How do I plant crops newly acquired in trade?

My village has squash and peppers natively and they're growing fine. Traders brought wheat and I traded for 100 units of it, then designated a farm, but when I try to assign it to growing wheat, only my squash and peppers are options. What's the trick?

ETA: A couple years later, a trader arrived with a single cherry tree seed. I bought it and planted it with no problems.