If Pot is legal in the place where the state fair is being held, then people have the right to show off their horticultural expertise even if it is considered a bit controversail. Many state fairs have beer, wine and liquor making contests, so there is no reason to treat pot any differntly .
If a state allows people to have pot plants, there is no reason it shouldn't be allowed at the state fair. People seem to forget that there are other uses for the plant other than smoking the leaves, in fact back in the day there were farmers who only grew pot for other uses and made a good living at it.
In states like Oregon where medical users of marijuana are allowed to keep up to six pot plants, people should be able to display their plants. Many people have never seen a pot plant so this could be seen as educational. So long as they are only displayed in areas of restricted access where children cannot see them,there shouldn't be any problem.
Yes, pot plants should be allowed at the state fair, as long as marijuana is legal in the particular state in which the fair is being held. Even if marijuana is legal only medicinally, the plants at the state fair can be closely monitored as they are used to educate the public.