Herbicides work by drying out the leaves of weeds. Unable to generate enough sunlight for food production, the plants die. The leaves of most plants are coated in a waxy material that protects them from diseases and insects, but that also causes herbicides to drip off. You can minimize this by mixing a surfactant, such as ordinary dish detergent, into the herbicide, which creates a bond between the product and the leaves.
While you can buy premixed herbicides in spray bottles, other types, such as concentrated liquids or powders, must be diluted or mixed with water and poured into a sprayer. In either case, add dish soap at a rate of 1 tablespoon per 1 gallon of liquid herbicide formula. Mix the dish soap in thoroughly before transferring the liquid to the sprayer and spray both sides of the leaves thoroughly on a dry, calm day when no rain is in the forecast.
Dish detergent mixes easily with liquid or powdered herbicides. It disperses evenly across the surfaces of leaves, breaking down their waxy coating and penetrating the pores. Purdue University Cooperative Extension Service suggests dish soap may dilute the effectiveness of the herbicide, and may combine with hard water to clog the sprayer.