Weed Killer That Is Safe for Dogs - Mix 1 gallon of white vinegar, 1 cup of regular table salt and 1 tsp. of dishwashing liquid, preferably something that is biodegradable. Spray the mixture on offending weeds to kill them without harming your dogs, kids or other pets. Once again, be careful where you spray, as this mixture is very strong and will kill any kind of plant it comes into contact with. The higher the acid content in the vinegar, the more potent it will be.