There isn't anything concrete about that...there aren't any provisions in the law that say if you are aware of marijuana growing wild on your property, but don't tend to it, you are innocent (or guilty). It would be the police officer's decision whether or not to charge you guys on one of the following charges (in my state at least): The cultivation, delivery or sale of 30 grams or less is a misdemeanor, punishable by up to one year in jail and a fine of up to $5,000. Cultivation or delivery of more than 30 grams is a felony, punishable by six months - three years in prison and a fine of up to $10,000. For cultivation or delivery of any amount of ten pounds or more the penalties range from 2 - 8 years in prison and a fine of up to $10,000.
If you are aware of the marijuana growing on your property, but do not attend to it, nor destroy it - you should be guilty of the cultivation of marijuana; that is, unless the area in which the marijuana is growing is proven to be inaccessable to you. If you are genuinely unaware of marijuana growing wild on your property, then you should be excused upon clearing your property of the wild marijuana.