B D Finch Aug 23, 2014:

@Tony - Other thoughts This is about a water weed, so it is water surface, not "ground surface" that is covered. As can be seen from the following reference, it can form "thick dense mats", so "thick" is not synonymous with "dense", it means the distance from the top surface to the bottom surface of the mat. Density would be the capacity of the mat to block light from penetrating it.

invasives.biodiversityireland.ie/wordpress/.../New-Zealand-Pigmyweed.p...
"New Zealand pigmyweed in flower with succulent leaves that form a collar on ... Forms thick dense mats which can float or be submerged."

Re thick hair, that's about the thickness of each strand of hair, which is not the same as its density and, obviously, not anything to do with its length.

Tony M Aug 23, 2014:

Some thoughts... My own instinctive (but non-expert!) interpretation seems to differ somewhat from that of other contributors, so in the interest of lateral thinking and brainstorming, I thought it might be useful to explain here what had occurred to me.

I don't think 'thick' refers to the actual height of the plants (i.e. the thickness of the layer), but rather to how dense it is ('dru'?) Cf. 'thick hair' = dense, not long.

And I have a feeling that following on from that, '100% cover' might not mean that the entire ground surface is covered (which already would be covered by my suggestion of the 'dense' meaning of 'thick'), but rather, that this weed is the only plant found there, to the exclusion of anything else. Cf. '50% coniferous / 50% deciduous'

As I say, simply non-expert musings, in case it's any help...