The high court, by a 6-1 margin, dismissed the appeal of Quebec prosecutors and upheld the acquittal of Nicole Rochon in a rare ruling from the bench on Tuesday.
The court said it would give its reasons by Thursday.
Rochon was originally convicted in 2006 of production and possession of cannabis for the purpose of trafficking.
The Quebec Court of Appeal set aside that verdict and acquitted Rochon.
Rochon had entrusted her 87-acre property to her son, and returned to find it was being used for an extensive marijuana operation.
On two occasions, she told her son to remove the crop, but he refused.
Rochon was arrested after police received a tip.
At trial, Rochon admitted to knowing about the situation, so the judge convicted her after concluding she was obliged to tell police.