Cyrtosperma johnstonii is a tropical plant native to the Solomon Islands in Oceania. Similar to its huge siblings It also has large, starchy rhizomes and gigantic leaves. This plant is grown across Southeast Asia as an ornamental, and despite its popularity, not too much is known about its origins or ecology. I decided to write quite a detailed plant profile in an attempt to condense as much information about this species as possible, so bear with me, it might get a bit heavy.
When young, its leaves have bright red veins, and as it grows older, it trades this colouration for sheer size. Its leaf blades or laminae can grow to over 1.5 metres in length, held up by towering, spiny pink and brown petioles. The suggests a total height of 3 metres, but I would argue that I’ve seen plants taller than this in Singapore, with leaves over 1.5 metres long. This plant is variable in morphology and flowers at different sizes. I have seen small, stunted plants growing in full sun, holding onto their painted juvenile leaves whilst three metre tall plants unfurl gigantic, plain green leaves in dappled shade a few metres away.
The laminae are almost always sagittate, or arrow-shaped. As the plant grows these change shape, with a general trend of the posterior lobes elongating and starting to look a bit like rabbit ears.
According to current information, plants in cultivation can only reproduce by vegetative suckers. It isn’t known whether it is because the species is completely infertile – which raises some questions, or whether it is self-infertile. I have seen a specimen with developing fruit before, but the stalk had bent over, so I imagine it was unsuccessful
I have found this plant to be hugely tolerant of most light conditions. In deep shade, plants lose their colouration more quickly and produce more spindly growth as one would expect. Light or dappled shade seems to suit the plant very well, and this is where I have seen the biggest specimens. The leaves of these plants are a deep green colour, which can make them harder to recognise.
This is a strictly tropical plant and does not do too well with extended periods below 18°C. I have found growth to be rapid here in the summer months where temperatures in my kitchen hit 25 degrees and above nearly every day. The most temperature sensitive parts of the plant are the roots and rhizome. When growing in a temperate climate, or even subtropical, it is crucial to keep them warm during cooler weather. I keep mine in a warm water bath at around 25-28°C, heated by a small aquarium heater.
There isn’t much, if any information on the habitat of C. johnstonii, but looking at the vast number of plants grown as marginals and bog plants it is safe to say that it loves water. I am also aware of plants being grown successfully under “normal” watering conditions in coconut chips.
If grown in drier conditions, leaf blades may dry out and shrivel at the tips. In severe cases, it can spread and destroy the entire leaf! In most cases, isn’t very noticeable as the leaves are so busy with colour and texture anyway, but if it bothers you, you might want to think about changing the way you grow it. My ambient humidity ranges from about 60% to 90% and I still get leaves drying at the tips, even after spraying with anti-transpirant (di-1-p-menthene). However, I have had success with semi-hydroponics in preventing leaf-tip burn, presumably due to increased availability of water.