain carbon and two others with the soil–carbon mixture. These should be containers holding about a pint to a quart. Punch holes in the bottom of the containers to allow drainage. Tin cans, paper milk cartons, or ice cream cartons are satisfactory for this purpose.
7. Plant five to eight oat seeds (or seeds of vegetable species of interest) in each container; cover seeds with about 0.5 inch of soil. Wet the soil with water but do not saturate. After emergence, thin to three plants to ensure maximum uptake or absorption of possible residue.
8. Place containers where they will be warm (about 70 to 75°F) and get as much sunlight as possible. Sunlight usually is essential for the development of atrazine injury symptoms. Artificial light is much less intense than sunlight and may not be satisfactory for symptom development.
9. Injury symptoms on seedlings should appear about 3 weeks after planting. If temperatures are below 70°F, more time is required. Water plants sparingly. Do not allow soil to dry out.
10. Severe triazine injury is characterized by drooping leaves and by leaf-kill that extends from the leaf tip toward the base. Leaf-kill indicates a significant amount of residue in the soil. A marginal residue content will stunt the oats’ growth without killing the leaves. Stunting can be determined by comparing the growth of oats in soil with carbon. Oats grown in soil with carbon should be normal and should show no atrazine injury or stunting, unless extremely high residues of atrazine are in the soil sample.
11. If the oats show any evidence of leaf-kill or stunting, plant an atrazine-tolerant crop in the field from which the samples were obtained.

Using Activated Charcoal

Activated charcoal (or carbon) can reduce herbicide contamination in specific areas (gardens, greenhouses, lawns, etc.) and can also be used as a root dip to protect transplants (tomatoes, peppers, strawberries, ornamentals, etc.) from triazine or substituted urea herbicides. Activated carbon can also be used to “clean up” pesticide spills.

Other herbicides that carbon can deactivate include trifluralin (Treflan), bromacil (Hyvar X), benefin (Balan), bensulide (Betasan and Prefar), DCPA (Dacthal), dichlobenil (Casoron or Norosac), EPTC (Eptam), 2,4-D, terbacil (Sinbar), and chloroacetamide herbicides such as metolachlor or dimethenamid.

Activated carbon, used in a wide range of applications in diverse industries, is made by heating or chemically treating organic matter to create a porous structure. This gives a large surface area within a relatively small volume. Most activated carbon products are purified by acid and water washes to remove impurities and are available in both granular and powdered form. Charcoal for outdoor grills and the like cannot be ground up to achieve the same pore structure characteristics of activated charcoal on a pound-for-pound basis.

The usefulness of activated carbon is based primarily on its ability to hold molecules within its vast pore structure. The phenomenon of adsorption can take place either in gaseous or liquid phase systems. Adsorption is often selective when applied to systems containing more than one component, for example when using activated carbon in gas masks to remove poisonous vapors, and as an antidote for ingested poisons.

Where to Obtain Activated Charcoal

Some garden supply centers carry packaged activated carbon that is designed for the uses outlined here. Activated carbon is used extensively in dry cleaning and water-purification. Usually, sources of activated charcoal can be quickly found by searching the Internet (e.g., GROW-SAFE http://buyactivatedcharcoal.com/activated_charcoal_products/soil_detox ), looking in the Yellow Pages under “Cleaners and Driers’ Supplies,” or by contacting a dry cleaning establishment. Some dry-cleaning carbons may contain additives that will make them unsuitable. Activated carbon is offered in containers of 1 to 50 lb. Small quantities of purified activated carbon are available at pharmacies and at chemical supply houses.

Charcoal Application

(Modified for Oregon conditions)

Applying activated charcoal should be considered as an emergency treatment for herbicide residues from previous crops, spills, or changes in crop rotations. The efficiency of deactivation depends on the soil’s organic matter content and physical condition, the herbicide’s activity, and the crop’s sensitivity. For differences between herbicides, see G. F. Warren, 1973, “Use of Activated Carbon to Inactivate Herbicide Residues,” North Central Weed Control Conference, 28:68–69.

If an area is contaminated with undesirable herbicide residues and a susceptible crop is to be planted, apply activated carbon at 200 lb/A (0.5 lb/100 sq ft) for each pound per acre of actual residue. The carbon can be mixed, at the rate of 1 lb carbon to 10 lb sand, and applied with fertilizer-spreading equipment or sprayed using large-capacity nozzles (0.5 gal/minute or larger). Carbon wets and suspends poorly. Local applicators are available in some areas such as the Willamette Valley of Oregon. Otherwise, a grower can add the carbon to a partially filled spray tank and use the remaining water to help mix the floating carbon while the agitator is operating.

