What are the two main types of herbicides?

The two main types of herbicides are classified based on their timing of application and mechanism of action: pre-emergent and post-emergent.

Pre-emergent herbicides are applied before the target weeds germinate. They work by creating a chemical barrier in the soil that inhibits seed germination, preventing the weeds from establishing themselves. This type of herbicide is crucial for controlling annual weeds that germinate in the spring.

Post-emergent herbicides, on the other hand, are applied after weeds have emerged. They target actively growing weeds, either systemically or contactingly, to effectively kill or damage them. This category of herbicide is important for dealing with existing weed populations that have already established themselves in a lawn or ornamental situation.

The other classifications mentioned in the options include choice related to the physical form such as liquid or granular, or based on their mode of action, like systemic and contact. However, the distinguishing feature of pre-emergent and post-emergent herbicides is their application timing, which is the primary basis of their effectiveness in weed management. Therefore, identifying these two main types is essential for anyone managing lawns and ornamental areas effectively.