Tree Planting and Care No. 15 UI Extension Forestry Information Series

Forest and Their Mode of Action Randy Brooks

Herbicides are currently one of the most widely used enhancing the toxicity of the material. Consult the label forms of weed control in tree plantings. Most forest for forestry applications, as some conifers may be weeds are not controlled as well by non-chemical affected by the addition of a surfactant. methods (scalping, burning, etc.) as by herbicides. The new silicon surfactants (e.g. SYLGARD 309) Herbicides have several advantages over other weed have been developed to specifically enhance the control methods due to selective and rapid control, performance of . These surfactants have retention of surface nutrients and organic matter, and their greatest effect when combined with water- the residual dead mulch that conserves soil moisture soluble, postemergent herbicides. Silicone surfactants and inhibits re-invasion of weeds. They are especially have the ability to penetrate leaf surfaces in less than effective on tough perennial weed species such as one minute. applications made prior to quackgrass, bromegrass, and Canada thistle. Selecting sudden rainstorms are no longer rendered useless. a herbicide will vary with species and growth habits of Some herbicides now come with SYLGARD already both the weed and the tree. Problem weeds should be pre-mixed. controlled during site preparation. They can be more difficult to control once trees are planted. Major Herbicides Labeled for Forestry in the Inland Northwest. The following herbicides are Many herbicides are labeled for use in tree plantings, labeled for forestry applications in the Inland North- and for site preparartion and general use in forested west. Their registered names are in italics and trade situations. Some herbicides can be applied directly names are in parentheses. Mention of specific chemi- over some trees. Direct applications can be harmless cals and trade names does not imply endorsement. to some trees at any stage of growth, harmless to others only when they are dormant, and lethal to some • (Velpar or Pronone) – Hexazinone is species at any time. Herbicides are very specific and a contact herbicide in its liquid form (Velpar) and a nonlabel applications are not only illegal, but may be residual herbicide in liquid and granular form lethal to crop trees and/or ineffective on weeds. Some (Pronone) which can be foliar and/or soil applied. herbicides must be applied as a directed spray to It affects the plant by inhibiting photosynthesis. It minimize or avoid spray contact with the trees. Herbi- controls broadleaf weed, woody species, and cides are used in various stages of tree growth. Know grasses, and is used for selective weed control in the existing weeds and use herbicides that control conifers, and for conifer release. It can be applied weeds while being safe for desired trees and shrubs. when target plants are actively growing. Dormant Be sure to read and follow the directions on the label. season application of Pronone granular over sensitive conifers will provide residual, soil active Surfactants. Surfactants (or adjuvants) are used to control for the next 1-2 growing seasons. enhance herbicide performance and are sometimes necessary for control of specific weeds. A surfactant • Imazapyr (Arsenal, Chopper, or Contain) – This enhances the coverage of a sprayed-on herbicide by is a nonselective, broad spectrum systemic herbi- reducing the surface tension of spray droplets and allowing greater contact and penetration, CONTINUED ON PAGE 2 cide with residual soil activity. It’s action is to • (Tordon) – Picloram is a restricted use inhibit enzymes used in synthesis of some amino herbicide. It is a highly translocated, selective acids. Imazapyr is readily absorbed through foliage herbicide for broadleaf weeds and woody plants. or roots. It is used to control most annual and It is active through foliage and roots and has a long perennial grasses, broadleaf weeds, and woody persisitence in the soil, requiring precautions to species. It can be applied pre- or postemergence avoid damage to desirable plants. for long-term total vegetation control on non-crop • (Rodeo, Accord, Roundup) – lands. Glyphosate is another one of the more common • Clopyralid (Stinger, Transline) – This is a highly herbicides. It acts in a plant by inhibiting amino translocated, selective herbicide. Control is acid production and protein synthesis. It is nonse- achieved by causing the plant to overexpend lective and used to control grasses, broadleaf, and energy by producing excess hormones. These woody plants. It is absorbed through the foliage chemicals are absorbed through a plant’s foliage. It and translocates to the roots, but has no apparent is used postemergence on broadleaf herbaceous soil activity. Because of this, it is applied weeds, mostly on Asteraceae, Fabaceae, and postemergence only and requires substantial foliage Polygonacae families, and does not harm conifers. contact for full effectiveness. Rodeo and Accord It provides excellent control of Canada thistle, but may require the addition of a surfactant. does not control grasses or sedges. • (Aatrex) – Atrazine is another restricted • Metsulfuron (Escort, Ally) – This is a selective, use herbicide that inhibits photosynthesis in the postemergence herbicide used at low rates to targeted species. It is a selective herbicide in control broadleaf weed and brush in non-cropland agriculture, but is used nonselectively in non-crop areas. It works within a plant by interfering with an areas. It is primarily root absorbed, although some enzyme which results in rapid cessation of cell foliar absorption does take place. Due to it’s long division in both roots and shoots. Escort is used for residual activity in the soil it provides season-long selective broadleaf weed and brush control in control for weeds, but precautions must be taken pastures, rangeland, and noncropland. It can be to avoid damage to desirable plants. used for site preparation or conifer release. • (Princep) – Simazine is also a restricted • (Garlon 3a) – This is a systemic, use herbicide that inhibits photosynthesis in the growth-regulating herbicide used to control woody targeted species. Similar to Atrazine, it is used and broadleaf perennial weeds in noncropland, selectively in agriculture. It provides complete forestland, range, permanent grass pasture, and vegetation control (broadleaf and grasses) in non- right-of-ways. It also mimics natural plant hor- crop areas when used at higher rates. It has long mones. soil residual activity (>1 year), but requires lots of moisture in the soil to activate it. • 2,4-D – Probably the most commonly used of all herbicides, 2,4-D is a selective herbicide used to • Sulfometuron (Oust) – Sulfometuron is a broad control annual and perennial broadleaf weeds. It spectrum herbicide. It acts in the plant to interfere mimics natural plant hormones. It is absorbed with an enzyme resulting in rapid cessation of cell through the foliage and translocated within the division. It controls grasses and broadleaf weeds plant. Applications are made postemergence. through foliar contact or root uptake, and can be Plants are most susceptible to 2,4-D when they are applied pre- or postemergence. It kills plants at young and rapidly growing. Actively growing low rates. Oust can be applied over the tops of conifers are very susceptible to 2,4-D. conifers if they are not lacking in vigor (stressed).

CONTINUED ON PAGE 3 • Pathway (Picloram+2,4-D) – Pathway is sold as and follow the directions on the label, and check to a combination of Picloram plus 2,4-D and has a make sure the chemical you are buying is registered forestry label. It can be applied on cut surfaces for your intended use and that you or other applicators (stumps) or injected under the bark of woody are licensed where required. species where it is translocated to the roots. Timing and rates of herbicides applications are very This information first appeared in Woodland NOTES, Vol. important. Timing depends on the herbicide being used 10, No. 1. and its persistence, along with other characteristics About the Author: Dr. Randy Brooks is an Area Extension discussed in the last issue. Remember, always read Educator - Forestry and Professor at the University of Idaho.

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