“Post-emergent” and “pre-emergent” are terms used on my blog, and many other lawn and gardening blogs. To clear things up, here is the difference.
Post-emergent herbicides work AFTER (post) the emergence of weeds. If you have weeds that are already growing in your lawn or garden, then you need a post-emergent herbicide.
Pre-emergent herbicides are just the opposite. Use pre-emergents BEFORE (pre) weeds begin to grow. When used correctly, these herbicides will stop weeds from growing in your lawn. Even though these products are great, do not expect 100% control. You will see some weeds grow here and there.
In my business, I like to use pre and post-emergent herbicides in conjunction for the best weed coverage using the least amount of herbicides. We will apply both herbicides at the same time to save water and increase efficiency.