Depth of soil needed for planting between pavers

I have started to dig out my new pathway. I have planned on digging it to 5cm and filling it with gravel around slabs. If instead, I decided to go with a ground cover plant like Isotoma Blue Star Creeper, how deep would I need the trench? I'm torn between the two ideas?

G.

 

The landscape architects I worked with used to specify 150mm for grassed areas, 450mm for landscaped areas (shrubs) and 300mm to rear garden areas for growing stuff. They thought more about shrubs tban vegetables.