The chemical category that involves substances that directly kill or stunt parts of treated weeds is referred to as what?

The appropriate term for substances that directly kill or stunt parts of treated weeds is contact herbicides. These herbicides work by physically coming into contact with the weed tissues, which leads to immediate damage and death of the plant's foliage. This category of herbicides operates on the principle that the active ingredient must penetrate the outer layer of the plant to be effective.

Contact herbicides typically eliminate only the parts they touch, which means that they do not translocate within the plant system, leaving untreated parts of the plant potentially unaffected. This characteristic is important in managing specific weed issues, especially for those that are present in isolated areas or between desirable plants where complete control is not necessary.

In contrast, selective herbicides target specific weed species while sparing desirable plants, and systemic herbicides are absorbed and distributed throughout the plant, affecting various parts over time. Residual herbicides provide extended control by remaining active in the soil for a period after application, preventing new weed germination. Understanding these distinctions is crucial for effective weed management in turf and ornamental applications.