Technical Abstract: The timing of weed seedling emergence relative to the crop is important in planning and optimizing the time of weed control, but very little work has been done to predict seedling emergence of tropical weed species, especially in low input and small-scale farms. We developed a simple model based on hydrothermal time to predict seedling emergence of tropic ageratum. Hydrothermal time at 2 cm soil depth was calculated from soil moisture and soil temperature simulated from several micrometeorological and soil physical variables. The model was developed using five years of field emergence data from a continuous corn/cassava production system in southwestern Nigeria. Percentage cumulative seedling emergence from the five-year data set was fitted to cumulative soil hydrothermal time using a Weibull function. The predicted cumulative emergence curve significantly matched observed field emergence (r**2 = 0.83). The model was validated with field emergence data from southeastern Nigeria (r**2 = 0.85) and Los Banos, Philippines (r**2 = 0.89). With such models, extension personnel working on tropical soils, especially in West Africa, may be able to provide additional advice to farmers on the appropriate time for the management of tropic ageratum.