Written by Heiko Muth on 30. 07. 2024 

The Indian star plant has been known and popular in the aquarium hobby for over ten years under the name Pogostemon erectus. However, the nurseries Dennerle (Germany) and Tropica (Denmark) have recently been labelling this aquarium plant as Pogostemon deccanensis. What is behind this? And what about the clearly different-looking "Pogostemon deccanensis", which has been available as a tissue culture plant for some time?  

Alleged Rotala verticillaris turns out to be Pogostemon 

Like many aquarium plants, the Indian star plant was first marketed under an erroneous name: Rotala verticillaris. In the 2000s, aquarists in the USA brought the plant to flower in emersed culture and realised that, with its magnificent candle-like inflorescences, it did not belong to Rotala but to the genus Pogostemon. Cavan Allen (USA) stated on the aquaticplantcentral.com website in 2009 that it was Pogostemon erectus. He mentioned in this context that in the book "Aquatic and Wetland Plants of India" (Cook 1996) Pogostemon deccanensis is listed as a synonym of P. erectus (in the spelling "Pogostemon erectum"). Under the name P. erectus, the plant spread worldwide in the aquarium hobby and trade around 2010. 

Rotala verticillaris is not a synonym of Pogostemon erectus, but a species of Rotala native to India and Sri Lanka. It is not clear to me whether this plant has also been kept in aquaria before or whether the name simply came into the aquarium hobby through misidentification. It is described as a plant with very narrow, whorled leaves, as is known from Rotala wallichii, but also various Pogostemon species. However, the latter have significantly different flowers and also belong to a different plant family (mint family, Lamiaceae; Rotala: loosestrife family, Lythraceae). Rotala verticillaris is the type species of the genus Rotala. Because its leaf whorls are reminiscent of the spokes of a wagon wheel, the first person to describe it, Carl von Linné, gave it the genus name Rotála in 1771, which means ‘wheel-like’ in Latin (rota = wheel). 
But back to the Indian star plant and the name Pogostemon deccanensis. 

Pogostemon erectus and P. deccanensis: Two similar species from southern India 

Christel Kasselmann (Germany) and Karen Randall (USA) visited known locations of Pogostemon erectus in the states of Kerala and Karnataka in south-west India in November 2014. They collected several specimens, some of which they herbarised and some of which they took back alive for culture experiments. However, the aquarium cultivation of the plants was not successful, unlike with "P. erectus" from the aquarium plant trade. Later, C. Kasselmann examined flowering plants of the latter, which Manuel Sauer (Germany) had sent her, and compared them with the P. erectus herbarium specimens from India. She came to the conclusion that "P. erectus" from the trade was Pogostemon deccanensis. She published this in the journal DATZ (Kasselmann 2019). 

As mentioned above, Cook (1996) considers this name to be a synonym of P. erectus. C. Kasselmann's investigations, however, are based on the later published monograph of the genus Pogostemon by Bhatti & Ingrouille (1997), in which P. deccanensis and P. erectus are described as distinct species. The examined "P. erectus" from the trade corresponds in its characteristics to the species description of P. deccanensis and the plant collected in India to that of P. erectus. 

Pogostemon deccanensis (Panigrahi) Press (1982), named after the Deccan Plateau in India, is therefore the correct botanical name for the Indian star plant from the aquarium hobby, which can therefore also be called the Deccan star plant. Like Pogostemon erectus (Dalzell) Kuntze (1891), this species is native to wetlands of south-west India.

Detailed information can be found in the aforementioned DATZ article (Kasselmann 2019), where the small differences between the two species are also specified. They primarily concern flower and fruit characteristics: Length of the flower spikes, shape and hairiness of the calyx, size of details of the corolla, length and hairiness of the stamens and shape and size of the fruits (nutlets). At 1.5 to 2 cm, the submersed leaves of Pogostemon erectus are significantly shorter than those of P. deccanensis, where they are 2 to 4.6 cm long. 

Flowering emerged Pogostemon deccanensis.

"Pogostemon deccanensis" from ADA: true P. erectus? 

Confusingly, however, another plant has been in cultivation for some time under the name "Pogostemon deccanensis". It came onto the global market years ago as a tissue culture plant via the ADA company. Compared to the aquatic plant identified as P. deccanensis, it grows more slowly in the aquarium and has higher requirements in terms of light intensity, nutrient and CO2 supply. The needle-like underwater leaves remain significantly shorter than those of P. deccanensis.  

It cannot be ruled out that this mystery plant is Pogostemon erectus as defined by Bhatti & Ingrouille (1997)! However, this could only be verified on the basis of flowering specimens in the terrestrial form, especially as there are a number of other aquatic Pogostemon species that are distinguished primarily by details of the flowers.  

We have provisionally named the ADA plant Pogostemon cf. deccanensis in order to distinguish it from the true P. deccanensis. 

Pogostemon cf. deccanensis, In-Vitro-plant.

Final remarks 

Unfortunately, aquarium plant names change from time to time. There may be various reasons for this, such as changes in classification or correction of incorrect determinations. The first identification of the Indian star plant was based on a publication (Cook 1996) in which P. deccanensis was not recognised as a separate species and could therefore only be Pogostemon erectus. However, the characteristics of the plants examined match the respective species descriptions in the later published Pogostemon monograph by Bhatti & Ingrouille (1997), so it makes sense to refer to the aquarium plant previously called "Pogostemon erectus" as Pogostemon deccanensis. 

However, this name is particularly confusing in the aquarium hobby because of ADA's "Pogostemon deccanensis" (for the time being Pogostemon cf. deccanensis), which is certainly misnamed and could, funnily enough, be a true Pogostemon erectus. As I said, the species affiliation of this plant would still have to be verified on the basis of its flowers. 

Submerged stem of Pogostemon deccanensis.

References

Allen, C. (15. Dec 2008): Rotala verticillaris? Nope! Aquatic Plant Central (APC)
https://www.aquaticplantcentral.com/threads/rotala-verticillaris-nope.58042/

Bhatti, G.R. & Ingrouille, M. (1997): Systematics of Pogostemon (Labiatae). Bull. Nat. Hist. Mus. (Bot.) 27: 77–147. 

Cook, C.D.K. (1996): Aquatic and Wetland Plants of India. A reference book and identification manual fort he vascular plants found in permanent or seasonal fresh water in the subcontinent of India south of the Himalayas. Oxford University Press (1996): 224. 

Kasselmann, C. (2019): Pogostemon deccanensis oder erectus. Welche Art pflegen wir? DATZ 72 (1/2019): 42-47.