Plan carefully. Using activated carbon is a poor substitute for a well-planned weed control program.

For use in grass seeding, see Section E. Grass Seed Crops in this handbook.

* Adapted from Herbicide-resistant Weeds and Their Management, PNW 437, a Pacific Northwest Extension publication (University of Idaho, revised 2011). Authors are Joan Campbell, University of Idaho; Carol Mallory-Smith and Andy Hulting, Oregon State University; Donn Thill, University of Idaho. Online at http://www.cals.uidaho.edu/edComm/pdf/pnw/pnw0437.pdf

Herbicide resistance is the inherited ability of a plant to survive a herbicide application to which the wild-type was susceptible. Resistant plants occur naturally within a population and differ slightly in genetic makeup, but remain reproductively compatible with the wild-type.

Herbicide-resistant plants are in a population in extremely small numbers. Repeatedly using one herbicide allows these few plants to survive and reproduce. The number of resistant plants then increases in the population until the herbicide no longer effectively controls the weed.

Resistant plants likely will persist in infested fields for many years, even in the absence of any additional selection with the herbicide. There is no evidence that herbicides cause the genetic mutations that lead to herbicide resistance.

Resistant plants may be resistant to other herbicides (imidazolinones as well as sulfonylureas, for example) that kill plants in the same way (same site of action or same group). This is called cross-resistance.

Weeds also can be resistant to herbicides with different sites of action (aryloxyphenoxy propanoates as well as sulfonylureas, for example). In Australia, a biotype of annual ryegrass is resistant to at least five different herbicide groups. This is called multiple resistance.

Herbicide resistance is not the natural tolerance that some species have to a herbicide. For example, wheat is tolerant to Hoelon because it can rapidly deactivate it. Wild oat can only slowly deactivate Hoelon, so the herbicide can be used selectively to remove wild oat from wheat.

The first identified herbicide-resistant weed—spreading dayflower (Commelina diffusa), resistant to 2,4-D—was identified in 1957, in a sugarcane field in Hawaii. Since then, more than 200 weeds resistant to one or more herbicides have been identified worldwide. Current information on the status of herbicide-resistant weeds can be found at http://WeedScience.org/in.asp

Herbicide-resistant weeds are now common in the Pacific Northwest:

• Kochia, prickly lettuce, and Russian thistle resistant to sulfonylurea herbicides (Glean, Amber, Ally, and other Group 2 inhibitors)
• Wild oat and Italian ryegrass resistant to Hoelon and other Group 1 (ACCase) inhibitors
• Powell amaranth resistant to triazines and other Group 5 inhibitors
• Yellow starthistle resistant to Tordon and other Group 4 inhibitors

• Wild oat resistant to Fargo and Avenge

The appearance of herbicide-resistant weeds is strongly linked to repeated use of the same herbicide, or herbicides with the same site of action, in a monoculture cropping system (for example, wheat after wheat) or in non-crop areas (railway or road rights-of-way, for example). To manage herbicides to delay and prevent the appearance of herbicide-resistant weeds, you must know in which chemical family a herbicide belongs and which herbicides have the same site of action.

The table below lists herbicides by group number and site of action, chemical family, common name, and trade name, and it gives examples of resistant weeds. Herbicide families that have the same site of action are the same group number.

A herbicide program to prevent resistance does not use herbicides from the same group more than once in three years.

Tank-mixing herbicides is not an effective resistance management strategy. If herbicides in the tank mixture control different weed species and have different soil residual characteristics, resistant weed biotypes will continue to be selected. For example if a long-residual (Glean) and a short-residual (2,4-D) herbicide are tank-mixed, both herbicides may control emerged broadleaf weeds. However, Glean will continue to control weeds throughout the growing season and could continue to select for resistant plants. Tank-mix only when a herbicide combination is required to control the weed spectrum or will reduce herbicide use rates. Tank-mixing for other reasons is not economically or ecologically sound.

Management can prevent or delay the appearance of herbicide-resistant weeds. The following practices can be used with the information on herbicide families provided in the table to form a herbicide resistance management strategy.

Preventing herbicide-resistant weeds

Herbicide rotation Avoid year-after-year use of herbicides that have the same site of action. At one time this meant not using herbicides from the same chemical family, but this is no longer the case. For example, two chemically different groups of herbicides, the sulfonylureas and imidazolinones, have the same site of action. For another example, Hoelon and Poast belong to different chemical families but kill susceptible grasses in the same way.

