Section 2: Variables Affecting Herbicides
Anchor: #i1021960Factors
Herbicide activity may be affected by many factors including:
- species of plant being treated
- soil type
- wind
- humidity
- rainfall
- temperature
- water quality
- mixing and application procedures and timing;
- maintenance supervisor
- individual herbicide applicator.
Type and Species of Plant to be Controlled
Proper selection of herbicides and proper application rates are dependent upon the type and species of vegetation to be controlled, as well as the condition of the plant species itself. Some plant species are more resistant to certain herbicides than are other more sensitive plants. The plant may be in either an active growth or a dormant growth stage. The plant may be a seedling or a mature plant. The plant may be in the process of budding, leafing, flowering or fruiting. Each of these conditions has a bearing upon how herbicides work and should be considered when deciding where and when to use herbicides.
For example, the best time to apply a foliar herbicide for the control of Johnson grass is when the species is in the early seed head or fruit production (“boot”) stage. This applies to most other pest species as well. In general, seedling plants (immature plants just emerging), are much easier to control than are older, more established plants.
Plants are categorized as annual, biennial or perennial.
Anchor: #i1022045Annual and Biennial Plants
Both annuals and biennials originate from seed (not from permanent rootstock). Annuals complete their entire life cycle in one growing season or year (seed to seed); biennial plants require two years to complete their life cycle.
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Perennial plants have extensive root systems and live for many years. Perennials produce seed as well to ensure survival of the species. Generally, higher rates of herbicides will control perennial plant species.
Anchor: #i1022065Aquatic Plants
Aquatic plants, which may be annual, biennial or perennial, are associated with water or wetland areas, and typically have large, glossy, waxy leaves or sometimes a hairy surface. For aquatic plant control, TxDOT uses only approved aquatic herbicides. These herbicides have been approved by the EPA for the broad-spectrum control of vegetation within or adjacent to aquatic sites. An aquatically approved surfactant must be added to the approved aquatic herbicide. Approved aquatic herbicides contain 54 percent glyphosate and 46 percent water (i.e. Rodeo and Aquamaster). All products used by TxDOT herbicide applicators have been extensively tested and researched by the Vegetation Management Section before being released for use.
Anchor: #i1022075Soil Type
Proper application rates as recommended within this document normally produce excellent results. Soil type, however, has an effect upon how well all herbicides work. Soil-active (residual) herbicides are more effective in soils low in clay or organic matter. In high clay soils, herbicide molecules attach (adsorb) to the clay particles and are not available for plant uptake. In heavy clay soils, therefore, the application rates may need to be increased.
The acidity or alkalinity of the soils also has an affect upon the performance of herbicides. For example, in relatively acidic soils, often found within parts of East Texas, Oust XP and Outrider decompose at a faster rate (and thus provide a shorter residual period), than they do in the more alkaline soils commonly associated with central and West Texas.
Anchor: #i1022093Wind Velocity
Wind will always disturb the spray pattern and blow chemical away from the target area. High winds can blow it several feet away from the target. The wider the pattern, the greater the effects of wind distortion. The best time to spray is before wind velocity rises (i.e., early in the morning). The addition of the proper drift control will help reduce drift. Drift control is relatively inexpensive and in many instances can reduce spray drift sufficiently for spraying to continue. However, if wind velocities rise to the point that the pattern cannot be kept on target, spraying should be discontinued.
For purposes of deciding whether to spray (and for record keeping), always carry a wind gauge in the spray unit.
Winds over five mph may cause drift. Spraying operations should cease when winds are of such intensity that you cannot keep spray on the target.
Anchor: #i1022113Humidity
Relative humidity is defined as the percent of moisture in the surrounding air relative to the maximum amount the atmosphere could hold. Generally, the higher the relative humidity at the time of application, the quicker plants will absorb foliar-applied herbicides. However, when relative humidity approaches 100 percent, rainfall will likely occur and the chance that the herbicide will be washed from the foliage increases. Consequently, herbicides should not be applied when rainfall is imminent.
Anchor: #i1022123Rainfall
Rainfall affects chemical control of vegetation in many ways. Rainfall acts as the vehicle carrying soil-active (residual) herbicides into the root zone of plants. To enter the root system of the target species, residual chemicals must be in solution. Excessive soil moisture may move (leach) the residual herbicide below the root zone and result in poor control. Moisture from rainfall, thawing cycles and snow may prevent the herbicide from entering the soil in concentrations necessary to achieve the desired degree of control. Moreover, excessive rainfall may lead to serious herbicide damage to vegetation outside the target area.
Do not spray herbicides during rainfall or when rainfall is predicted within one to two hours after application. Rain will wash herbicide off the foliage of target species before it can be absorbed by the plant. Conversely, after a rain, dust on the foliage will have been washed off and foliar-applied herbicides can be easily absorbed by the plant. Allow the foliage time to dry after a rain before conducting spraying operations as wet foliage may also yield poor results.
Anchor: #i1022138Temperature
Temperature affects the action of herbicides and the degree of control achieved as well. Do not apply herbicides when the soil is frozen, when rain or snow is falling or when there is snow on the ground. High temperatures, typical during the hot and dry summer months, cause many plant species to become dormant. When dormant, plants will not absorb the herbicide and control will be reduced.
Anchor: #i1022148Water Quality
Water is an important factor affecting the action of all herbicides, especially Roundup Pro. Always use the cleanest water available to mix herbicides, as impurities and soil particles in the water will bind or “tie up” the herbicide molecules and deactivate the chemical. Additionally, Oust XP and Outrider herbicides decompose faster in acidic water. For those instances where acidic water must be used, mix smaller batches to prevent these herbicides from becoming ineffective.
Impurities in the water, such as sand or clay particles, may also clog filters and damage centrifugal pumps, electric solenoids and nozzles.
Anchor: #i1022163Application Methods and Timing
Proper application methods and timing are essential to the success of any spraying program. Both residual and foliar-applied herbicides must be applied uniformly. Foliar-applied herbicides must be applied uniformly to the foliage surface of the target species.