Vegetation Control with Herbicides

The herbicides selected for weed and brush control depend on the species composition, proximity of crops, degree of control required, and whether it is for site preparation or conifer release. The commonly used control methods and typical herbicides used are described below.

Major Herbicides and Common Brand Names of Forest Management Products

Herbicide Name

Common Brand Names

2,4-D ester

Weedone LV-4, Weedone LV-6 and others

aminopyralid + metsulfuron


aminopyralid + triclopyr



Aatrex 4L, Atrazine 4L, Atrazine 90 and others


Transline and others




Vista XRT


Rodeo, Accord XRT ll, Roundup Custom, and many others


Velpar L, Velpar DF

imazapyr (2 lb ae/gal)

Chopper, Polaris SP, Rotary 2 SL, and others

imazapyr (4 lb ae/gal)

Arsenal Applicators Concentrate, Polaris AC, Imazapyr 4 SL and others


Esplanade F


Escort, MSM and others

penoxsulam + oxyfluorfen



Tordon 101, Tordon K and others


Segment II


Oust XP, Spyder, SFM and others

triclopyr ester

Garlon 4 Ultra and others

triclopyr salts

Garlon 3A, Vastlan and others

Foliage Application

Foliage spraying is extensively used for conifer release, site preparation, and directed spot treatments with a relatively small group of herbicides. Products containing aminopyralid, 2,4-D, clopyralid, fluroxypyr, triclopyr, imazapyr, glyphosate, metsulfuron, and picloram are used for most foliage applications on both deciduous and evergreen trees, shrubs and weeds.

Several products can be used selectively for over-the-top broadcast conifer release. Clopyralid, fluroxypyr, 2,4-D, triclopyr, imazapyr, and glyphosate have good to moderate conifer selectivity when used over the right conifer species at the correct rate and season. Most applications occur when conifers are dormant, usually before budbreak in the spring or after bud set in the late summer. Selectivity for conifers depends on managing a host of variables such as season, timing, dosage, target species and conifer species to be released.

Conifer release works well in the late summer and fall because conifers are becoming dormant and several vegetation groups are sensitive to treatment. Deciduous shrubs and trees are effectively controlled by glyphosate and imazapyr. Blackberries are sensitive to triclopyr and fluroxypyr at this time. Both Scotch broom and some evergreen brush species can be treated with triclopyr. Alder, madrone and manzanita species are sensitive to 2,4-D. Elderberry is well controlled by clopyralid over the whole growing season including late summer. Because many of these species are deciduous, the window for application starts when conifers set a dormant bud and effectively ends with leaf fall. Applications should be timed so that target species are still active enough to absorb the herbicides applied.

Foliar release treatments in the spring before conifer break bud are more limited but can still be effective on some vegetation groups. Scotch broom and evergreen species like snowbrush ceanothus, madrone and manzanita are controlled by triclopyr or 2,4-D. Alder can be controlled with an early foliar application of 2,4-D as Douglas-fir buds are breaking. After budbreak new conifer growth will be very sensitive to most release herbicides and severely injured.

Site preparation applications made before conifers are planted utilize the same products on target species as conifer release above but at higher rates and during the growing season. In addition, some products are registered only for site preparation. Products containing aminopyralid such as Capstone (aminopyralid + triclopyr amine) and Opensight (aminopyralid + metsulfuron) are registered for brush control during site preparation as well as directed spraying. Metsulfuron is a common site preparation component often mixed with other herbicides and has strengths controlling Rubus species, snowberry and ferns. While not widely used, picloram products are federally restricted and labeled for broad spectrum brush control during site preparation. Tank mixes of these products are common and offer broad spectrum control on diverse plant communities found on forest sites.

Directed spraying on individual plants can be accomplished with nearly all of the above herbicides. Applications made in conifer plantations should avoid overspray onto seedlings to avoid potential conifer injury. Ester forms of triclopyr and 2,4-D are also volatile and vapors can move off in warm weather and potentially damage conifers or other plants. Products containing aminopyralid and imazapyr require extra caution since they have soil activity and heavy dosing near conifers can result in damage even without foliar contact.

Surfactants and other adjuvants may be added to spray mixtures to enhance foliage activity. However, each commercial herbicide product may or may not have its own emulsifiers and wetting agents. Follow label recommendations on their use. Adjuvants do not always increase efficacy and some surfactants can decrease selectivity of herbicides when sprayed over conifers. In general, adjuvants should be used carefully if conifer selectivity is desired, unless experience shows conifer tolerance with the herbicides in use.

