It's not what it sounds like, I swear

The burning marijuana plant is a standard marijuana plant that has been set aflame by the flamethrower, a molotov, a grenade or an explosive barrel.

Once set on fire, the plants will burn fairly quickly and the fire will spread with ease. This is the basis of the Kicking the Hornet's Nest mission, in which Willis tasks you with burning down one of Hoyt's pot fields.

And no, you don't "burn" any marijuana. Leave that stuff to Oliver.