Short-residual herbicides Using herbicides that do not persist in soil for long periods and are not applied repeatedly within a growing season reduces the selection of herbicide-resistant weeds. However, repeated applications within a growing season of a herbicide with no soil activity (e.g., Gramoxone) has resulted in weeds resistant to the herbicide.

Crop rotation Because different crops may require different herbicides, rotating crops can increase herbicide rotation. But with the large number of sulfonylurea and imidazolinone herbicides available for use in many different crops, crop rotation alone may not be enough to avoid weed resistance to herbicides. This also is true for other herbicides with the same site of action.

Cultivation In row crops, cultivation can be an effective tool for eliminating weed escapes that may represent the resistant population. Fallow tillage controls herbicide-resistant and herbicide-susceptible weeds equally as long as seedlings of the two biotypes emerge at the same time. Do not use the same site-of-action herbicide in fallow as was used to control weeds in the crop.

Accurate record keeping To have an effective herbicide rotation or tank-mix system to prevent resistance, you must know which herbicides have been used in the past, at what rate, and how often.

Clean seed Plant certified seed to prevent introducing herbicide-resistant weed seeds.

Integrated weed management This concept is important for all weed control, not just management of herbicide-resistant weeds. Integrated weed management uses all the tools available to control weeds, including cultural, mechanical, and chemical methods. An integrated approach to weed management, whether it is in crop or non-crop land, is an important environmental and economic consideration.

Dealing with herbicide-resistant weeds

Monitor fields for weed escapes Weed escapes are not necessarily resistant, but they may be. A resistance problem may not be visible until 30% or more of the weeds are no longer controlled. See whether escapes are only one species or a mixture. If they are a mixture, the problem is more likely related to environment or application. If they are only one species, the problem is more apt to be resistance, especially if the herbicide controlled the species in the past, and if the same herbicide has been used repeatedly in the field.

Keep weeds from spreading Prevent known resistant weeds from flowering and producing seed. After using machinery in fields or areas with known or suspected infestations of herbicide-resistant weeds, thoroughly clean the equipment to reduce the spread of resistant weeds from one field or area to another. Always plant clean seed.

Change crops and tillage systems Crop rotation and altered tillage practices can affect the weed populations. Alternating spring and winter crops means that the field will be tilled at different times each year. During one of the field preparation operations, resistant as well as susceptible weeds will be killed.

Change herbicide program If weed resistance occurs, herbicides with other sites of action and other weed management practices must be used.

Recognizing herbicide-resistant weeds

Irregular patches of a single weed species in the field are an indicator of herbicide resistance, especially when:

1. No other application problems are apparent.
2. Other weed species are controlled adequately.
3. There are no, or minimal, herbicide symptoms on the single weed species not controlled.
4. There has been a previous failure to control the same species in the same field with the same herbicide, or a herbicide with the same site of action.
5. Records show repeated use of one herbicide or of herbicides with the same site of action.

What to do if you suspect herbicide resistance

• Do not re-spray the field with the same herbicide.
• Report your suspicion to university research or Extension personnel, or to the Extension educator in your county.
• Collect plants or seed that can be used to confirm resistance has developed.

Managing herbicide-resistant crops

Herbicide-resistant crops are a recently developed tool for the control of weeds. These crops are resistant to herbicides that are lethal to nonresistant varieties of the same crop species.

Crops resistant to specific herbicides have been developed through genetic engineering and through traditional selective breeding. Examples include Clearfield wheat, which was selected for resistance to imazamox, and Roundup Ready canola, which was genetically engineered to be resistant to glyphosate. Used properly, herbicide-resistant crops can be valuable tools to manage difficult weeds, but they also have two inherent risks that need to be considered before planting: the emergence in subsequent growing seasons of herbicide-resistant volunteers, and the potential for herbicide-resistant crops to cross with weedy relatives.

Volunteer herbicide-resistant crops as weeds Consider whether the herbicide-resistant crop typically is a volunteer crop in years after its cultivation and, if so, whether herbicide options are available in the crop rotation to remove herbicide-resistant volunteers. For example, glyphosate is commonly used to control volunteer crops before planting a rotational crop. Glyphosate will not effectively control Roundup Ready crops, however; some other herbicide or non-chemical control measure is required to control the glyphosate-resistant volunteers. Evaluate the impact of using these other herbicides or non-chemical control measures for your operation. Impacts could be increased cost, or increased soil erosion or moisture loss due to increased tillage.

