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Poaceae family

Cenchrus echinatus
Linnaeus
, Sp. Pl. 2: 1050.  1753.

Zèb kolan   
Indigenous very common pantropical herb of open dry areas, especially coastal.

FLA also lists Cenchrus brownei and Cenchrus incertus as being present in St. Lucia. I have sent off several collections and all have been determined to be Cenchrus echinatus. The herbarium specimens of the two other species appear to me to be Cenchrus echinatus.
Thus I believe that only Cenchrus echinatus has been collected in St. Lucia and may be the only Cenchrus species here.
Inflorescence variable in colour.