Acabamos de encontrar esta página no New York Daily, do título:
What are they smoking?! When scenes call for pot or cocaine, Hollywood turns to stash of faux drugs
mais tarde, diz:
Turns out that many cocaine look-alikes are ingredients you might use in a cake, like powdered sugar, powdered milk and baking soda, while herbal tobacco fills in for genuine cannabis. [...] When faux pot plants are needed, as they were in "Weeds," things got costly.
“In seasons two and three, we were growing hydroponically indoors,” Benabib says. “We needed literally hundreds of the plants, and they are very expensive. They cost an arm and a leg.”
Claro que também há exceções como Easy Rider : de acordo com o IMDB neste filme eles estavam fumando maconha real.
EDITAR
Há também este artigo : isso explica que
Since "Weeds" also involved a fair amount of growing, not just selling, the art department also had to supply “plants.” Entire grow houses were outfitted with plastic plants — like the fake flower you might use to spruce up your cubicle, except weed, and lots of it. A late-series episode that finds Nancy Botwin’s onetime supplier growing in Humboldt required planting nearly a hundred plastic plants in the ground. The effect is strikingly realistic, except for the fact that actors couldn’t prune plastic: “If you look close when they’re trimming ’em, they’re not really doing much.”
(a fonte em negrito é minha).