HERBICIDES IN CALIFORNIA FORESTRY Joseph M. DiTomaso University of California, Davis Few herbicides are available for use in forestry. The compounds most commonly used include 2,4-D, triclopyr, glyphosate, atrazine, hexazinone, and sulfometuron. One additional compound, imazapyr, is currently in the registration process in California and is expected to obtain a forestry label by late 1997 or early 1998. Based on their physiological activity in plants, these seven compounds can be classified into 3 categories, growth regulators (2,4-D, triclopyr), photosynthetic inhibitors (atrazine, hexazinone), and amino acid inhibitors (imazapyr, sulfometuron, glyphosate). The physical properties of each of these herbicides is included in Table 1. Among these compounds, those that are categorized as weak acids are more likely to adsorb to soil particles in more acid conditions. In contrast, these compounds will exist as anions in alkaline soils and will leach more readily through the soil profile. With the exception of the ester formulations of2,4-D and triclopyr, all other forestry herbicides have very low vapor pressures and, consequently, are not likely to volatilize during or after application. Those that are esters can cause rapid injury to the treated foliage when air temperatures are above 75 to 80°F. This rapid injury can damage the translocating tissues in the plant and prevent movement of the herbicide to underground portions of the plant. This will result in poor control of resprouting trees and shrubs. Those compounds which have a soil half-life of over 1 month will provide good to excellent preemergence control of weeds. These include atrazine, hexazinone, sulfometuron, and imazapyr. Glyphosate, 2,4-D, and triclopyr are considered postemergence herbicides with little or no soil residual control of weedy species. The mode of action of forestry herbicides used in California is outlined in Table 2. Those herbicides which are translocated via the xylem are typically soil-applied and are primarily absorbed by the roots system. Phloem mobile herbicides are more effective when applied to the foliage. Two herbicides, sulfometuron and imazapyr, move easily in both the phloem and the xylem and, thus, tend to be very mobile in plants. The formulation of an herbicide can determine the application technique (Table 3). For example, ester formulations oftriclopyr and 2,4-D are effective as basal bark treatments, whereas amine formulations of the same herbicide are not applied to bark. Some herbicides, such as triclopyr, imazapyr, and to some degree glyphosate, are quite versatile and can be applied using many different techniques. While some herbicides are primarily used for site preparation.(e.g., hexazinone), others are most effective for conifer release (e.g., imazapyr, 2,4-D). Many herbicides registered for forestry use are non-selective and control grasses, as well as herbaceous and shrubby broadleaf species. These include hexazinone, glyphosate, and imazapyr. Others are often considered non-selective, but only control herbaceous species (atrazine, sulfometuron), and yet others are selective on only broadleaf species, both herbaceous and woody (triclopyr, 2,4-D). Nearly all forestry herbicides damage conifers when applied directly to seedlings in the non-dormant stages. A susceptibility chart. is provided for these seven herbicides on several problematic weeds of forest plantations (Table 4). This list was compiled by William et al. (1996) for work conducted in Oregon. The response of some of these species may differ in California. Consequently, the list should be used as only a rough guide. 113 Table 1. Physical, soil, and toxicological char~c;teristics of forestry herbicides used in California. Characteristic atrazine hexazinone 2.4-D tricloovr 2lvohosate sulfomet . - Trade name Aatrex Velpar Weedone & Garlon 3A Accord Oust Arsenal Pronone others Garlon 4 Roundup Pathfinder II Chemical family Triazine Triazine Phenoxy Picolinic none Sulfonylurea Imidazolinone acid Chemical type neutral neutral weak acid weak acid weak acid weak acid weak acid Volatilization low low low- amine, low- amine, low low low high - ester high - ester '"""" '""""~ Solubility in water 33 33,000 900- amine 2,100,000 - 15,700 300 11,272 (ppm) 100 - ester amine 23 - ester Leachability slow rapid very little very little none slow moderate Soil decomposition slow slow rapid rapid none slow slow rate Soil half-life 2 months 2-3 months 10 