Weeds are basically any type of plant that is growing where it is not desired to grow. There are two main types of weeds that we treat with pre-emergent and post-emergent herbicides: broadleaf (dandelion) and grassy (crabgrass).
Pre-emergents are applied to lawns in the spring and fall to prevent the development of weed seeds. They don’t interfere with germination but instead stop the formation of new root cells in baby weed plants. Timing is critical so pre-emergent herbicides must be applied before weeds surface to have any real effect. If you wait until after the weeds surface, the herbicide cannot do its job. Established plants are safe, as their root system is already developed and the plant is strong and healthy. Pre-emergent information indicates that it is the sensitive root tissue of newly developed seedlings which is killed off, resulting in complete plant death. Perennial weeds develop thick persistent adult roots that re-sprout in spring, which makes them difficult to control with a pre-emergent formula. Annual weeds are in two classes: winter and summer annuals. The timing of a pre-emergence weed killer for each type must match the germination period for the variety of weed. Biennial weeds, like dandelions, are not controlled by a pre-emergent because they produce seed that develops almost year around. Pre-emergent weed killers require water to activate them and carry the chemical down to the root systems of newly sprouted weeds. In the spring, they should be applied when air temperatures reach 65-70°F for four consecutive days. In the fall, they should be applied when nighttime lows reach 55-60°F for four consecutive nights
If the chance to apply a pre-emergent is missed, a post-emergent product can be applied. Post-emergents work by destroying already established weeds. Post-emergent weed killers either attack the foliage or flow systemically to the roots of the weed. They come in spray on formulas or as granular applications. They are most beneficial in areas that have literally been taken over by weeds and caution must be used when applying in order to prevent drift of the spray or contact with non-target plants. Some post-emergents are selective, meaning they target specific weeds, while others are nonselective, which means they destroy everything green in their path, whether it’s weeds, grass, flowers or shrubs.