Minecraft wiki entry for Flower_farming, associated text: *The 2x2 design has no center, making it harder to use/build. This applies to any even size.
**A 15x15 area guarantees that no flower will fail to generate, but note that the earthquake collection method cannot be used (there are too many blocks for pistons to push). A somewhat complex water flow system is necessary for automatic collection. In Bedrock Edition, you can tap a flower w/ bone meal and it will multiply. Sadly, in Java Edition, you can only use this technique on two block high flowers.