Volunteer crops are considered a problem largely within 1 year of harvest. However, certain species have extended dormancy, which could result in multiple years of a herbicide-resistant volunteer crop problem even without seed production by the volunteer crop.

Gene flow from herbicide-resistant crops to weedy relatives Rarely, the trait that confers herbicide resistance in the crop can move into weedy relatives through cross-pollination, resulting in a herbicide-resistant weed. Consider nearby weedy and native relatives of the herbicide-resistant crop as well as their propensity to cross-pollinate. Self-pollinating crops, such as soybean, are considered low-risk for gene flow to weeds or other crops. But a crop such as Roundup Ready, Clearfield, or Liberty Link canola could pollinate nearby herbicide-susceptible canola as well as weedy relatives of canola, resulting in volunteer canola plants or weeds that may be resistant to several herbicide families.

Crops that may pose problems Herbicide-resistant crops at risk for gene flow or volunteer-management problems would include some or all of the following traits:

• The crop cross-pollinates with nearby relatives that are problem weeds, or with other crops.
• Crop seed shatters or vegetative propagules are left in the ground after harvest, resulting in volunteer crops in subsequent years (for example, canola or potato).
• Herbicides for managing volunteer crops are limited to ones in the same family to which the crop is resistant.
• Crop seed is viable in soil for several cropping seasons.
• Using the herbicide-resistant crop increases your reliance on herbicide families that would be applied multiple times per season or several times during a cropping system.

Herbicide Rotation

To avoid selecting for herbicide-resistant weeds, do not use herbicides from the same group more than once in three years. Rather, rotate to a different group every year of the production system.

The proper procedure for cleaning a spraytank depends on several factors, including the composition of the spraytank, pesticides used, and sensitivity of the crop that pesticides will be applied to following a tank cleaning. In some cases, triple rinsing with water will be sufficient, for example if glyphosate is used. Typically, a detergent should be added to the water. Removal of many herbicides from spray equipment requires the use of ammonia or approved tank cleaners. Specific directions are included on herbicide labels and should be consulted and used.

Some pesticides are more difficult to remove from spraytanks than others. These pesticides often have very low use rates (e.g., Aim and Sandea), or may stick to residues of other chemicals that remain in the sprayer. In some cases, additives such as crop oils or nitrogen solutions may allow release of previously used herbicides.

There are numerous recommendations about cleaners for specific herbicides. Cleaners usually fit into the categories of detergents, ammonia, chlorine bleach or commercial tank cleaners.

Ammonia increases the pH of the solution, which increases the solubility of some herbicides and the potential to remove them from the spraytank. Ammonia is commonly used to clean tanks.

Commercial tank cleaners generally raise the pH of the solution and act as detergents.

Chlorine bleach lowers the pH of the solution, which speeds degradation of some herbicides. Chlorine bleach is not usually recommended as a cleaning agent.

Never mix ammonia with chlorine bleach; this mixture creates dangerous vapors.

A Standard Triple-Rinse Procedure for Cleaning Spraytanks

1st rinse: Drain remaining pesticide from the spraytank, hose down the interior surfaces of the tank. Then flush tank, hoses, boom and nozzles with clean water for 10 minutes.

2nd rinse: Fill the tank with water, add detergent or other recommended cleaner, and recirculate for 15 minutes. Spray some of the rinsate though the boom and nozzles, then drain the tank.

Labels for very low-rate herbicides, such as Aim, or growth regulators, such as Landmaster and 2,4-D, often recommend that the cleaning solution be allowed to stand for a few hours in the sprayer, sometimes as long as overnight.

Remove the nozzles and screens, and clean them separately.

3rd rinse: Drain the cleaning solution from the tank, rinse with clean water, then spray rinsate though boom. Repeat steps 2 and 3 for difficult to remove herbicides.

Validating tank cleaning methods

A bioassay can sometimes be used to test whether a spraytank has been thoroughly cleaned. The simplest method is to collect rinsate from the final rinse, then, using a sprayer or spray bottle, apply it to plants known to be extremely sensitive to the herbicide in question, then compare the effects on untreated versus treated plants. For instance, rinsate from a tank holding 2,4-D could be applied to tomato plants. A second option would be fill the cleaned tank with water and spray the water on a small area of the crop that will be treated.

The disadvantage of these bioassays is that symptoms will often take a few days to develop; in the case of sulfonylurea herbicides, it may take two weeks. Another drawback is that water may not remove remaining residues in the spraytank in the same manner as herbicides.