From Browning's Childe Roland:
So, on I went. I think I never saw
Such starv’d ignoble nature; nothing throve:
For flowers—as well expect a cedar grove!
But cockle, spurge, according to their law
Might propagate their kind, with none to awe,
You ’d think; a burr had been a treasure trove.
I don't get the meaning of this sentence. I understand that the burr here is
A bur; a seed pod with sharp features that stick in fur or clothing. (Wiktionary)
But why a "treasure trove"? On first reading this line, I imagined that "cockle" and "spurge" were the names of some small birds, and for them even burrs were good, maybe as forage. But on finding that these are the names of weed plants, I could no longer make sense of the bolded sentence.
And why "had been"? Why does the author use the Past Perfect?