Rick L. Cantrell
Senior Director, Forest Policy and Sustainable Agriculture
American Forest & Paper Association
1111 19th Street, NW Suite 800
Washington, DC 20036
(Formerly with the University of Tennessee Agricultural Extension Service)

Note: Chemicals contained herein are labeled for forestry uses based on the most up-to-date information available to the authors. Use of trade names is solely for identification. No endorsement of the products named is implied by the Clemson University Cooperative Extension Service nor is any discrimination intended by the exclusion of products or manufacturers not named.

INTRODUCTION

USING THIS GUIDE FOR HERBICIDE PRESCRIPTIONS

FOREST HERBICIDE PRESCRIPTION FLOWCHART (Figure 1)

SIGNIFICANCE OF INPUT TOPICS USED FOR PRESCRIPTIONS

Crop Species
Dominant Weed Species
Other Factors Affecting Efficacy
Environmental Concerns
Other Critical Factors
Factors Affecting Application Methods

ARRANGEMENT OF TECHNICAL INFORMATION

SITE PREPARATION

Registered Herbicides (Table 1)
Products x Crop Trees (Table 2)
Species Susceptibility (Table 3)
Herbicide Characteristics
Application Methods (Table 4)
Recommended Treatments (Table 5)

RELEASE

Registered Herbicides (Table 6)
Products x Crop Tree (Table 7)
Species Susceptibility (Table 8)
Herbicide Characteristics
Application Methods (Table 9)
Recommended Treatments (Table 10)

HERBACEOUS WEED CONTROL

Registered Herbicides (Table 11)
Products x Crop Trees (Table 12)
Species Susceptibility (Table 13)
Herbicide Characteristics
Application Methods
Recommended Treatments (Table 14)

Selected References and Literature Cited

Appendix A - Herbicide Application Records Sheet ...................................pdf format

Appendix B - Common and Scientific Names of Weed and Crop Species Included in This Manual

Appendix C - Herbicide Terminology

Appendix D - Chemical Companies

Numerous herbicides are registered for forestry use in the Southern United States. These are described in detail on product labels and technical brochures. In addition, forest herbicide use has been systematically summarized in various guides. However, from a practical standpoint, most sources do not present information required to effectively prescribe herbicide treatments. The Chemical Expert System for Silviculture (CHESS) developed at Virginia Polytechnic Institute and State University is a very effective and valuable prescription writing tool, but requires an adequate computer, software and a capable operator.¹ A need still exists for a concise, written reference of herbicide prescription information with general accessibility for both the field and office.

The objective of this work is to describe important variables influencing forestry herbicide prescriptions and the characteristics of individual products that may interact with those variables. The information is arranged so as to clarify conditions under which particular products are most effective, and to help eliminate the misuse of products in situations where they clearly do not fit.

This manual is intended to serve as a guide for the novice practitioner involved with occasional forest herbicide use and as a quick reference for more experienced users. For simplicity, we focused upon major herbicides used for forest site preparation, conifer release, and herbaceous weed control.

Alternative Vegetation Management Practices

Before proceeding, a few comments are appropriate regarding alternative vegetation management methods. Vegetation control should be conducted to fit within and enhance the overall objectives of a sound forest management plan. An effective land manager should determine whether chemical control will meet these objectives and at a similar or lower cost than other control methods. Currently, alternatives are available primarily for site preparation (mechanical methods and controlled burning) and timber stand improvement (controlled burning), while pine release and herbaceous weed control are accomplished almost exclusively with herbicides.

The suitability and cost effectiveness of herbicides compared to mechanical site preparation can be affected by a number of factors. Mechanical site preparation has traditionally been used extensively because it offers a dependable method of clearing a site for either machine or hand planting. However, use can be restricted somewhat by wet weather, poor drainage or extremes in topography. Other problems include negative impacts on erodible or fragile soils and the potential for abundant hardwood sprout development.

Herbicide treatment alone or in conjunction with controlled burning has limited impact on most soils and is not restricted by terrain. Herbicides will control hardwood resprouting when used correctly and are generally less expensive than mechanical treatments. However, chemical site preparation is often restricted to sites that are to be hand planted and is ineffective on sites with abundant herbicide-resistant species. Herbicide use is sometimes restricted by environmental concerns, and requires technical training by the user.

Many of the above features are easily assessed, but quantitative yield data relating vegetation control to increased crop tree growth and cost effectiveness is not readily available. An acceptable level of vegetation control is generally an arbitrary condition based on perceptions of the practicing forester. Nonetheless, making a reasonable decision as to whether the use of herbicides is appropriate requires an understanding of their effects and limitations.

