Vegetation Control with Herbicides

The methods selected to apply herbicides for weed and brush control depend on the species composition, proximity of crops, degree of control required, and available equipment. Method and season of application also have a major influence on degree of selectivity. The "best" method to use will depend on the project scale, terrain, type and size of vegetation, and will range from aerial applications to treating individual stems. The commonly used control methods and typical herbicides used are described below, with special emphasis on objectives, formulations, and equipment peculiar to each.

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

Herbicides sold as commercial products vary in their concentration of active ingredients. Different brands of a given active ingredient may have varying concentrations. When calculating how much product to use for a given application, make sure the calculation of dosage starts with amount of active ingredient (ai), and add whatever amount of product is required to provide that amount of active agent. This section describes applications of many chemicals in various applications. If a product has 4 lb ai/gal, and an application calls for one lb of the active portion of the product/acre, then the rate of product used is one quart per acre. Do not confuse these terms, lest over- or under-dosage results. Some hormone-type products are formulated as amine salts (water soluble) or esters (oil soluble, often water emulsifiable). Active ingredients are described in terms of percent ester or amine salt, and also as acid equivalent (ae). Use ae for determining the proper mixture when diluting for application.

Release treatments are designed to free conifers from competing vegetation. Several products can be used selectively for over-the-top broadcast conifer release. Most applications occur when conifers are dormant, usually before budbreak in the spring or after bud set in the late summer. Fluroxypyr is registered for selective blackberry and brush control in conifer plantations including ponderosa pine. Clopyralid is useful for elderberry and thistle control. Triclopyr and 2,4-D can be used selectively (but avoid high rates; see label) over Douglas-fir seedlings but will injure ponderosa pine and noble fir. Glyphosate and imazapyr can be used selectively to favor conifers only in late summer or early fall. Glyphosate is highly effective only on deciduous and herbaceous species in full foliage development. In fall, glyphosate will selectively remove brush and herbs from conifers. In midsummer, glyphosate damages conifers and brush severely. Imazapyr is active on maple, alder, and other brush as a growth inhibitor. It also damages conifers when applied to foliage during the growing season. Imazapyr is well adapted for trunk injection or spot treatments. Triclopyr salt formulations may be injected as a concentrate as well.

Site preparation applications before conifers are planted utilize the same products as conifer release but at higher rates and during seasons that would injure seedlings. Some products are registered only for site preparation. Aminopyralid, is registered for use to control broadleaf weeds and some brush in forests or on forest roadsides (non-crop) use. The product with aminopyralid alone, Milestone, is permitted only on roadsides, whereas the combination of aminopyralid and triclopyr amine (Capstone) is permitted for use in forests for site preparation, directed spray and cut surface applications (see the label for specific uses). The combination of aminopyralid and metsulfuron (Opensight) has received supplemental labeling in Oregon, Washington and Idaho. Aminopyralid has soil activity; overdosing near conifers can result in conifer injury through root uptake. Planting four months after application is recommended.

Picloram and metsulfuron nearly always harm conifers when applied directly so use is confined to site preparation. Picloram is federally restricted because of its mobility and high potential for damaging crops if it appears in irrigation water; use it with this in mind. Note restrictions on the label when grazing treated areas. Metsulfuron is used at rates much lower than rates for most other products and is useful at present only for site preparation and directed spraying. It is highly effective on blackberries, ferns and deciduous brush when applied before timber harvest to prevent postharvest sprout growth.

Grass and weed control in conifer plantations is covered at the end of this chapter.

Foliage Application

Foliage spraying is extensively used. Selectivity for conifers (release sprays) will depend on managing a host of variables such as season, timing, dosage, and species to be released or controlled. Brush fields that are to be converted to conifer stands are best suited to summer foliage spraying before planting. Species such as vine maple (see tables later in this chapter for recommendations) may be satisfactorily controlled by this method, but only with highly systemic products such as glyphosate and imazapyr. Manzanita, Ceanothus spp., madrone, and other persistent-leaved brush species lend themselves to satisfactory control with foliage treatments of growth regulator products (e.g., 2,4-D or triclopyr esters) at any season from late dormancy until late midsummer, but treatments often work best in early spring.

