The codex entry for Greenhouse (both tiers) lists the recipe for flowers in the recipe list. Flowers do not show up anywhere else in the codex.
The list on the left (all items/buildings) does not contain flowers.
But you can click on the flower icon in the recipe of the Greenhouse to get to the codex entry for it. From there you can even go into the statistics for it.
This appears inconsistent.
Version: Update 3 | v0.7.6a (b466)