Herbicide application in Washington – An Overview

Some common herbicides used to control weeds. Background photo courtesy of James Atland at Oregon State University: http://oregonstate.edu/dept/nursery-weeds/feature_articles/top_10/Top_10_reasons.html. Images of herbicide containers obtained from various commercial sources. Edited by D. Duncan.

Introduction:

Many of us are well aware of the dangers of using chemicals to combat undesirable or dangerous vegetation in our gardens, farms, restoration sites and nurseries, as well as the controversy surrounding their use. There has been a heightened, widespread realization as early as the New Yorker’s 1962 serialization of Rachel Carson’s Silent Spring, that pesticides are not only hazardous to ecosystem fidelity, but also to our own well-being and sustenance. With the advent of chemical control came necessary regulations, which require some degree of education to properly follow. Should you conclude that there is no other way to control your weed infestation by tolerance or manual/mechanical removal, it is always better to be well informed than to make grave mistakes by which you may not only suffer the ecological consequences, but also legal ones.

Locally, the Washington State Department of Agriculture regulates pesticide application, ensuring that these chemicals are used safely and legally under the Washington Pesticide Control Act (15.58 RCW), the Washington Pesticide Application Act (17.21 RCW), the General Pesticide Rules (WAC 16-228), the Worker Protection Standard (WAC 16-233) and pesticide and/or county specific regulations.

Prior to applying any restricted use herbicide where you work or on a commercial scale, it is necessary to obtain the appropriate pesticide education and license. You can get various types of licenses from the WSDA depending on where you will be applying, what kind of organization you are working for, and if you are self-employed.

Before application:

The Nature Conservancy has an online guidebook explaining precautions (chapter 5) you must take prior to and during application of herbicide. First, determine your work plan for the target weed (i.e. assess the tradeoffs of applying herbicides; develop a rotation schedule to minimize the occurrence of resistant weeds). Evaluate the site conditions, because they will dictate what chemicals you will use and even what license you need to perform this operation. Be attentive to the herbicide properties, or “Mode of Action”, and choose the herbicide appropriate to the species and conditions (i.e. you may want to minimize drift, or affect only particular plants and not others). The overriding principle, however, is to absolutely minimize the amount of net negative impact you have by applying the herbicide. If you are likely cause more short and long-term damage than the invasive threatens to do or than mechanical removal involves, then it is time to reconsider your interest in using chemical methods. Further details on herbicide mode of action can be found in chapter 6 of the Nature Conservancy’s guidebook

Types of herbicides:

Herbicides fall into several categories:

• Systemic: Chemical affects the entire plant system, thus completely killing it.
• Contact: Affects only the parts it comes in contact with, but leaves the rest of the plant alive, such as the roots.
• Selective: Generally only affects certain types of plants, most often dicots, that they are targeted for (not necessarily on a species level).
• Non selective or broad spectrum: Can affect all vegetation it contacts.

They can be categorized into finer detail by their site of action. Herbicides in the same families often have the same site of action, or mechanism of detrimental effect. For further details on site of action effects, see the above linked page organized by Bob Hartzler, extension weed management specialist, Department of Agronomy, Iowa State University.

Deciding what to apply

For aquatic applications:

The following formulations, according to the Washington Department of Ecology, have been approved for aquatic use. You must acquire a National Pollutant Discharge Elimination System permit to apply any of these, in addition to possessing the appropriate license from WSDA. All of these herbicides are considered restricted for aquatic use.

Active ingredient	Family	Trade Names	Type	Forms	Used on:	Time to take effect
2,4-D	Phenoxys	Aquakleen, Navigate (granular), and DMA*4IVM (liquid)	Selective systemic	Granular or liquid	Eurasian watermilfoil and other broad leaved species.	Relatively fast-acting
Diquat	Bipyridyliums	Reward	Non-selective contact	liquid	One season control of submersed aquatics. Avoid turbid water and dense algae. Effective against Brazilian elodea.	Fast-acting
Endothall	Dicarboxylic acid	Aquathol	Non-selective contact, can be used selectively in low concentrations	Granular or liquid	Removes exotic species in smaller areas.	Fast-acting
Fluridone	unclassified	Sonar, Avast!	Broad spectrum systemic. Carefully used in low concentrations for selective treatment.	Pellet or liquid	Submersed plants like pondweed and Eurasian watermilfoil with little water movement and extended time for treatment.	Damage shows in 7-10 days, but kills in 6-12 weeks (slow-acting)
Glyphosate	unclassified	Rodeo, Aquamaster, Aquapro, Aquastar, Aquaneat,	Broad spectrum systemic, but can be used selectively with good application	Liquid	Floating leaved plants like water lilies and shoreline plants like purple loosestrife. Not for submerged plant. Used on spartina and reed canary grass.	Several weeks
Imazapyr	Imidazolinones	Habitat	Non-selective systemic	liquid	Used to control spartina.	Slow-acting
Triclopyr (TEA formation)	Carboxylic Acids	Renovate3	Selective systemic	liquid	Effective against Eurasian watermilfoil and broad leaved species like purple loosestrife.	Relatively fast-acting

