Scaly pholiota, mora tapinella and glowing mushroom grow on their own logs in your planters, pretty much independently from the soil beneath them, because of their nature and how they normally grow in the wild.
Considering that creep cluster is a creeper plant and those mainly prefer solid surfaces, it now has its own rock to creep across. 
...And it also makes it properly harvestable. I love it when practicality matches logic.
(Come to think of it, shouldn't glowing mushroom grow on a rock too? We usually find them on cave walls...)


Recommendations:

• This fits perfectly with Simplicity of Seeding by wSkeever
• Creep Cluster Collision Fixes by sync67 completely solves the harvesting issue and retains the original creep cluster look, with tweaks. You can use that instead, or use it together with my mod - if you want to retain the rocky creep cluster look in your garden that this mod provides. Just load my mod after Creep Cluster Collision Fixes (let my mesh win the conflict).
  

Updates:

v1.1: I trimmed the bottom part of the rock mesh so it doesn't clip with a large box planter. It may also look neater in the ground garden because the model is smaller. It will still clip with the small planter, so avoid planting it there.  ...Or plant it there as well, if you don't mind the clipping. It still works as intended.

v1.2: The smallest model is resized further to fit into the small pot planter. It can be easily picked both from small and large planter, but in a ground garden it is difficult to pinpoint the plant itself because it was severely shrunk down. However, the action area for harvesting above the plant is huge, so it's easy to detect the harvesting prompt. This version is best suited for those who prefer homes with indoor greenhouses.



Specifics:

The model was taken from the world model of creep cluster that grows on rocks, with a few necessary tweaks to the mesh so it would fit well into the planter.

The rock will appear as soon as creep cluster grows for the first time, and it will stay in your garden/greenhouse until you clear the planter.

The rock has no collision, so it won't block you while you move through the garden.

Both the rock and creep cluster use vanilla texture paths. That means you can use a retexture for creep cluster, and the rock itself will use your mountain/landscape texture (mountainslab02.dds and mountainslab02_n.dds). I'm using Northfire's Photoreal Mountains (2K version).

Harvesting amount, growth and regrowth time work normally.

You can install it mid-game, and even when you've already planted creep cluster in your garden.

Requires Hearthfire DLC, obviously.


Enjoy :)