The seeds cost 175 coins to plant, and can be harvested every day for 455 coins. They also award 3 XP per plot. However, the seeds must be bought in packets of 50 within the Greenhouse (for 9,250 coins) until the second level of mastery has been acquired on it.
Harvesting Sun Poppy has a 10% chance of yielding Perfect Bunches, which could be used as a decoration. Sun Poppy lived up to 14 days in a perfect bunch and could not be sold.
Farm Trivia: Sun Poppies can't help but say "I'm Happy"(and yellow)!