For terrestrial applications:

Numerous restricted and non-restricted use herbicides have been approved for terrestrial use, relative to those approved for aquatic ecosystems (though many are still restricted). The following table shows some commonly used herbicides, their trade names and their use on some common weed species in Washington:

Active ingredient	Rest-ricted use?	Trade Names	Type	Forms	Some species it is used on:
Imazapyr	No	Plateau, Arsenal, Chopper, Habitat, Stalker, Contain	Non-selective systemic	acid & salt	Bull thistle, poison hemlock
2,4-D (amine) and its combinations	Yes	Barrage	Selective systemic	Granular or liquid	Tansy ragwort (spring before flowers), musk thistle, black knapweed, scotch thistle, English Ivy, bull thistle, poison hemlock, Scotch broom in some areas, hoary cress, black henbane
Amitrole	Yes	ATA, ATZ, Amitrol 240,	Non-selective systemic	soluble powders, soluble concentrates, suspension concentrates, water dispersible granules, liquid solutions, and wettable powders	Perennial sowthistle, hoary cress
Atrazine	Yes	Arta-Pril Aatrex, Atrazine, Bicep, Cheat Stop, Cycle	Selective systemic	dry flowable, flowable liquid, water dispersible granular liquid, and a wetable powder	Perennial sowthistle
Bromacil	Yes	Hyvar, Krovar	Selective systemic	granular, liquid, water soluble liquid, and wettable powder formulations	Perennial sowthistle
Chlorsulfuron	No	Telar, Glean F.C.	Selective systemic		Bull thistle, hoary cress
Clopyralid	Yes	Confront, Curtail, Curtail M, Lontrel, Redeem R&P, Stinger, Battleship, Millenium, Trupower, UHS Chaser Ultra	Selective systemic	Liquid	Musk thistle, bull thistle (when mixed with triclopyr)
Dicamba	No	Banvel, Clarity, Diablo, Dicamba DMA, Kamba, Sterling, Trooper, Vanquish Often found in mixes.	Selective systemic	liquid concentrate, granules	Tansy ragwort (can be used in the flowering stage), Perennial sowthistle, musk thistle, black henbane, black knapweed, scotch thistle
Diuron	Yes	Karmex, Direx, Krovar	Non-selective systemic	granular	Perennial sowthistle
Glyphosate	Yes	Many trade names	Non-selective systemic	Liquid	Japanese knotweed (cut and inject), black knapweed, butterfly bush if mixed with metsulfuron methyl),
MCPA amine	Yes	Chiptox (sodium salt), MCP amine, Rhomene, Rhonox (ester), Sword (ester), Wildcard	Selective systemic		Perennial sowthistle, hoary cress
Metsulfuron methyl	No	Escort	Selective systemic	dry flowable formulations	Scotch thistle, bull thistle, butterfly bush if mixed with glyphosate, black henbane
Picloram	Yes	Tordon	Selective systemic	Pellet or liquid	Tansy Ragwort, musk thistle, black knapweed, scotch thistle, bull thistle, black henbane
Simazine	No	Princep, Sim-Trol, Simazine	Selective systemic	wettable powder and granules	Perennial sowthistle
Triclopyr	No	Crossbow (mixed with 2,4D), Garlon, Pathfinder II, Remedy, Renovate, Tahoe, Turflon ester	Selective Systemic	Granular or liquid	Tansy ragwort, Scotch broom is some areas, English ivy, Himalayan blackberry.

This list is a not complete one; it is simply an example of the numerous types of herbicides that can be used on certain species, depending on mixture, time of year and even on the stage of the plant. Timing, as with all herbicide application, is essential to successful eradication. Oftentimes, you may also need to prepare the plant with a surfactant or by cutting so that the herbicide will actually be taken up by the plant. In many cases, repeat applications will be necessary. Additionally, remember to investigate how the chemical you have mixed or applied will behave. Will it drift? Will it foam in your spray tank? Agents that control such problems do exist; it depends on the chemical. Many are also available as mixes, and some plants in some areas are affected only by these mixes. Always read the label to determine appropriate timing, mixing, concentration and methods of application.

Herbicides in the container nursery and greenhouse:

Eclipta growing under and around containers. Photo courtesy of James Atland, North Willamette Research and Extension Center, Oregon State University: http://oregonstate.edu/dept/nursery-weeds/feature_articles/sanitation/sanitation_page.html

Weeds growing in nurseries and greenhouses not only have detrimental effects on the species you are growing, but also act as reservoirs for insects like aphids, mites and other pathogens. Healthy, weed and pest free nursery plants are of particular importance; weeds affect their marketability and value in restoration projects. It is highly recommended that you employ an integrated pest management program in your nursery or greenhouse. Most effective weed management programs use sanitary practices in conjunction with proper preemergence herbicide application for optimal weed control.

