Are aquatic poppys and fiber plants only in caves?

[Lightningcrush]

I have looked around for them on the rock walls and i have never found them.

[Cress]

No, you can find them outside as well. Not sure if biome affects this.

[Lagmanor]

No, they should be spawning all around, just like the other plants or minerals. It's just much harder to find them, since there's no scanner for plants.
Try looking for some through the ship weapons every time you can see the walls of the cave and your vessel is not in danger, wish you luck!

Kinda off-topic, but I've noticed that it's much harder to locate slime bacteria naturally growing. Not sure if it's made for the sake of balance (so you don't get too many antibiotics) or we were just so unlucky to not encounter any while in first two biomes.

[Cress]

Originally posted by Lagmanor:

Kinda off-topic, but I've noticed that it's much harder to locate slime bacteria naturally growing. Not sure if it's made for the sake of balance (so you don't get too many antibiotics) or we were just so unlucky to not encounter any while in first two biomes.

I have also noticed these are much rarer than the others.

[Lagmanor]

Originally posted by Cress:

I have also noticed these are much rarer than the others.

Well, now we moved into the third biome, which flora is more "mushroom", so maybe there we'll finally encounter those. For now the weirdest thing we found are some exploding cave formations.

[GGLoner]

They're definitely harder to find outside but if I recall, in the later areas, they tend to also be near mineral patches. So as long as you find some minerals, there's a good chance there's some plants within the same distance apart as well.