The choice of season for controlling these species is determined in part by the availability of spray equipment and the presence or absence of susceptible crop trees. Check product labels carefully because optimum application times vary. For instance, conifer release works best in the fall when applying glyphosate to control alder, salmonberry, and thimbleberry. The optimum season for control of target species is usually midsummer, but that is also when many conifers are most sensitive. Optimum timing differs by herbicide used, and between site preparation and conifer release treatments. If spraying for selective release, never spray phenoxy herbicides over ponderosa pine in early spring, and do not attempt to release Douglas-fir when shoot elongation is more than an inch or two in length, or not yet hardened off.

Herbicides 2,4-D, aminopyralid, clopyralid, fluroxypyr, triclopyr, imazapyr, glyphosate, metsulfuron, and picloram are the herbicides used for foliage applications. For application rates, see the tables later in this chapter. Glyphosate plus imazapyr gives excellent control of mixed deciduous brush species. Picloram, aminopyralid, and metsulfuron are highly toxic to most conifers when applied directly, but seedlings planted the next season after treatment normally are unaffected. Water is almost always used as a carrier; in late summer a small amount of an oil additive may be added to emulsifiable products including mixtures containing 2,4-D or triclopyr (see labels).

Adjuvants 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. Many (but not all) surfactants decrease selectivity in conifers. In general, adjuvants should be used carefully if conifer selectivity is desired, unless experience shows conifer tolerance.

Application The choice of whether to spray foliage from the air or with ground equipment depends on the size of the job, stream buffer requirements, the equipment available and product registration (some products do not allow aerial application). For most small spray jobs, small equipment is most satisfactory. A backpack sprayer with adjustable cone nozzles can apply sprays at 3 to 10 gal/a quite uniformly (waving-wand technique) on cover less than 8 ft tall, if the applicator can move relatively freely. Note that most product labels show a minimum volume per acre that sets the lower limit. This technique is much faster than the spray-to-wet procedure, and wastes much less chemical.

The waving-wand backpack method of application is uniquely suited for treating small clearings and brush beneath a canopy of conifers or hardwoods being prepared for harvest, but is also well suited for larger areas and riparian zones where accessible on foot. These low-volume applications have great promise for allowing planting immediately after logging, if the pre-harvest application contains foliage-active products such as glyphosate/imazapyr, and soil-active materials such as sulfometuron. Preharvest treatment prevents perennial species from sprouting, and greatly simplifies postharvest applications by preventing immediate green-up. When using the waving-wand technique for site preparation or release, it's important to keep the nozzle pointed above the target vegetation so the light spray comes down directly onto the foliage rather than cross-wise. Foliage is naturally designed to capture light from above, and this orientation also is excellent for catching a light spray. Wearing required personal protective equipment (PPE) as listed on the label is critical. For directions on this technique, read Handbook of Weed and Insect Chemicals for Forest Resource Managers, by M. Newton and F.B. Knight (Timber Press). For broadcast application rates, see recommendations in the following sections of this chapter for boom-less nozzles on ground rigs and aerial equipment. Volumes of total spray per acre suitable for aerial application work well, even at the lowest level. Always check and follow the label for minimum volume per acre allowed. Low volumes minimize labor cost as long as distribution is good. A recent publication describing the waving wand method appears in

Aerial applications use 5 to 10 gal/a of spray mixture, as described on the label. Ground equipment is suitable for small jobs, but the labor required can be excessive on jobs of more than 40 acres; aerial application may be preferred. On large jobs, aerial spraying is a less expensive way to apply herbicides.

Herbicide selectivity and species composition Certain herbicides are effective on different mixtures of brush species and herbs. However, forest weed communities are far more diverse than agricultural fields, so several products may be required in a tank-mix. For example, if a recent cutover has developed a cover of grasses and forbs such as tansy ragwort and thistles-plus vine maple, elderberry, and salmonberry in the draws-a single product will merely change species composition. A site-preparation mixture for such a target group might include sulfometuron, glyphosate, and clopyralid, each at a dosage capable of removing a target species. This may be a costly prescription, but is much less costly than repeated applications, each of which may be inadequate. In tank-mixes, however, each product must not exceed the rate of application given on the label.

Dormant Application

Make most dormant applications when conifer buds have begun to swell but have not opened in late winter or early spring and before deciduous brush has leafed out. 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 at 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 bud swell may be best for Scotch broom.

