Unfortunately, pre-emergent treatments aren't 100 per cent effective. Pre-emergent applications are applied uniformly over the entire yard directly on dirt and rock landscaped areas to control weed seeds before they sprout (emerge). This material is designed to stop the seeds from germinating, but does not affect pre-existing or established plants. Any vegetation or weeds that already are present will not be harmed by the pre-emergent application.
Most manufacturers labels state that an application of a pre-emergent will last for 6 months. We recommend treating your yard twice per year, at least 6 months apart.
You can still have some weeds, because at any given time, there can be hundreds of thousands of seeds on your property, just waiting for the right amount of rain, light and temperature to start growing.