In a year full of conspiracy theories this has to be one of the dumbest. There are already thousands of Chinese nationals and Chinese friendly Americans who would sell us out for chump change throughout the land. Why not discretely send them seeds to scatter instead of a silly random mailout?
Maybe a lot have be planted over a widespread area in a very short amount of time for the effect to really take hold. Of course when you are willing to enter the kook fringe such scenarios are easy to dream up.