Foliage applications mainly rely on helicopters or backpack sprayers on forest sites. The waving-wand backpack method of application is uniquely suited for treating small clearings and low brush. It allows for a calibrated broadcast application for site preparation and conifer release provided terrain is moderate. A recent publication describing the waving wand method appears in

Dormant Application

Dormant applications are made to bare stems and branches of trees and shrubs in late winter and early spring when conifer buds have begun to swell but have not opened. Water-soluble products generally are not used at this time. Dormant applications are used where Douglas-fir or true firs are established and require release from brush that is susceptible during this season. Unless the brush species retain green foliage during winter, oil is used for the herbicide carrier on bare deciduous brush stems. Emulsions containing oil may be as effective on the persistent-leaved evergreen brush. Pines, especially ponderosa, are sensitive to dormant sprays, particularly those containing oil, after the end of January. Sprays containing oil appreciably increase the cost of treatment, leading to decreased reliance on dormant sprays. Spraying well before conifer bud swell may be best for Scotch broom control.

Triclopyr esters and 2,4-D ester are the most frequently used products in dormant applications. They are generally applied in low volumes of oil or water-oil mixtures. There are no known substitutes for oil on stems of certain deciduous brush species and Scotch broom when there are no leaves. Water is appropriate on evergreen brush; adding a surfactant or 5% oil may improve results. Consult labels for proper mixing order and rates to avoid mixing problems.

Basal Bark Application

The basal application method is generally used to selectively treat individual woody plants. This method also lengthens the period of time that brush can be sprayed; basal applications are effective from January to November. For basal treatments, mix a triclopyr ester herbicide with oil and apply completely around the lower 15 inches of a tree trunk or brush stem, soaking the trunk liberally to the ground line. Basal treatments usually control even larger hardwood trees with thick bark. Application is easiest if the base of the tree is scraped bare of moss and debris before treating. While allowing for selective brush control, this method can use a large amount of herbicide and oil. In some cases, frill or stem injection can be faster and use less material.

Basal sprays are applied with oil as a carrier, using either one of the basal carrier oils, kerosene, or plant seed oils. Diesel oil can also be used since it is readily available, but is not as good of a carrier on some species because it can damage plant tissue and interfere with herbicide uptake. Mixtures with water have not been effective. Rates are 1% to 5% by volume of triclopyr ester herbicide (e.g., 1 to 5 gal Garlon 4 Ultra/100 gal) in oil. The concentration used for basal sprays is much greater than for foliage sprays. For success, the stem must be soaked and thoroughly covered throughout the treatment area. Note that when trash is kicked away from the base, thinner bark is often exposed. If all of this is treated, thicker bark above may not need coverage. Results from basal treatments are not apparent immediately. Often the tree will leaf out and die back one or two years before finally dying. Incompletely covering the stem circumference leads to sprouting, and the entire tree is likely to survive.

Low volume basal bark treatment

For susceptible woody plants with stems less than 6 inches in diameter at the base, mix 4% to 30% by volume of a 4 lb ae triclopyr ester product in oil. Concentrations of 10 to 15 percent have provided good control on many Pacific Northwest species. Apply with backpack sprayer, using low pressure and a solid cone or flat fan nozzle. Spray basal parts of brush and tree trunks to thoroughly coat lower stems, including root collar area, but not to the point of runoff. Vary herbicide concentrations with size and susceptibility of species treated. Apply at any time from January to November except when snow or water prevents spraying to the ground line. Avoid treating when bark is soaking wet with water.

Thin-line basal bark treatment

To control susceptible woody plants with stem diameters less than 6 inches, apply an undiluted triclopyr ester product in a thin stream to all sides of lower stems as allowed on the label. Direct the stream horizontally to apply a narrow band of herbicide around each stem or clump. Up to 0.5 oz of chemical will be needed to treat single stems, and from 1 to 3 oz to treat clumps of stems. Use an applicator metered or calibrated to deliver the small amounts required. A D2 nozzle gives the desired stream. This is effective on sprouts with smooth exposed bark. Thinline applications can be effective but can use more product and as a result cost more than a low volume basal treatments.

Cut Surface Applications: Hack-and-Squirt, Stem Injection and Cut Stump Treatment

Individual trees or resprouting cut stumps can be treated using stem injection or a hack and squirt. In the commonly used hack-and-squirt method, the tree trunk is frilled or cut at intervals around the trunk using an axe or machete and herbicide is applied with a spray/squeeze bottle or similar applicator. Cut spacing can vary depending on the herbicide and dilution with water. A complete frill or girdle can be effective, but is labor intensive and often not needed.

