I just harvested my first crop. The description says that the fruit makes excellent fertilizer. I'm not sure if that means it will make plants faster to evolve, but that would be great. Also, the game has generated two wild growfruit trees that I've seen so far. One is across the street from Desert Bloom park and the other is in the area to the left of the pond in Magnolia Blossom park. I'm sure they will disappear once the challenge ends, but you can get more seeds there once they become harvestable. The fruit seems to appear fairly quickly, but I didn't take note of the exact harvest time. It's no more than 48 hours, probably less. The fruit sells for a base price of $5.