Table 3: Common herbicides, their properties, and application. Table courtesy of Oregon State University Extension Service: http://eesc.orst.edu/agcomwebfile/edmat/EM8823.pdf

Site preparation:

Consider weeds at every developmental phase of your nursery and greenhouse. Always keep container plants on ground covered with gravel, plastic or woven weed fabric. When preparing the site, you may want to treat the area with an herbicide like dichlobenil (Casoron) before you install any of these ground covers to prevent perennial weeds before they germinate; some weeds are very capable of germinating in debris on top of the barrier, or through it, whether it gets damaged or not. For field grown nursery crops, you might treat the site with glyphosate 1-4 weeks before planting to allow the weeds to completely take up the chemical (if they are present). No preemergence herbicide will kill all weeds, so first decide if your predominant weed species are broadleaf or grass (see table 3 above). Once again, use common sense – don’t use herbicide if you don’t have to, for example, if your operation is very small and/or you can easily control all the weeds with hand-pulling or mechanical methods. Many weeds, such as the common groundsel, also develop herbicide resistance, so exercise caution with your herbicide choice, and always administer them as the label directs.

Application of preemergence herbicide before laying down a weed barrier

Photo courtesy of the High Tunnel Project in New Jersey:

http://aesop.rutgers.edu/~horteng/hightunnel5.htm

Weed germination prevention in container crops:

First, select a preemergence herbicide that your container crops will tolerate, and is “designed” for the weeds you are most likely to encounter. Wash your pots with pressurized water before potting plants. Irrigate your newly potted plants several times (the equivalent of 1 inch of precipitation) before applying the herbicide to settle the substrate. If the plants have already been potted and infested with weeds, pull all weeds or re-pot before using a preemergent herbicide. Chemical labels will also tell you how to best apply, so always read the label and follow the instructions to prevent weeds effectively.

Herbicides can form a chemical barrier for container media, preventing weed infestation. Photo courtesy of James Atland, North Willamette Research and Extension Center, Oregon State University: http://oregonstate.edu/dept/nursery-weeds/feature_articles/sanitation/sanitation_page.html

Sanitary Practices:

Weeds characteristically propagate themselves with abundant seed production – if you can limit the number of seeds that access your operation, then you will significantly reduce the probability of them passing through your physical and chemical barriers. Always check in between containers to ensure debris hasn’t accumulated and regularly sweep the weed barrier when not in use (unless it is gravel – in that case, add new rock or use a low solubility preemergence herbicide such as prodiamine). Eliminate weeds in non-crop areas such as along roadways and around buildings in your operation. Be careful when moving container plants around to avoid poking holes in weed fabric, or moving other equipment through the covered area. Weeds can very quickly infest holes in your weed barrier. After potting you plants and creating a chemical barrier, do not poke holes in the soil or disrupt the barrier in any way. Weedy bark and other substrate piles serve as a source of weeds as well, so keep them covered; you may have to use more costly techniques like steam pasteurization, solarization, composting, and fumigation to kill seeds in these piles. Wind-breaks like an impermeable fence, tall hedgerow, and even a wall of 5 gallon container plants have been used to effectively prevent weed seeds from blowing in from neighboring lots. Regularly inspect all corners of your operation to catch weeds before they seed.

Irrigation:

After you have applied a preemergence herbicide, proper irrigation is essential to adequate incorporation. Using a drip system is often not enough to do this, as it relies on capillary action. Overhead irrigation or rain are ideal methods of incorporation, as they adequately blanket the soil with water. However, rain is sufficient only if it occurs in the exact quantity needed (1/2 inch precipitation or more, continuously) and very soon after spraying. Irrigate immediately after applying the herbicide – if not, the soil will dry and the chemical will bind to the soil, inhibiting the potential for further movement. This generates a shallow chemical barrier, which is not conducive to weed prevention.

On the other hand, be careful not to routinely water your plants too much. This practice is known to reduce herbicide efficacy, though reasons are yet to be found. Due to the properties of preemergence herbicides, it is unlikely that they can get washed away. It is hypothesized that high levels of water simply facilitate the marginal survival of weeds that somehow break through the chemical barrier.

References:

http://agr.wa.gov/

http://cfpub.epa.gov/npdes/

http://eesc.orst.edu/agcomwebfile/edmat/EM8823.pdf

http://www.ecy.wa.gov/programs/wq/plants/management/aqua028.html

http://www.hort.uconn.edu/ipm/greenhs/htms/ghwedcntrl.htm

http://www.nwcb.wa.gov/

http://oregonstate.edu/dept/nursery-weeds/feature_articles/sanitation/sanitation_page.html

http://oregonstate.edu/dept/nursery-weeds/feature_articles/top_10/Top_10_reasons.html

http://www.pesticidesafety.uiuc.edu/newsletter/html/200006c.html

http://www.pfc.forestry.ca/biodiversity/broom_e.html

http://pnwpest.org/pnw/weeds

http://tncweeds.ucdavis.edu/handbook.html

http://www.weeds.iastate.edu/reference/siteofaction.htm

http://weeds.ippc.orst.edu/pnw/weeds

http://www.weedscience.org/in.asp

Compiled by:

Ivona Kaczynski 2005