Hikaru Sulu dispensing weed spray
An herbicide, or weed spray, was a chemical designed to kill unwanted or undesirable plants.
According to Leonard McCoy, one of his "great-great-granddaddies, way back, had the finest garden in the South." Familiar with one of this ancestor's weed killer recipes, McCoy brewed up a batch for the landing party's return trip to the surface of Phylos, in order to rescue Spock from the Phylosians in 2269. James T. Kirk, Hikaru Sulu, and McCoy were each armed with a chemical sprayer containing McCoy's concoction, which they used to neutralize attacking swoopers. Upon their success, McCoy exclaimed, "Well, how about that. Great-granddaddy's weed spray still works!" (TAS: "The Infinite Vulcan")
When Jean-Luc Picard overheard Boothby complaining about the weeds that kept popping up in the pittosporum on the grounds of Starfleet Academy, Picard suggested that Boothby could use a good herbicide to kill them, rather than pulling the weeds by hand. Boothby, who was not keen on the idea, analogized the suggestion to Picard exploring space on a holodeck instead of a starship. (TNG: "The First Duty")