Growth Cycle

• Roundup must be applied when the plant is in active growth. The Roundup label recommends spraying annual weeds when they are small, but perennial weeds when they are approaching the mature stage.
  
  

Considerations

• Spray on a calm day. Spraying when there is a breeze risks having the herbicide land on desirable plants. It also increases your risk of inhaling or coming into contact with the herbicide, which should be avoided.
  
  

Potential

• Roundup works quickest on sunny, warm days. The label states that use of the product on cool or cloudy days may slow plant response to the herbicide.
  
  

Water Effects

• Apply Roundup to dry leaves. The amount of time before the chemical is "rain-proof" varies from 10 to 30 minutes, according to the Scotts website. Be sure to wait the specified time before using sprinklers on the area.