Pre-emergent herbicides are typically used to prevent weeds from germinating and emerging in landscapes and gardens. The length of time that a pre-emergent herbicide will last depends on several factors, including the type of herbicide, the formulation, the application rate, the soil type, and weather conditions. Some pre-emergents can last for several months, while others may only last for a few weeks.
The longest lasting pre-emergent herbicides contain active ingredients like siduron, imazaquin, or oxadiazon. These herbicides can provide weed control for up to six months. However, it is important to note that even the longest lasting pre-emergents need to be re-applied after heavy rains or irrigation events in order to maintain their efficacy.