Safety for Grass

• Though soaps are not inherently harmful to grass, laundry detergent specifically contains ingredients that damage grass, though not necessarily kill it. When using laundry detergent as part of an insecticidal spray, dilute it heavily with water. The University of Rhode Island recommends a dilution of no more than 3 percent laundry detergent, or slightly less than 2 tbsp. of detergent for every quart of water.
  
  

Water

• According to the American Rhododendron Society, water is an effective insecticide without the damaging effects of laundry detergent. Water-stressed plants are often more susceptible to insect pests, and spider mites and aphids can be kept in check with regular watering. Water your lawn before trying other methods.
  
  

Preventing Damage

• Water can also stop the damaging effects of laundry detergent. Just as soaking grass with water dissolves harmful salts, it also dilutes laundry detergent and disperses it in the area. Soak the grass with 1 gallon of water for every 1/2 cup of detergent you estimate you spilled. This produces a 3-percent dilution of detergent, which is safe for grass.
  
  

Killing Grass Naturally

• If you want to kill grass, use salt instead of laundry detergent. Unlike laundry detergent, salt is sure to kill grass rather than damage it. Cover the unwanted grass with salt to dry it out and deprive it of nutrients. Whiteness in the soil is a sign the salt is working.