Herbicide Classification Herbicides are classified according to timing and method of application and chemistry. Select a proper herbicide based on weed sensitivity as well as turfgrass tolerance, and also when and how you want to control the weed population.

Herbicide Classification Foliar Absorbed Herbicides Absorption of the active ingredient occurs in leaves. Postemergence (POST) control only Examples: glyphosate, 2,4-D, dicamba Root Absorbed Herbicides Absorption of the active ingredient occurs in roots. Preemergence (PRE) or sometimes POST control Examples: DNA crabgrass herbicides, atrazine

Herbicide Classification Selective vs. Nonselective Herbicides Selective herbicides control or suppress certain plants without negatively affecting the growth of other plants. Selectivity may be due to differential absorption, translocation or physical (morphological) differences between species.

Herbicide Classification Selective vs. Nonselective Herbicides Selective herbicides may be detoxified in plants or just not converted into its active ingredients. Examples: 2,4-D, mecoprop, dicamba control many broadleaf weeds but many turfgrasses are tolerant. PRE crabgrass herbicides are safe on many turfgrass species also.

Herbicide Classification Selective vs. Nonselective Herbicides Nonselective herbicides control plants regardless of species. Examples: Glyphosate, glufosinate and paraquat are nonselective, foliar absorbed herbicides. (Note – glyphosate is selective to certain turf species at low rates)

Herbicide Classification Contact vs. Systemic Herbicides Contact herbicides kill only the portion of green tissue that is contacted. Therefore, uniform spray coverage and particle size are essential for adequate control. (the entire plant must be sprayed) Example: If a drop of paraquat (a nonselective, contact herbicide) were placed on a leaf tip, only the leaf tip would show injury symptoms.

Coverage with Granular vs Spray

Herbicide Classification Contact vs. Systemic Herbicides Contact herbicides do not kill perennial underground structures such as rhizomes, tubers and bulbs. Repeat applications to regrowth may eventually drain the plants’ underground resources, but it is better to try to control perennial weeds with systemic herbicides. Use contact herbicides on annual weeds.

Herbicide Classification Contact vs. Systemic Herbicides Systemic herbicides are extensively translocated in a plant’s vascular system from point of absorption (leaf or root) to sites of action. Example: If a drop of glyphosate (nonselective, systemic herbicide) were placed on a leaf tip, the entire plant would express injury symptoms due to translocation throughout the plant.

Herbicide Classification Contact vs. Systemic Herbicides Systemic herbicides are transported throughout the vascular system along with nutrients, water and organic materials necessary for plant growth. Systemic herbicides require days to weeks for total control. (herbicide must travel throughout plant) Systemic herbicides are more effective on perennial weeds than contact herbicides.

Herbicide Classification Contact vs. Systemic Herbicides Contact herbicide examples MSMA for POST grass control Bentazon for POST yellow nutsedge control Paraquat for nonselective weed control

Herbicide Classification Contact vs. Systemic Herbicides Systemic herbicide examples Glyphosate and glufosinate for nonselective weed control 2,4-D, mecoprop, dicamba for broadleaf weed control Atrazine and simazine for annual bluegrass and broadleaf weed control

Herbicide Classification Herbicide Timings Herbicides can also be characterized as… Preplant Preemergence Postemergence

Herbicide Classification Herbicide Timings Preplant herbicides (soil fumigants) are used in areas where a new turf stand is to be established. They are nonselective herbicides that kill desirable and undesirable plants! Soil fumigants kill seeds.

Herbicide Classification Herbicide Timings Preplant herbicide examples include… Methyl bromide (to be completely banned in 2005 because it is an ozone depleter) Metam-sodium Dazomet

Herbicide Classification Herbicide Timings PRE herbicides should be applied and activated prior to weed seed germination (either annual or perennial). PRE herbicides form a thin barrier at or right below the soil surface. Emerging seedlings contact the treated zone, absorb the herbicide, then die. PRE herbicides in turf are selective herbicides.

Weed Control With Preemergence Herbicides Herbicides do not prevent weeds from germinating, they kill weeds as they grow through the herbicide treated zone.

Weed Control With Preemergence Herbicides Large gaps in herbicide barriers result in weed escapes.

Herbicide Classification Herbicide Timings Weeds that have already emerged before application or activation are not affected by PRE herbicides because their primary growing point has escaped treatment. PRE herbicides must be activated with approximately 0.5 inch of irrigation or rainfall.

Herbicide Classification Herbicide Timings PRE herbicides are relatively immobile, persistent and nonvolatile in soils. However, if left on the soil surface for an extended period of time without activation by water, these herbicides can volatilize or photodecompose. PRE herbicides do not control perennial weeds emerging from vegetative structures.

Herbicide Classification Herbicide Timings PRE herbicides (especially crabgrass and goosegrass herbicides) are very effective as granular applications. Particle sizes are uniform and small enough to ensure adequate soil coverage as long as your sprayer is properly calibrated and you’re applying the correct amount!!!

Herbicide Classification Herbicide Timings PRE herbicides DO NOT kill seeds. They don’t keep seeds from germinating. They kill seedlings as they germinate! PRE herbicides kill weeds before they are visible. This lack of dead and dying weeds is more aesthetically pleasing.

