I just added the units to a Cluster of Ferns located at the Spring Festival grounds on New Brommel. While doing so I saw that the ferns only require 1 resource per unit instead of 2. Generally structures only require 1 resource per unit when dealing with special resources, i.e. seeds, bulbs, permits, etc. Since there are no special resources for the ferns it should probably require 2 resources per unit unless the developers had a specific reason for breaking out of the pattern for other structures.