The reason Pre Emergents are so important right now is that your lawn is in a race. Not against time but against the ground temperatures. There is not a specific time that the summer weeds will start growing. Instead the germination and growing process is based off the weather and more importantly the ground temperatures. Once the soil reaches an average of 55 degrees the seeds will start their life cycle. If you get your pre emergents applied before this happens you will stop this process and prevent the weeds from growing this summer.