I've set up a 15*15 grid of wheat with 9 beehives (4 spaces between hives, 2 spaces between hives and edge of field. I've noticed that some of the tiles are repeatedly growing significantly faster than other tiles. At time of this post there are tiles that have grown 2 crops while others have yet to grow one (I've only harvested and replanted once). All the tiles are covered by all the hives, so I'd think they should all grow at about the same rate.
EDIT: Is grow speed calculated using how many beehives are around when the crop is planted? I have 4 squares off to the side unconnected to the rest of the field that were planted before any hives were built (a couple days ago), and they have yet to produce a single harvest. The slower growing fields are older than the faster growing fields.