Easily overlooked and regionally endangered, very little is known about the habits and needs of our endemic small onion orchid Microtis oligantha. As this year’s Victoria University of Wellington Summer Scholar at Te Papa and Ōtari-Wilton’s Bush, Ben Carson aims to make the acquaintance of these unusual orchids.

“Oligantha!” We had spent around twenty minutes crawling around the sun-brazen hilltop, straining our eyes against summer grasses only to find a toothpick-sized plant not yet in flower. This was my first encounter with Microtis oligantha and despite its inconspicuousness – or because of it – I was instantly charmed. Prior to this, I had only come across Microtis unifolia, the common onion orchid which has denser, longer floral spikes and longer leaves than the more understated M. oligantha.

What is an onion orchid?

Onion orchids belong to the genus Microtis and can be found across Australia, East Asia, and New Zealand. The genus was described by botanist Robert Brown (1773–1858) in 1810 and its name derives from Greek meaning “small ear”, referring to the shape of the flowers.

The plants have a single leaf that emerges from their underground tuber from around November, flower over summer, and fruit by March before disappearing again. Onion orchids often have a weedy habit and are associated with disturbed habitats

There are three species in New Zealand: M. unifolia, M. parviflora, and M. oligantha. Microtis unifolia and M. parviflora are common across Australasia, while M. oligantha is only found in New Zealand and tends to prefer montane grassland environments.

Interestingly, M. unifolia has 88 chromosomes whereas M. parviflora and M. oligantha have only 44, suggesting that a genome duplication has contributed to their divergence and different habits (Dawson, 2007).

Regionally Endangered in Wellington

In complete contrast to Microtis unifolia, only a single population of M. oligantha was reported in the Wellington region, creating some concern around its long-term survival. Currently, M. oligantha is classified as a Regionally Endangered species and several actions have been suggested to assist with its conservation. Human development and introduced species are regarded as threats to M. oligantha and other plants in Wellington. Strangely, sheep and cattle grazing may allow this orchid to persist in sites where other vegetation would take over.

Due to its conservation status, I was tasked with investigating how to grow M. oligantha so we can increase the number of populations and establish back up collections at Ōtari Native Botanic Garden. More specifically, I wanted to find out what conditions are needed for seeds of M. unifolia and M. oligantha to germinate, focusing on their fungal partners and nutrient requirements. I hope that this work can shed light on why M. unifolia is so much more widespread than M. oligantha and help us to conserve them.

Onion orchids and their fungal friends

All orchids depend on mycorrhizal fungi. These live inside the orchid root and help the plant to take nutrients from the soil. This partnership can be highly specific – seeds of some orchid species will not germinate unless the right mycorrhizal fungus is present in the soil. Orchid seeds are referred to as “dust” because they are miniscule and contain few energy reserves; by associating with fungi, they acquire the nutrients they need to grow.

To find out what fungal partners Microtis oligantha associates with, I extracted tiny coils of fungi from its roots and placed them on nutrient media. I only had to wait four days before observing little fungal rosettes on the plates! Soon, DNA from the fungal threads – hyphae – will be analysed to find out what these fungi are likely to be.

What have I found so far?

Much of my first month involved learning the tricks of the trade at the Lions Ōtari Plant Conservation Laboratory, practicing on ever-abundant M. unifolia. I first set about plating seeds in culture with fungi that partner with other native orchids. I also placed M. unifolia seeds on different types of nutrient-rich media and incubated them at different temperatures.

So far, the most advanced seedlings – somewhat unsurprisingly – are those grown in combination with fungi. After only three weeks, M. unifolia seedlings grown with one of its fungal partners had already started to produce leaves! In contrast, other native orchids such as spider orchids (Corybas) can take months before reaching this stage, even with fungi.

Furthermore, seedlings in oatmeal agar and plain agar controls, which have almost no nutrients available to them, are also doing well! (Remember I mentioned their weedy behaviour?) Instead of stunting their development, plain agar seems to be conducive to the early growth of M. unifolia. However, it remains to be seen whether they can advance to the next germination stages without nutrients nor the ability to photosynthesise.

I have also set up experiments to determine the best way to store seeds, involving a combination of desiccation and freezing, and trialled chemical staining methods to assess seed viability.

Once I am armed with baseline knowledge about M. unifolia, I will carry out similar tests on M. oligantha. I am also waiting for the fruits of M. oligantha to mature before I carry out field surveys and germination trials – and will share my findings here soon!

Unassuming, but full of surprises

In so many ways, Microtis seems to be an exception to the orchid family’s rule. Relatively unshowy and quick to germinate, these industrious plants continue to yield surprises. It’s fair to say I’m obsessed now! Leaving the lab at the end of the day, I feel a refreshing bewilderment that I get to advance our understanding of these eccentric orchids. I can’t wait to see what they do next.

A huge mihi to Karin van der Walt (Ōtari-Wilton’s Bush) and Carlos Lehnebach (Curator Botany Te Papa) and Peter Ritchie (Victoria University of Wellington) for supervising me this summer; I have been so inspired by your passion and dedicated mahi for our flora. Thank you to Jennifer Alderton-Moss for guidance in the lab and Sally Paine for helping keep track of everything. Finally, I am grateful to the Deane Endowment Trust for funding my scholarship and presenting me with this invaluable opportunity – thank you!

Further reading

• Bates, R., (1984). The genus Microtis R. Br. (Orchidaceae): a taxonomic revision with notes on biology. Journal of the Adelaide Botanic Gardens, v. 7(1), pp. 45-89.
• Crisp, P. (2020). Conservation status of indigenous vascular plant species in the Wellington region. Greater Wellington Regional Council. March 2020.
• Dawson, M. I., Molloy, B. P. J., & Beuzenberg, E. J., (2007). Contributions to a chromosome atlas of the New Zealand flora—39. Orchidaceae. New Zealand Journal of Botany, v. 45, pp. 611-684. https://doi.org/10.1080/00288250709509743
• De Long, J. R., Swarts, N. D., Dixon, K. W., & Egerton-Warburton, L. M. (2013). Mycorrhizal preference promotes habitat invasion by a native Australian orchid: Microtis media. Annals of Botany, 111(3), 409-418. https://doi.org/10.1093/aob/mcs294
• Moore, L. B., (1968). Taxonomic notes on New Zealand monocotyledons. New Zealand Journal of Botany, v. 6(4), pp. 473-492. https://doi.org/10.1080/0028825X.1968.10428586