Which is the collective noun for thistle?

There isn't a widely recognized collective noun specifically for thistles.

However, you could use a general collective noun like:

* Group of thistles

* Patch of thistles

* Stand of thistles

* Cluster of thistles

Or, you could get creative and use a more descriptive phrase like:

* A prickle of thistles (playing on the prickly nature of the plant)

* A crown of thistles (referring to the thistle as the national flower of Scotland)

* A fortress of thistles (highlighting their dense and defensive growth)