Variation in the life-history of weeds, for instance the timing of emergence or flowering, can determine whether management succeeds or fails to controlling crop losses and further weed seed inputs into a farming system. Greater knowledge about the timing of life-history events may help us more effectively utilize both chemical and mechanical control measures. Scientists and nature buffs have for some time recorded observations of when different species emerge and flower. Those prone to recording such observations are called “phenologists” from the Greek meaning “the study of appearance”. Although knowledge about the average timing of phenological events can be helpful in assessing appropriate management strategies, we are also interested in knowing if we can predict deviations from mean behavior on the basis of the weather patterns in any given season. One approach to basing phenological prediction from weather data is to use thermal-time (e.g. growing degree days) or hydro-thermal-time models for particular species.