How It Works

• A pre-emergent herbicide stops weeds before they ever have a chance to grow by affecting the seed. The seed will still sprout, but the root system will not be able to grow, causing the weed to die quickly. Pre-emergent herbicides do not affect existing plants and lawns, but they are not selective when it comes to seeds. Grass, flower and other seeds sown where a pre-emergent has been applied will be affected in the same way as the targeted weeds.
  
  

Chemicals

• There are a number of pre-emergent herbicides available to homeowners, including pendimethalin, which is rated as excellent for effectiveness; oxadiazon, which is sold as Ronstar, and is rated as excellent for effectiveness; bensulide, which is sold as Betasan and Pre-San, and is rated as good to excellent for effectiveness; Benefin, which is sold as Balan and is rated as good for effectiveness; siduron, which is sold as Tupersan, and is rated as good for effectiveness.
  
  

When to Apply

• In order to be effective, pre-emergent herbicides must be applied before the weeds begin to grow, or while they are still seeds. This means that the herbicide should be applied in early spring to kill spring weeds. To kill fall weeds, the herbicide should be applied in late summer. It cannot be applied too far ahead, such as in fall to kill spring weeds, because it will break down before the weeds have a chance to emerge.
  
  

Application

• Pre-emergent herbicides should be applied uniformly in order to be most effective. To do this, apply one half of the herbicide at the rate indicated by the manufacturer by spreading it in one direction, such as left to right, and then apply the other half in the other direction, such as up and down, across the lawn.