With Forestry, you can automate wheat farming by using a Farm to create the farm area and plant and replant the seeds and a Combine to harvest the grown plants. You can apply a Forester to instanly grow the wheat with the Catalyst you add to it.
This machine will place farmland in a 15×15 hollow square (with a 9×9 area in the middle) and plant seeds on it when powered by BuildCraft energy. It needs have seeds in it to operate, but it only needs dirt until its finished the construction.
The Combine will harvest any fully grown wheat in an area of 21x10x21 (X;Y;Z coordinates) and eject in a BuildCraft pipe or inventory (like a chest) on any side, it needs to be powered with BuildCraft energy.
<note>You can use the Farm and Combine to automate potato and carrot farming too, just replace the seeds in the farm with them, the combine will harvest them, although the catalyst doesn't, so you can use the Forester.</note