Herbicides Triclopyr esters and 2,4-D ester are the most frequently used products in dormant applications. They are generally applied by helicopter in low volumes of oil or water-oil mixtures. Other types of ground apparatus are not well adapted to this type of treatment. 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.

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 the 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. Spring applications may produce the best top kill, whereas summer and fall sprays may give better sprout control. Winter treatments may require a higher volume of spray with higher concentration as well.

Herbicides Basal sprays always are applied with oil as a carrier, using either one of the basal carrier oils now available, 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. Rates are 4 to 20 lb ae of triclopyr ester herbicide (e.g., 1 to 5 gal Garlon 4 Ultra) to each 100 gal of oil used, or 1% to 5% by volume. 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 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 gal of a 4 lb ai triclopyr ester in enough oil to make 100 gal of spray mixture (4% to 30% by volume). 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 wet 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 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 hack-and-squirt method, the tree trunk is frilled or cut at intervals around the trunk using an axe or machete. Cut spacing can vary depending on the herbicide and dilution with water. A complete frill or girdle can be effective, but is labor intensive. 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 0.25 tsp (one milliliter (ml) or cubic centimeter) of water-soluble herbicide solution per cut are usually adequate. Special injector hatchets and stem injectors are available to make the cut and inject the herbicide at the same time.

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 using either of two methods. One method uses the same herbicide in oil as for basal treatment. Research with bigleaf maple and Oregon white oak show results are best if the stump is thoroughly treated with a basal spray solution (triclopyr ester in oil) around the ground line and the cut perimeter any time after cutting, but preferably before it resprouts.

The second method for stump treatment is far less costly. Apply 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. A tablespoonful is adequate for almost any stump less than 1 ft in diameter when uniformly distributed in a thin line around the perimeter of the live wood. More is required for stumps over 12 inches in diameter. 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 cut and treat stumps within 1 hour as the cut surface will tend to dry out and suberize and not effectively absorb herbicide. Triclopyr ester undiluted can be used as an alternate treatment on stumps that have dried out or where a significant delay has occurred.

Herbicides Imazapyr, glyphosate, triclopyr salts, or picloram, used singly or in combination, are most effective and easiest to apply using the hack and squirt method. Esters do not work as well, except for triclopyr ester when applied to stumps cut 1 to 30 days after cutting. 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 also this requires more precise application to meter out very small volumes per cut. On hardwoods other than maples, triclopyr salt formulations may be diluted to half strength with water. Triclopyr is very effective on many species. Imazapyr is highly effective on maples and on almost all other hardwoods, and may be diluted without losing effectiveness on most species. A dilution of 50% v/v in water gives excellent control of maples and other associated species. Glyphosate may be used at half to full strength and is effective on most hardwood species except bigleaf maple. 2,4-D amine is effective on alder, cherry, and madrone when used at full strength. Be sure to check labels for current correct dilutions on the product being used.

Application For stem injection or hack and squirt, use one milliliter (ml) of the diluted solution for each cut. Use glyphosate during summer and fall for best results with cut spacings of one per inch of diameter. Use picloram in any of these seasons in mixture with 2,4-D, but control will be best if picloram is applied in summer. Triclopyr is effective in spring, fall, and winter if trees are completely frilled. In summer, space cuts for triclopyr about 3 inches between centers, or about one cut per inch of diameter. Imazapyr is effective all year but best performance is from mid-summer into winter while spring applications can be more variable. Imazapyr cut spacing can be 6 inches (about one cut per two inches of diameter) on trees up to 12 inches in diameter, and 3 inches on larger maples; use the more concentrated material in winter.

Application may be made with various types of tree injectors, or with a hatchet and squeeze/spray bottle. Tree injectors are tools designed to place chemicals into trees without frilling, girdling, or felling. The objective is to inject the chemical directly into the live tissue of the tree. Injector hatchets permit reaching around trees from one side, and should make this a very useful and low-cost method, particularly since modern chemicals do not require injection at the base of the tree.

The hack-and-squirt method is slower than the injector method, and is less precise in metering chemicals. Animal health syringes attached by tubing to small reservoirs that can be adjusted to 1 ml increments have shown usefulness. The method offers the advantage of a lower investment cost for small jobs.

Injecting/frilling conifers is probably the cheapest 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 trees whereas glyphosate involves less risk.