Was this for sure on public land?

I've been involved in several operations seeking to eradicate people growing pot and running meth labs in the national forests. If it was national park or national forest, don't call the sheriff or local law enforcement agency, call the law enforcement division of the NFS or NPS. Since these plants were tall, and in containers - and the beat up car drove by - the plants are probably gone by now.

We can debate the rightness or wrongness of these activities - but this is public land, places where you would take your family, and they have no business or right to grow drugs on public land. They do it, and drive an old beater car or truck, because if they get caught, the law can confiscate everything they are using for the illegal activity. That way, they're not out much.

Also, you can rest assured that each individual garden plot is one plant shy of felony charges. Each plot will be misdemeanor level - these guys aren't dumb. In most places, they aren't looking for trouble, and are liable to abandon any garden they think may be compromised -- they have a dozen or more other gardens in the area. Typically, these are not guys growing pot for personal use. These are typically folks with some ties either to gangs or organized crime -- they're not wise guys or gang bangers, but they typically are suppliers.

For future reference, note the location, get the hell out of there, and call it in to the national park service law enforcement division or national forest service law enforcement division -- these guys aren't "Ranger Rick" -- they are sworn Federal Law Enforcement. You can always report in anonymously. If you think that your vehicle has been seen, then you'll have to make a decision. These guys generally seek to avoid trouble and will be non-confrontational... but that depends a lot on what part of the country you are in. 99 times out of 100, if they see you in the area and think that you've found the plot - they'll either go in immediately to harvest the plants, or more likely, they'll abandon it and never go back.

Don't do anything to jeopardize your safety, and don't try to take matters into your own hands.