Herbicide Classification Herbicide Timings POST herbicides are applied directly to the emerged weed, and are usually more effective on seedlings. Plants have two growth stages, vegetative (more easily killed when rapidly growing) and reproductive. POST herbicides CAN be used on grass and broadleaf weeds, but MUST be used on sedge and kyllinga species, wild garlic, and many other perennial weeds. (Yellow nutsedge is an exception; metolachlor provides PRE control).

Herbicide Classification Herbicide Timings POST herbicides usually require multiple applications for adequate control. They can be foliar or root absorbed, selective or nonselective, contact or systemic. Example: 2,4-D is a POST, selective, systemic, foliar absorbed herbicide.

Herbicide Classification Herbicide Timings POST herbicides are more effective when applied as liquid formulations rather than granular formulations (liquids stick to the weeds). If granular products are chosen, apply to wet foliage so the herbicide will stick to the weeds and not roll onto the soil where they are ineffective.

Herbicide Classification Herbicide Timings POST herbicides are more effective when… Applied to actively growing weeds Temperature range from 60 to 85 degrees Good soil moisture Applied BEFORE mowing Mowing is delayed 1-2 days after application

Herbicide Classification Herbicide Timings POST herbicides must penetrate a waxy leaf cuticle to get inside of the plant. When plants are actively growing, the cuticle is thinner allowing for easier penetration. When plants are drought-stressed, the cuticle becomes thicker to conserve plant moisture and herbicide penetration is more difficult.

Herbicide Classification Herbicide Timings POST herbicides usually should be avoided when temperatures exceed 90 degrees. The weeds could be drought-stressed and also the herbicide could become volatile and drift to nontargeted sensitive plants. Example: 2,4-D injures tomatoes, cotton, tobacco.

Herbicide Classification Herbicide Timings Delay mowing for several days prior to a POST application to allow for more available foliage for herbicide absorption and to ensure that the weeds are actively growing. Delay mowing for several days after a POST application to allow for herbicide translocation throughout the plant.

Herbicide Classification Herbicide Timings Do not apply POST herbicides in the rain or irrigate immediately after application. Depending on the individual label, herbicides can be rain-fast (inside the plant) in as little as one hour or as long as 12 hours. If a POST herbicide gets washed off to the soil, it becomes ineffective.

Herbicide Classification Herbicide Timings Herbicide retention on a leaf is affected by… Leaf size Leaf shape Leaf surface area Leaf angle

Herbicide Classification Herbicide Timings Good herbicide retention on a leaf occurs when… Leaf size is LARGE Leaf shape is WIDE Leaf surface area is NON-WAXY Leaf angle is HORIZONTAL (FLAT)

Herbicide Classification Herbicide Timings Poor herbicide retention on a leaf occurs when… Leaf size is SMALL Leaf shape is NARROW Leaf surface area is WAXY Leaf angle is UPRIGHT

Herbicide Classification Herbicide Timings Wild garlic and nutsedge species are examples of weeds that have narrow, waxy, upright leaves. Many herbicide particles tend to miss the plants entirely, or bounce or slide off the leaves. For POST herbicides to be effective on these types of weeds, spray adjuvants are needed.

Herbicide Classification Herbicide Timings Adjuvants are chemicals added to a pesticide formulation or tank-mix to improve mixing and application, or enhance pesticide performance. Check the herbicide label to determine if spray adjuvants are needed.

Herbicide Classification Herbicide Timings Spray adjuvants include… Surfactants Methylated seed oils Crop oil concentrates

Herbicide Classification Herbicide Timings Surfactants consist of several classes of chemicals that reduce the interfacial tension between water and solids (plant material) or other liquids.

Herbicide Classification Herbicide Timings Classes of surfactants Wetting Agents – used to increase the wetting capacity of water (make water wetter) Spreaders – allow pesticides to form a uniform layer or coating over the treated surface Stickers – cause pesticides to adhere to plant foliage and reduce spray runoff or washoff

Herbicide Classification Herbicide Timings Types of surfactants Anionic – negatively charged, readily leach from soil Cationic – positively charged, binds electrostatically to soil causing waterproofing Nonionic – no charge, best type of surfactant to use

Herbicide Classification Herbicide Labels It is important to read and understand the label of any pesticides that you use, not just herbicides!!! For example… PRE and POST herbicides generally should not be applied to newly seeded, sodded or sprigged turfgrass until it becomes well established.

Herbicide Classification Herbicide Labels Immature turfgrass seedlings can be affected by supposedly tolerant herbicides. Tolerance ultimately depends on root growth. This and other important information will be stated in the label. Individual labels will give their definition as to what is an established turfgrass.

Herbicide Classification Herbicide Labels Some PRE crabgrass and goosegrass herbicide labels state that applications should not be made to turfgrass seeded the previous fall. Examples: benefin, oryzalin, benefin + trifluralin

Trimec Classic – POST Broadleaf Herbicide Fescue seed can be sown 3 to 4 weeks after application Do not apply to newly seeded turf until 2 nd or 3 rd mowing Confront – POST Broadleaf Herbicide Do not reseed until 3 weeks after application Newly seeded turf should be mowed 2 to 3 times before application Millennium Ultra – POST Broadleaf Herbicide Do not apply to newly seeded turf until well established