Another thing I have learnt while shooting cyanotype.
Here is a scene I shot this afternoon:
Here is the same scene as a cyanotype (reflected, unwashed, and still a negative):
And, indeed, checking the research, I find that one of the lowest natural UV albedos is from grass, at 0.02. (https://www.tandfonline.com/doi/pdf/10.3846/1648-6897.2008.16.83-88)
So my little hypothesis appears to have been borne out by independent research, and a source of other information about things I cannot see when taking the cyanographs, which will help me plan future exposures.