威拔是三氮井系列的殺草劑，殺草作用機構為阻礙光合作用，主要是經由雜草的根部吸收，而當雜草在生長旺盛期間亦有甚顯著的接觸效果。雜草萌芽後至旺盛生長期間施用，對大部分的雜草都有顯著效果，在較低溫度下（7～10C），接觸作用較緩慢，土壤中本藥易被微生物分解，在土壤之表面上不容易被日光分解，半生期為1~6個月不等。

Velpar is a photosynthetic inhibitor. While the primary mode of entry is through the plant roots, there is a marked contact effect from application made during active plant growth. Optimal efficacy on most weed species is obtained when Velpar is applied after seedling emergence and during the early period of active plant growth. Contact effect is slower under cooler temperatures (3 to 4 weeks for contact kill at 7 to 10C). Rainfall is needd to move the active igredient into the root zone of weeds, If this does not occur the tougher weeds, particularly perennials, may recover from the contact effects and continue to grow. Low growing perennial weeds and woody vegetation are best controlled when Velpar is applied to the actively growing foliage at a time of reasonable rainfall to ensure root uptake as well. The primary mode of disppearance is decomposition by soil microbes, Velpar is not readily decomposed by sunlight on the soil surface. Degradation take place mainly after rainfall moves it into the soil. The half-life is 1 to 6 months depending on soil and climate.