¹Chemical Expert System for Silviculture (CHESS) can be obtained by writing to Virginia Tech, School of Forestry, Blacksburg, VA 24061.

Using This Guide for Herbicide Prescriptions

The herbicide prescription process involves matching a suitable herbicide treatment to a specific silvicultural objective. It can be confusing in that several products may be registered for a general purpose with each exhibiting desirable or undesirable traits for specific site conditions. Although it may not always be possible (or necessary) to select a single “best” treatment for a given site, selection(s) should still be based on a series of logical actions and decisions.

Three functions of this guide are to:

(1) direct the user through a logical sequence of considerations required for the prescription process;

(2) provide an understanding of these considerations;

(3) provide convenient access to pertinent technical information about specific herbicides.

Arrangement of Technical Information

The prescription process, presented as Figure 1, consists of a flow chart of the hierarchy of considerations necessary for herbicide prescriptions. The chart directs a series of actions in response to various input topics in the order that they normally occur when formulating a prescription. In order to use the flow chart effectively, the user must understand the significance of each input topic. These are described immediately following the chart under the heading “Significance of Input Topic Used for Prescriptions.”

Technical information on registered herbicides can be sequentially accessed from Figure 1. Note that in the figure, colored blocks labeled “Site Preparation,” “Release,” and “Herbaceous Weed Control” correspond to major sections of the manual. Refer to the table of contents for easy access to each section. Technical information within each section is formatted to match shaded blocks in the remainder of the flow chart. These include:

(1) a table of registered products;

(2) a table of registered products and crop trees for which they are labeled;

(3) a table of species susceptibility;

(4) herbicide descriptions;

(5) a table of application methods.

A sixth table is included at the end of each section as a reference source. This table lists treatments that are commonly recommended or used for specific conditions. The treatments cover many of the site conditions encountered in the southeast but are not all-inclusive. Information in the manual and product labels should be read carefully before buying, applying or disposing of any herbicide.

The user of this guide must understand that it was not designed to make decisions, but to provide order and access to information required for those decisions. The flow chart directs the user to registered products, crop species for which they can be used, and weed species on which they are effective. Beyond this point, distinguishing features necessary for prescriptions must be obtained through careful reading of individual product descriptions.

Significance of Input Topics Used for Prescriptions

Crop Species
It is unlawful to use herbicides or other pesticides on crop species other than those specified on the label. The pesticide label is a legally binding document. Violation of the label can result in damage to crop trees and/or litigation. Consult tables 2, 6, and 10 for product specific information.

Dominant Weed Species
Before prescribing any herbicide, a survey must be conducted to identify dominant weed species on a site. A stand (woody or herbaceous) will generally be comprised of three to four major species. An herbicide should be considered acceptable as long as resistant (relatively unaffected) species are only a minor component.

Species susceptibility is addressed in tables 3, 7, and 11 for site preparation, pine release and herbaceous weed control. The tables categorize weeds as either highly susceptible or “resistant.” This approach is somewhat simplistic. However, data necessary to accurately specify intermediate gradients of control are unavailable. We therefore chose to emphasize resistant species (in bold type) in order for the reader of this guide to avoid using herbicide products where they clearly will not work.

Other Factors Affecting Efficacy
Other factors in addition to species that affect herbicide efficacy include soil type, application timing, and weather. Implications are discussed below and specific features of each product are outlined in the same order within the product description sections of the guide.

Soils

Soil-active herbicides may be strongly impacted by soil texture, percent organic matter, and internal drainage. Texture and organic matter determine the adsorptive capacity of a soil. Microscopic colloids (clay and organic), have a large surface area in proportion to volume and tend to adsorb herbicides in proportion to their content. Thus, fine textured soils (clays) or organic soils have a higher tendency to bind herbicides, making them less available for uptake by root systems of target vegetation. Generally, product labels provide a range of application rates for various soil types. Frequently, higher rates of an herbicide are required on soils high in clay or organic content. On the other-hand, for pine release or herbaceous weed control in young stands, care must be exercised so that injury to the crop trees does not result from over-application on sandy or coarse textured soils. A user should read the label carefully, know the soil type, and make sure that applications are conducted according to specifications.

Effects of internal drainage on herbicide activity have not been widely studied. However, it is known that certain products do not work well on poorly drained sites.

Timing

Application timing is critical with several herbicides. Soil-active herbicides are generally applied in the spring to early summer when rainfall required for activation is likely to occur. Similarly, foliar products are applied after full foliar development in the spring. However, there are exceptions. A few products can be applied throughout the growing season or in early to mid-fall.

