Season

• Roundup is designed to be used during spring, but can be used well into the summer. Watch the grass and weeds for growth in the area where you want to eliminate them. Once the threat of frost has passed, the weeds should begin growing regularly. This is the ideal time to spray the Roundup to kill the grass and weeds. Soak the blades of grass and weeds for the most effectiveness.
  
  

Weather Conditions

• Scotts advises against spraying the Roundup on your unwanted weeds and grass when it's windy. The wind can push the Roundup where you did not intend for it to go. In addition, days that call for heavy rains are not good days to use the Roundup. A light rain that falls 10 minutes after the application of Roundup won't be a problem, but heavy rains may wash away some of the herbicide.
  
  

Temperature Considerations

• Ideal outdoor temperatures for using Roundup to kill your grass and weeds are 60 degrees F and up, although temperatures above 85 F can cause stress to the lawn. Scotts Miracle-Grow Company also says it is possible to use Roundup between 50 F and 60 F and still get positive results.
  
  

Safety Factors

• The chemicals in Roundup are hazardous to humans and domestic animals. Take all safety precautions when you are ready to use Roundup. Place children and pets inside, and dress in long shirts and pants. Wear safety goggles, gloves and a mask. Once the application has been successfully applied, go inside and immediately wash your clothing. Do not allow your children or pets to go back outside until the Roundup has completely dried.