Hi Walter. It seems unlikely to be C. purpurascens in that location. It normally prefers to grow next to water or in damp places. If possible, could you return & pull or dig up this specimen & dispose of it safely?
It seems the Cenchrus species are difficult to distinguish. The spikelet bristles of C setaceus are plumose, whereas the bristles of the plant I found are definitely not. Also, regarding habitat it seems that C. purpurascens, although commonly called Swamp Foxtail, is not confined to moist places ( http://keyserver.lucidcentral.org/weeds/data/media/Html/cenchrus_purpurascens.htm) . I still think it could be C. purpurascens
Yes, from a photo C. purpurascens & C. setaceus can be difficult to distinguish. Since C. setaceus is a weed we want to eradicate I suggested it be removed. Anyway, C. purpurascens also has weed potential near that drain, so removing it would do no harm. My experience of C. purpurascens locally is that it prospers & can spread rapidly in places that have intermittent wet conditions. It was planted at ANBG some decades ago & quickly became such a problem that they had to eradicate it from the gardens. Either way, removing the plant is a safer option.
Ok. thanks David. I went passed the locality thismorning and found it had been mown, so couldn't see the single C. purpurascens. I'll keep an eye on it.