Weather Conditions

Local weather conditions should be considered when selecting either soil-active or foliar products. Soil-active products require rainfall to activate and move the herbicide into the soil profile. Too little rainfall can result in low herbicidal activity, and too much can wash herbicides off-site or deep into the soil profile, below the active rooting zone of the target species. Likewise, foliar products may be washed from leaf surfaces when applied immediately before rainfall. Minimum time intervals between application and rainfall are recommended for some products.

It is speculated that dry weather during the previous fall and current spring can stress vegetation and increase herbicide susceptibility. This may increase the risk of damaging crop trees during pine release or herbaceous weed control.

Applications of certain translocated foliar products appear to be most effective when plants are at peak physiological activity. Lower herbicidal activity might be expected from applications conducted during extremely cool, wet springs.

Environmental Concerns
Environmental concerns generally involve human safety aspects and the potential economic and ecological impacts that result from misapplication or movement of product beyond the target site or vegetation. Crop species such as cotton, soybeans, grapes, tomatoes, and tobacco are very sensitive to certain products and can be damaged when exposed to spray drift or volatilized chemical. Similarly, movement of soil-applied products with surface flow following rainfall can be detrimental to off-site vegetation and/or contaminate groundwater, streams, ponds, etc. Significant environmental features such as toxicity, volatility, and behavior in soils are included in product descriptions.

Other Critical Factors
Other critical factors feature miscellaneous product-specific information highlighting positive and negative traits as well as important warnings. These are included under the last heading in the product descriptions and should be read carefully.

Factors Affecting Application Method
The practicality of a herbicide prescription depends on both a high level of efficacy under a given set of conditions, and on availability of a suitable application method. Application methods registered for specific herbicides are listed in Tables 4 and 9, and on page 48. For practical reasons, only those methods listed on product labels are included. Additional methods may be legal as long as the label does not prohibit their use. However, we advise a potential user to check with either a representative of the product manufacturer or with the Cooperative Extension Service before using methods not included on the label. Some of the factors affecting choice of application method are discussed below.

Acreage

Tract size can influence choice of application method, cost of application, and potential for contracting the work to a commercial applicator. Applicators using aircraft or heavy equipment have a general desire to work large tracts of land because of the fixed costs associated with moving and setting up equipment. Their cost per acre declines with the number of acres treated and they are often unwilling to treat small tracts (50 acres or less) unless located near a larger operation or grouping of smaller tracts. Fortunately commercial applicators that use ground crews with backpack sprayers or other hand-held devices are available in many areas. They are less affected by tract size and will often work smaller tracts at a reasonable cost.

Topography

Rough terrain or inaccessibility can preclude the use of ground crews or equipment. Aerial applications are often the only practical means of treating such lands.

Stand Density and Structure

Ground applications of herbicides require a site that machinery or ground crews can move across with reasonable ease. When applying foliar or soil-active products at a rate per unit area, machinery such as rubber tired skidders or track vehicles must be capable of moving at a uniform speed. Such applications are practical only on sites with few hardwoods left after harvest.

Backpack applications for site preparation or pine release are affected by stand density and height. Stands with a high density of hardwoods and other understory growth are difficult to traverse and are costly in terms of treatment time. Application is also difficult if target stems are greater than five to six feet in height. Controlled burning can sometimes open the stand and kill stems to the groundline, where after resprouting they can be treated with relative ease.

Aerial applications are affected by stand structure. Lower components of multistory canopies can be difficult to cover with foliar treatments due to shielding from taller trees. Partial solutions to this problem include increasing total gallons of spray per acre, using solid formulations or products with both foliar and soil activity.

Sensitive Areas

The location of sensitive areas such as crops, endangered species, water sources, wetlands, grazing lands, dwellings and public areas can have a direct effect on application method as well as herbicide selection. Movement of herbicide from the target area to any of these is a label violation subject to litigation. This can occur through drift of fine spray particles during application, movement of volatilized product during or after application, and movement of products in surface water. Potential problems can be avoided by using proper equipment and selecting products that do not volatilize or move with surface water.

Aerial applications should be used cautiously and avoided completely in high risk areas. Applications should be conducted under ideal weather conditions (wind speed less than 5 mph, and temperatures lower than 85 degrees Fahrenheit). Nozzles that deliver herbicides in large droplet sizes should be used in combination with drift control agents. Also, the presence of a helicopter or fixed wing aircraft near any sensitive area is likely to draw considerable attention to a spray operation regardless of risks. Advance notification of the operation to the local population can be used to determine any objections. In such cases alternative application methods or mechanical treatments may be advisable. Foliar ground applications are generally less prone to drift than aerial. However, similar precautions should be used. At times it may be advisable to treat buffer areas surrounding a tract by hand or other ground methods and broadcast treat only the interior portion of the site.

