Passage: Weed flora. Traditionally this classification was applied to plants regarded as undesirable, and studied in efforts to control or eradicate them.  Today the designation is less often used as a classification of plant life, since it includes three different types of plants: weedy species, invasive species (that may or may not be weedy), and native and introduced non-weedy species that are agriculturally undesirable.  Many native plants previously considered weeds have been shown to be beneficial or even necessary to various ecosystems.

Question: What type of plants was this classification typically used for?
undesirable