When I first saw this herbaceous plant at the end of May in a park, I thought it was Oldenlandia auricularia. But the flowers did not seem to match. Then I remembered seeing a similar looking but rarer herb (Oldenlandia verticillata) in April 2016 at a park. Again, the flowers did not match well. I then browse through the 2009 Singapore flora checklist (page 173-177) focusing on the herbaceous plants in the family Rubiaceae. Two suspects were identified, Diodia ocymifolia and Diodella sarmentosa. According to The Plant List website, Diodia ocymifolia is a synonym of Spermacoce ocymifolia.

As I was not able to find any picture of the 2 Diodia species taken in Singapore from online and offline reference resources, identification of the herb became rather difficult despite narrowing down to 2 species. Overseas pictures of Diodella sarmentosa were available in some websites. However, they did not offer much help as both species appear to be quite similar in appearance.

Besides the 2009 Singapore flora checklist, the search landed me with 4 additional records from Singapore that listed the name Diodia ocymifolia while only one of them listed Diodella sarmentosa. The most recent record was in a 2014 Checklist of Vascular Plants of Pulau Ubin published by NParks. The second record was a 2011 publication on flora survey of another island known as Pulau Semakau. The third one was a 1997 vascular plant checklist [1] of the Nature Reserve of Singapore. The oldest but most critical one was a 1985 publication [2] which included descriptions of both plants. It described the leaves of Diodia ocymifolia as oblong-lanceolate while that of Diodella sarmentosa as oblong-ovate. Based on this slight distinction, I had named the herb that I found as Diodia ocymifolia, currently known as Spermacoce ocymifolia (Slender Buttonweed). Moreover, there were more recent sightings reported on this herb compared to Diodella sarmentosa.

Spermacoce ocymifolia is an exotic (foreign) plant species that is considered naturalised here.

Reference:

[1] Chew PT, Suran S, Ibrahim A. Checklist of vascular plants in the Nature Reserves of Singapore. Gardens' Bulletin Singapore 1997;49(4):161-223.

[2] Keng H. Annotated List of Seed Plants of Singapore (IX). Gardens' Bulletin Singapore 1985;38(2):149-174.