Assume that it is ditch weed and not a grow op, or a field of some other common plant: Under these circumstances, whoever owns the property is probably unaware of the plants existance. However, you don't need to be aware of the plants to be arrested and charged. Posession is 9/10ths of the law. If you call the police they will likely arrest an innocent civilian who will likely spend time in federal prison and it will be YOUR FAULT. If you can live with the knowledge that you ruined an innocent person's life, then nothing anyone here says to you can change your mind. Know that karma is a real bitch, though. You have no idea what the consequences might be.
Assume that it is a grow op: The only way you could determine if the party is guilty is by staking out the crop. This is highly dangerous even if it isn't a grow op because many people will shoot trespassers on private land, and you will probably be arrested if found on public land after hours. And of course, there is the time and cost involved in volunteer survailence. Even if you catch someone growing pot, you have no way of knowing if they are a medical patient or not. You may be taking away someone's meducine who needs it to live.
A field of pot isn't hurting you or anyone you know, as long as it isn't on your property or a friend's. Anyone who uses cannabis does so of their own free will, so letting it go only hurts voluntary users. You have nothing to gain personally by calling the police. The only reason why the thought even occured to you is because you have been brainwashed by our politicians and media into believing that the government knows what is best for everyone. If you can come to a logical conclusion that a field of pot is causing you or your community REAL HARM (something that I doubt is possible), then man up and go pull the plants up yourself. Calling the authorities when there hasn't been any violence, burglury, or other such offence makes you a traitor to your family, community, and yourself. The police have real jobs to do.