Advice on a cob herb spiral?

I built a herb spiral a while ago with rocks but with time, it started to 'shrink' a bit - the soil contents coming through spaces in the rocks, herbs a bit poorly and then started looking a bit mini-me.

Today I had the idea of building a cob spiral. Anyone done this and could offer a few tips?

I'm not sure cob would stand up in the rain on its own, and I think plant roots might find their way into it. I also think worms might burrow through it. Perhaps if you whitewashed it on all sides, those problems would be less pronounced (whitewash reacts with clay to form Roman cement).

Holding the rocks together with lime mortar might be more appropriate.

There might also be a species of ground-cover plant that will do well in the spaces between stones, but not so well in soil. Some varieties of medic might be worth looking into, perhaps.

Thx Joel, reckon I'll go with the lime mortar