How to plant cattail seeds?

The shallow water planting mechanism was introduced recently.
As opposed to normal planting, you don’t have to prepare the ground, but planting is done directly in shallow water. I don’t know if you can do the planting manually or if it can only be done via the placement of a planting zone.

There was also a bug report recently where someone failed to plant the seeds for some reason, but I don’t know if that issue was resolved.