Each herbicide used and species controlled will have different cut spacing ranging from 1 cut /1 inch of stem diameter (roughly spaced 3 inches on center) to 1 cut/3 inches of stem diameter (see table: Herbicide Recommendations for Control of Listed Species). Cuts must be through the bark and into the sapwood; chips remain connected to the tree to form a small cup or frill. The cut should be treated with concentrated salt-formulated herbicide solution immediately after the frill is made. Few species require complete frilling. Spaced axe cuts treated with one milliliter (ml) (0.25 tsp or cubic centimeter) of water-soluble herbicide solution per cut are usually adequate. For stem injection, special injector hatchets and stem injectors are available to make the cut and inject the herbicide at the same time using similar rates.

Season can be important in cut-surface applications. Certain herbicides, including 2,4-D or triclopyr salts, are most effective during the early summer. Picloram, glyphosate, and imazapyr, are most effective when applied from midsummer to leaf fall. Rising sap in late winter/spring dilutes the herbicide and may prevent movement to the roots resulting in variable performance.

Cut stumps may be treated by applying concentrated or diluted water-soluble herbicides (imazapyr, glyphosate or the salt form of triclopyr) to only the cambium perimeter (one inch of wood just inside the bark) of the freshly cut stump surface and any exposed cambium where bark may have been ripped or damaged down the side. Treated stumps may sprout weakly the second year if treated during the growing season or fall. Spring stump treatment is not as successful. Try to treat stumps within 1 hour of cutting as the cut surface will tend to dry out and not effectively absorb herbicide. Triclopyr ester herbicide applied undiluted as above can be used as an alternate treatment on stumps that have dried out or where a significant delay has occurred.

Imazapyr, glyphosate, triclopyr salts, or picloram are used with the hack and squirt method or stem injection. Most herbicides can be used diluted with water, and some products allow using the undiluted material. A dilution of 50% by volume in water works well for most herbicides. Be sure to check the label on maximum concentrations and rates. For products allowing undiluted applications, users can save on carrying material but this also requires more precise application to meter out very small volumes per cut. Animal health syringes attached by tubing to small reservoirs that can be adjusted to 1 ml increments have shown usefulness.

Injecting/frilling conifers can be an inexpensive way to thin stands of fewer than 1,000 conifer trees per acre. Total kill is not required, and minimal dosages permit good development of untreated trees. Heavy dosages, particularly of picloram and imazapyr, offer danger of flashback damage to untreated leave trees whereas glyphosate involves less risk.

Special Considerations

Besides federally restricted-use products like picloram and atrazine, some forestry labeled products can have restricted uses in an individual state. A list of ingredients that have restricted uses in the Pacific Northwest are located in the beginning of this handbook. Users should consult the label and the respective state department of agriculture for additional restrictions.

Definitions of treatment boundaries and stream buffers must be very clear to avoid trespass and resource contamination. Forest treatment areas are often irregular and border other forest ownerships, crops, or water sources. It is always wise to consult neighboring landowners before spraying and take the needed precautions to avoid chemical trespass.

Some commonly used herbicides are injurious to conifers at high doses or during the growing season. Control difficult weed problems before planting, as part of site preparation. Use maximum rates only with experience or in exceptional circumstances.

Invasive Weeds in Forests

Exotic invasive weeds have many opportunities to establish and spread in forests. Once introduced, they are very costly to eradicate and can inflict serious harm on forest ecosystems. Many forest invasive species are plants that have been propagated and distributed commercially. English holly, English ivy, gorse, Scotch broom, pampas grass, butterfly bush and false brome have been planted as ornamentals and have escaped to cover large areas. Himalayan and evergreen blackberries are popular for recreational picking. Others, like knapweeds, starthistle, mare's tail, and exotic thistles, are widely distributed nationally and even globally. For specific weed information and geographic locations contact the state department of agriculture. For control of these and other species, see section "Control of Problem Weeds" in this handbook.

Invasive plants are often spread by machines, including logging equipment and various motor vehicles. One of the most important conduits for invasive plants is roads. Controlling invasives on road shoulders is one way to prevent spread. Requiring washing of all equipment brought in to do logging and road work before entering and leaving the property is another preventive procedure. Landowners may also have additional herbicide control options on roadsides since they are considered a separate use site from forest management sites. Herbicides useful for controlling plants on roadsides and non-crop sites can be found under the section "Non-Cropland and Right-of-Way" in this handbook.

Recommendations for Directed Spot Spray, Tree Injection, and Basal Bark Treatment

Hand applications of herbicides can be very effective in controlling individual plants or small areas of weeds. The information below lists applications for directed foliage spraying using common herbicide products. Following this is a species specific table with information on foliage sprays, basal applications and cut surface stem treatments. Spot foliage treatments can use larger volumes of spray solution than broadcast sprays. The table assumes volumes applied at 50-100 gal per acre. Users should always read the label of the products they are using to make sure they don't exceed any per acre maximum listed and adjust the herbicide concentration accordingly. When spraying around conifers or other desirable plants, avoid foliage contact especially during active growth.