This is a shout out to Urban Earth in Midtown Memphis, TN, and Germantown Hardware in Germantown, TN. Both are great places for all your DIY lawn and garden needs.
First, lets discuss what a pre emergent does exactly. Pre emergents create a barrier in the soil at the level where weed seeds germinate (if applied properly) that prevent SEEDS from creating roots which effectively kills the plant as the seed germinates.
Pre Emergent products do not simply kill seeds. This is an important distinction since it lets you know that the pre emergent must be present at the time germination happens or shortly afterwards. Typically, this means that the pre emergent application must be made before SOIL temperatures reach 55 degrees. This is different than AIR temperatures. So, in Memphis, you typically want to get your pre emergent down before March 15th. It does depend on the weather, though. With an exceptionally cold winter, for example, you have a little more time to get the products on your yard and effectively create a weed barrier.
One way to tell when the soil temperatures are nearing 55 degrees is when the forsythia bushes start to bloom. Make sure to apply by the time the forsythia blooms or shortly after. Now, I have a forsythia bush. If you dont, however, you can always use a meat thermometer to be precise about your timing. Precision is not necessary, though, as pre emergents last in the soil from between 40 days to 9 months depending on the active ingredient, weather, and amount of organic material in the soil. Early applications are better than late applications.
It is also important to note that pre emergents do not stop weeds that are coming up from the roots of a plant that has previously emerged (perennial weeds). An example of this is nutsedge. Nutsedge propagates through seeds that come from the top of the plant and from nutlets in its root system. Pre emergents might stop the nutsedge seeds that fell from the plant to the ground as they germinate, but won’t stop emergence of nutgrass from the root system of last years plants.