What is the term for applying herbicide directly to the leaves or foliage of plants?

The term for applying herbicide directly to the leaves or foliage of plants is foliar application. This method involves spraying or applying the herbicide to the above-ground portions of the plant, allowing the active ingredients to be absorbed through the leaf surface. Foliar application is particularly effective for targeting weeds that are actively growing, as the herbicide can be quickly taken up and distributed throughout the plant, leading to decreased growth or death of the target species.

In this context, other methods have specific applications that make them distinct from foliar application. For instance, soil application involves introducing herbicides into the soil, where they are taken up by the roots, which differs from targeting the leaves directly. Partial application refers to applying products to only a portion of an area or plant, lacking the specificity of targeting leaves. Seed treatment focuses on the application of substances to seeds before planting, which is unrelated to treatment of mature foliage. Thus, foliar application is the most accurate term for what is described in the question.