Pre-emergence herbicides do not kill growing plants that have developed green color. Postemergence herbicide kills plants after they are green and growing actively. It may be used to kill annual weeds that have emerged from the soil as well as some perennial weeds. Directions usually indicate the best method for spray application of these liquid materials. Fertilizer spreaders may be used for granular herbicides but should be calibrated and set according to directions.
Selective herbicide is more toxic to some plants than others when used at proper concentrations.
A 50W indicates a 50 percent chemical in a powdered form that can be mixed with water for spraying. Mineral soil is sand, clay, silt or other soil with less than 10 percent organic matter. Organic soil contains more than 10 percent organic material. Naming herbicides Home gardeners often find the names of weed-killing materials confusing. Chemical name is a long, usually difficult name that describes the chemical structure of the material.