Soil-active products in liquid or granular form can be applied from the ground with little risk of drift. Products that will move with surface water should not be used on sites that drain into water supplies or any other sensitive area.

Wildlife Habitat

Land managers should be aware of possible impacts of herbicides on wildlife. Wildlife is often featured as a forest management objective, and certain animal as well as plant species are protected under the Endangered Species Act.

Normally, impacts on wildlife due to direct exposure to forestry herbicides are negligible. Forestry herbicides are not highly toxic to animals, are applied at relatively low rates, and do not persist for long periods in the environment. The reader is referred to Cantrell and Hopper (1989) for a more detailed discussion of this topic.

Herbicides impact wildlife primarily by affecting habitat. This impact may be positive or negative. The degree of habitat modification depends upon the component of vegetation at which the treatment is directed and the type of application. Some herbicides primarily control woody species, some control herbaceous grasses and forbs, while others can temporarily eliminate nearly all vegetation on a site when broadcast at highest labeled rates. Even in the most extreme case the effects are temporary. Sites treated with broadcast applications generally revegetate within a year after treatment. The land manager should select treatments based on his objectives. Herbicide applications can be manipulated to favor wildlife. Selective application methods can be used to create habitat diversity and enhance wildlife food and cover. The reader is referred to Hurst (1989, 1988a, 1988b) and Witkowski, et al, (1988) for additional information.

Site

Preparation

Effective site preparation prior to planting pines will strongly impact the overall species composition of the developing stand. Herbicides can be used to implement a wide range of site preparation intensities. Excellent vegetation control can be achieved with broadcast applications at labeled rates of several broad-spectrum products. Lower rates and/or selective ground application methods are available for less intensive vegetation control. The user should base product, rate, and application selections on management objectives. The following section of this manual will help with the technical aspects of the decision process.

Table 1
Herbicides Registered for Forest Site Preparation
in the Southern United States (1989).

¹Velpar ULW is available only as a package deal in which the herbicide dealer prescribes the rate, contracts the application, evaluates results and retreats if necessary.

²OneStep is a prepackaged mixture of 8.36% imazapyr and 22.13% glyphosate.

³For injection only.

Table 2
Herbicide products registered for forestry site preparation,
crop tree species for which they are labeled, and recommended
time intervals between application and planting.

¹Time intervals between application and planting are not specified on several labels. Applications early in the growing season generally provide for a sufficient time interval for dissipation of the product prior to planting the following planting season.

²The time interval is rate dependent; see label for details.

Table 3
Susceptibility of woody species to various site preparation herbicides.¹

¹Information in this table is based on published data and on personal communication with various vegetation management specialists. As with all “susceptibility” tables, caution must be exercised when using this information. A concentrated effort was made to only include weed and crop species that are consistently controlled by, or consistently “escape” a particular herbicide.

²Highly susceptible indicates that the weed or crop species is very sensitive to the herbicide indicated at labeled rates; resistant indicates that the weed or crop species will be relatively unaffected by the herbicide indicated at labeled rates.

³Escort (metsulfuron) is labeled as a tank mix with either Arsenal Applicators Concentrate (imazapyr) or Accord (glyphosate) in order to improve control of species listed as “highly resistant.” Escort by itself is not recommended for broad-spectrum woody brush control.

Herbicide

Characteristics

Affecting Prescriptions

Table 4
Application methods used to apply herbicides registered for
forest site preparation in the southern United States.¹

¹Application methods represent those listed on product labels. Generally, other methods can be legally used unless the label specifies otherwise.

Release

Release is a silvicultural treatment used to reduce the level of hardwood competition in young pine stands. The objective is to reduce pine mortality in situations where dense brush is overtopping pines and/or increase pine growth by controlling competition. The necessity for pine release often results from less than adequate site preparation, and hardwood control is generally more difficult once pines are established on the site. Hardwoods can be selectively controlled with certain herbicides that have greater activity on hardwoods than pines. Although selective broadcast applications of soil or foliar active products offer convenience, the user should be aware that pines are likely to sustain a certain amount of injury. The objective is to injure hardwoods to a greater degree so that pines can recover and dominate the site.

In addition to selective products, certain nonselective products are registered for directed spray applications with backpack sprayers. All of these products are described in this section. The user should pay close attention to details regarding pine selectivity, labeled application methods, and crop tree species for which individual products are labeled.

Table 6
Herbicides registered for pine release in the southern United States (1989)

¹Registered for directed sprays only.
²Registered for basal bark sprays.
³Registered for frill and injection.

