Preemergence Herbicides

Preemergence herbicides (e.g., oryzalin, pendimethalin, and prodiamine) are applied to the soil surface and then are incorporated by rainfall or irrigation. One-half to one inch of water is required to move these herbicides into the top layers of the soil where they kill germinating seeds of both annual and perennial weeds. Most preemergence herbicides recommended for use in landscapes have limited mobility in soils and do not move into the root zone of established weeds or ornamental trees and shrubs. Thus, they are ineffective on emerged weeds and should be applied prior to anticipated flushes of weed emergence (e.g., winter rains or summer monsoons). Some preemergence herbicides with moderate soil mobility should be used cautiously on sandy soils to avoid injury to sensitive ornamental plants. Preemergence herbicides should be applied to a clean soil surface free of organic debris for best results.

Soil sterilants

(e.g., imazapyr and prometone) are a subgroup of preemergence herbicides that are distinguished by their great soil mobility, long-term persistence in the soil, and ability to kill all plant growth. These chemicals move downward and laterally with water movement in soils and will injure or kill desirable plants. Nearby trees and ornamental plantings can be injured or killed when roots grow into soil treated with soil sterilants. In general, these soil sterilants cannot be used safely where desirable vegetation is grown and are not recommended for use in urban landscapes. There is no antidote for plants affected by soil sterilants and it is not possible to inactivate sterilants already in the soil. Time, rainfall, and irrigation over several years will gradually degrade the chemical. Homeowners should never use a chemical sterilant without consulting the Cooperative Extension Service or a professional landscaper. One widely available product that causes problems for some homeowners is Ortho's Ground Clear Triox® Total Vegetation Killer₁ with contains 0.25% glyphosate and 0.08% imazapyr. Ortho's product literature clearly states that this product "Eliminates unwanted vegetation out of driveways, walks, patios and fence rows". This Triox formulation is appropriate for this use as long as there are no desirable plants in these areas and water does not move the herbicide into areas where desirable plants are growing. Some homeowners mistakenly use this product underneath and around desirable vegetation which is subsequently injured or killed by imazapyr.

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