I pounced upon something that I have never witnessed before. I have grown the Indian borage plant (Plectranthus amboinicus), both the all green version as well as the variegated one, but I have never seen its flower with my own eyes before. My observation was also echoed in PROSEA’s entry on the Plectranthus – Plectranthus amboinicus rarely flowers in Malesia. It is a popular herb grown by many Singaporeans for the use as a treatment for coughs and sore throats.
I visited Oh Chin Huat Hydroponic Farm over the weekend and at their medicinal herb garden, I caught a glimspe of the flowers borne by the Indian borage. Numerous light blue flowers were borne on a terminal spike. Individual flowers were small and has a distinct lip with four yellow spots. They are reminiscent of those produced by the so-called colourful foliage coleus plant which has now been reclassified under the new genus Solenostemon.
This brings us to the three closely related genera in the mint family, Lamiaceae – Plectranthus, Coleus and Solenostemon. At present, there is still much disagreement about generic delimitation in these three genera.
In my favourite reference PROSEA, under the entry for Plectranthus, it was stated that, strictly speaking, Coleus is often considered as distinct from Plectranthus, on the basis of the fused bases of the calyx filaments. However, this is a variable and unreliable character. It is not unusual to see Coleus is still considered as a synonym of Plectranthus. In some herb books, one will see the Indian borage known as Coleus amboinicus.
Those colourful foliage plants now classified under the genus Solenostemon have calyx segments that are about equal whereas Plectranthus species have a a distinctly 2-lipped calyx.
Sounds confusing, isn’t it? Frankly, I have taken the above distinctions for granted as a gardener for a long time as I have never went into looking at the plants closely. It is perhaps time that I go pick some flowers borne by these three genera so as to be able to take a closer look at them.