Table 7
Herbicide products registered for conifer release and
crop tree species for which they are labeled.

¹Do not apply broadcast treatments to stands less than 3 years old.

²Products are applied only as directed sprays with backpack equipment. Application to crop trees will cause injury or mortality.

³Broadcast applications are registered for loblolly, shortleaf, and Virginia pine. Grid applications are registered for loblolly, longleaf, slash, and shortleaf pine.

¹Application methods represent those listed on product labels. Generally, other methods can be legally used unless specified otherwise on the label.

The practice of controlling herbaceous weeds in young pine plantations is fairly new. It has gained a means of increasing pine survival following regeneration and for accelerating early pine growth. Although these benefits are well documented and appear impressive, the utility of herbaceous weed control is somewhat limited. Herbicides registered for this use are primarily effective against annual grasses and forbs that are commonly pioneer invaders of newly disturbed sites. Applications work best in conjunction with intensive site preparation methods that expose mineral soil. Some of the products are also effective for converting former agricultural fields to pine. Products must be applied accurately by air or with agricultural boom type sprayers that are operable only on sites cleared of obstructions. Thus, herbaceous weed control is often not practical on forest sites with residual hardwoods from the previous stand, or on sites with herbicide resistant grasses and forbs.

The following information should provide general guidelines on how to use registered products appropriately.

Table 11
Herbicides registered for herbaceous weed control in
pine outplantings in the southern United States.

1Oustar is prepackaged mixture of 63.2% hexazinone and 11.8% sulfometuron methyl.

Herbicide

Characteristics

Affecting Prescriptions

Most products registered for herbaceous weed control are applied at a rate per unit area of surface treated, for example, pounds or ounces per acre. Most often they are applied in bands or strips centered over crop tree rows. These must be accurately applied with a well calibrated boom type sprayer. The only exceptions are Accord and Poast, which can be direct sprayed around seedlings.

Table 14
Recommended herbicide treatments for herbaceous weed control
in young pine plantations.

This table was printed by permission of the Auburn University Silvicultural Herbicide Cooperative, School of Forestry, Auburn University, AL 36849-5418. Prescriptions in the table are from AUSHC Information Note 95-3. Recommendations are based on labelled rates, studies reported in the literature, and experience with current operational forestry practices. Other combinations may be available. These rates will usually work under the specified conditions and research has shown them to provide increases in growth and/or survival under many but not all conditions. Rate recommendations do not supercede label restrictions - read and follow label instructions before application.

¹Highest range rate provides best weed control. Combinations are usually prescribed using a high rate for one herbicide with the low rate for the other.

²Up to 10% loblolly mortality could occur at high range of this rate.

³Higher rate recommendations for established vegetation may cause some pine damage in order to control herbs.

⁴Apply in August as a pre-plant treatment.

⁵Soils commonly found in the coastal plain of the southeastern United States that are characterized by ashy gray sands over dark sandy loams. A soils expert should be consulted for positive identification.

• Cantrell, R. L. 1985. A guide to silvicultural herbicide use in the southern United States. Auburn University School of Forestry, Alabama Agric. Exp. Stn., Auburn, AL. 592 pp.
• Cantrell, R. L. and G. M. Hopper. 1989. Forest herbicides are safe to use—Here’s why. Ren. Res. Notes. Univ. Tennessee Agric. Ext. Serv. SP 368. 8 pp.
• Hurst, G. A. 1989. Forestry chemicals and wildlife habitat. Forest Farmer. 48:10-11.
• Hurst, G. A. 1988a. Vegetation following imazapyr for site preparation. South. Weed Sci. Proc. 41:201. Hurst, G. A. 1988b. Vegetation responses to hexazinone for site prep. South. Weed Sci. Soc. Proc. 41:210.
• Miller, J. H. and R. J. Mitchell, 1988. A manual on ground applications of forestry herbicides. USFS Mgmt. Bull. R8-MB21.
• Walstad, J. D. and F. N. Dost. 1984. The health risks of herbicides in forestry: a review of the scientific record. Special Pub. 10. College of Forestry, Oregon St. Univ. 60 pp.
• Williamson, M. and J. H. Miller. 1987. Hand application methods for commonly used forestry herbicides in the south. USFS, Atlanta, GA. 15 pp.
• Witkowski, C. A., G. P. Sick and J. D. Fenwood. 1988. Herbicide risk to wildlife—a new look. U. S. Poster Presen., SAF Nat. Conv.; Rochester, NY. 9 pp.
  

Appendix C
Herbicide Terminology

Appendix D
Chemical Companies