Last night, I was playing Agricola offline against an AI opponent. I built the Seasonal Plants improvement.
I had two fields in my farmyard. On turn 14, I did a Sow action, placing a vegetable on one field and a grain on the other.
After confirming that I was planting these fields, I was given a screen with a little "+1" token in the bottom right. My understanding is that this was the interface for the Seasonal Plants, and that it would let me choose which field to plan to.
However, as I dragged the +1 token, neither of my fields lit up. I was unable to attach the +1 to either the grain or vegetable field.