I have spent part of the last four summers working for OBNDDC (Oklahoma Bureau of Narcotics and Dangerous Drugs Control) using the UH-1H as a platform for insertions, rapeling, stabo, as well as spot spraying marijuana plants. I always fly with a load of Narc agents and these guys are one tough bunch. For spraying we use a 30 gallon tank with pump and a 100 ft. hose with a single solenoid controlled nozzle. I will hover over the plant(s) and an a couple of agents will lower the hose/nozzle, position the Huey, and spray. The herbicide is Round Up mixed with red dye. This last summer we eradicated about 2500 plants, which is a lot less than years past. Sometimes we land and the Narcs cut the plants down or use backpack sprayers and haul it out. We have killed/cut some plants that are 14 ft tall.
As a side issue I usually spend several flight hours training Narc. agents from all over the US in rapel and stabo operations, usually in the late spring. The OBNDDC runs an Air Assault school in Oklahoma every year.
Not much else to add except that the Company I run does Engineering work on various helicopters for STC's. We have in the past developed the kits and obtained FAA Certification on Hueys with PT6C-67D engine, LHTEC T800 engine, Tail Rotor Enhancement Kits, etc.