days <l month none >l month 1-5 months LDso rats (mg/kg) 3,090 1,690 764 713 5,600 >5,000 >5,000 Table 2. Mode of action of forestry herbicides used in California. Characteristic atrazine. hexazinone- - 2.4-D- tricl------ - lvohosat ---lf1 - t - ' .. -- ---- ------ . - Trade name Aatrex Velpar Weedone& Garlon 3A Accord Oust Arsenal Pronone others Garlon 4 Roundup Pathfinder II Absorption mainly roots, mainly roots, mainly mainly foliar only mainly roots, both roots and some foliar some foliar foliar, some foliar, some some foliar foliar roots roots Translocation xylem xylem phloem phloem phloem both xylem both xylem I-" and phloem and phloem I-" V'1 Mechanism of action photosynthesis, photosynthesis, growth growth aromatic branched- branched- D 1 protein on D 1 protein on regulator, regulator, amino acid chain amino chain amino PSII PSII nucleic acid nucleic acid synthesis, acid synthesis, acid synthesis, synthesis synthesis EPSP ALS (AHAS) ALS (AHAS) synthase Breakdown in plants varies with varies with slow rapid very slow varies with varies with species species species species Table 3. Formulations, application techniques, timing, selectivity, and phytotoxicity of forestry herbicides used in California. Herbicide Formulation Application technique Timing Selectivity Phytotoxicity to conifers atrazine Aatrex - liquid broadcast applications by spring or late fairly non-selective toxic to conifers in flowable, needs ground summer to early fall for herbaceous non-dormant stages agitation weeds, not effective on woody species hexazinone Velpar- water aerial or ground as PRE late winter to early non-selective, tow toxicity to many dispersable liquid, or POST, spring in rainbelt, herbaceous and conifers, including Pronone- granular stem injection fall before soil woody species ponderosa pine and freezes or spring in Doug fir snowbelt 2,4-D both water soluble aerial or ground direct foliar during broadleaf safe on Doug fir during amine or ester broadcast or direct foliar, late spring or early herbaceous and dormant season, toxic basal with ester, cut summer woody species to most other conifers ........ stump, hack and squirt ........ triclopyr Pathfinder II and aerial or ground foliar late summer to early broadleaf toxic to conifers in °' Garlon 4 - ester, treatment, direct spray, fall after conifers herbaceous and non-dormant stages, Garlon 3A-water basal bark, thin line, harden off, basal woody species esp. 2-3 needle pines soluble amine dormant stem, cut stump almost anv time glyphosate Accord and aerial or ground foliar late summer to early non-selective, toxic to conifers in Roundup - water treatment, direct spray, fall herbaceous and non-dormant stages soluble cut stump, stem injection woody species sulfometuron Oust - dispersible aerial or ground PRE or fall or spring non-selective on toxic to conifers in granules, needs POST treatment herbaceous species, non-dormant stages, agitation limited shrub some tolerance in control Doug fir, no injury to redwood imazapyr Arsenal - water aerial or ground foliar, late season, almost non-selective, both best on Doug fir, toxic soluble direct or spot spray, year round for herbaceous and to most other conifers isopropylamine possible broadcast, cut directed, cut stem, or woody species salt stump, hack and squirt, dormant treatments stem injection Table 4. Susceptibility of forestry weeds. From William, RD. et al., 1996. Pacific Northwest Weed Control Handbook. Oreson State Universiti:· Corvallis, OR. SJ!ecies atrazine hexazinone 2z4-D triCloJ!lr &!ll?hosate sulfometuron imazaelr Conifers Douglas fir R R 1-R I R 1-R l True firs I-R R S:-1 I R 1-R l Pines R R S-1 s 1-R 1-R l Shrubs Alder l 1-S s l 1-R s Blackberry l R s s l l Ceanothus spp. I s s 1-R s Cherry R s I s s Chinquapin R I s 1-R Elderberry R R s s s Madrone R I s 1-R s Manzanita spp. R s R R Bigleaf maple R R I-S I R s Vine maple R R 1-S s R s Poison-oak R R s s s Tanoak R I 1-S R R l Thimbleberry I I l s l l Herbs Bracken fem l l R R s s Annual grasses s s R R s s s Perennial grasses S-R s R R s s s Broadleaf herbs I S-1 s s s s s R = resistant to highest rate. I = intermediate or variable, S = sensitive, killed by medium or low rates. Literature Cited William, R.D., D. Ball, T.L. Miller, R. Parker, K. Al-Khatib, R.H. Callihan, C. Eberlein, and D.W. Morishita. 1996. Pacific Northwest. Weed Control Handbook. Cooperative Extension. Washington State University and the University ofldaho. 378 p. 117 .