This plant is the only known wild specimen in the world, found on just 300m of roadside

Topic:Native Species

The seeds of the Woods Well Spyridium have been stored at the SA Seed Conservation Centre at the Adelaide Botanic Garden.

In short:

The number of known Woods Well spyridium growing in the wild has shrunk to one solitary plant.

It has grown in crushed limestone on the side of the road in the Coorong.

What's next?

Ecologists will continue to hunt for other plants in the wild and conduct trials around seed germination.

Woods Well on the Coorong, along the South Australian coastline, is very much a 'blink and miss it' type of place.

There is a smattering of houses and paddocks on one side of the road between Meningie and Salt Creeks, and remnant scrub edging the waters of the ecologically struggling South Lagoon on the other.

Surprisingly, nestled in the scrub is the extremely rare plant Woods Well spyridium.

The Woods Well Spyridium plants at the Adelaide Botanic Gardens have been grown from seeds taken from the plants found at the roadside in the Coorong.

The critically endangered small mallee shrub grows to about 1.8 metres in size and produces tiny, delicate white flowers. It is a very rare plant.

Robbie Andrew, from the Limestone Coast Landscape Board, said that it was likely the plant's population had reduced over the years due to land use and clearing.

Robbie Andrew with the only known wild specimen of Woods Well spyridium.

Just why it ended up growing only on a section of roadside, in not particularly fertile crushed limestone, is not yet entirely understood.

There is some thought that things like ground disturbance might make the seeds germinate. So, potentially, maybe when the road was redone, some of that old road base was pushed around up onto the bank, and that disturbance made some seeds that were still in the soil germinate and grow there. This is just a theory.

'But in terms of plant numbers, as far as we know, we're down to only one plant left in the wild, which is pretty concerning.'

First spotted on roadside in 1973

The clutch of Woods Well spyridium growing at this roadside was first noted in 1973, but it was not until 2012 that the species was officially described as unique.

'It's a pretty hard plant to identify because it doesn't have any really distinctive features,' Dr Andrew said.

An initial collection of seeds from the remaining 35 roadside plants was made in 2006. Half were retained in the seed bank at the Adelaide Botanic Garden, and half were sent to the Millennium Seed Bank at Kew Gardens in London. This was an important step in preserving the plant's diversity.

Bradley Bianco with one of the plants being grown in the Seed Orchard at the Adelaide Botanic Gardens.

Another 3,000 seeds have been collected this year, but those initial collections proved extremely important in terms of preserving diversity as the known population at the side of the road shrank.

'Every time we've gone back to reassess the population to make further seed collections, there's fewer and fewer plants,' said Bradley Bianco, who is a flora ecologist with the SA Seed Conservation Centre at the Adelaide Botanic Gardens.

Several specimens of the plant are now growing in the botanic gardens' seed orchard, alongside other rare and endangered plant species from around the state.

When seeds were first collected from the spyridium at the roadside in 2006, there were 35 plants still living, but that number has reduced to one single plant.

However, it has proved difficult to germinate, even under the best of circumstances.

'I guess that kind of explains why the plants in the wild, if they're dropping the seeds, they're having trouble germinating, because even in ideal conditions that we can provide for it, it's hard to germinate,' Dr Andrew said.

Mr Bianco said there were 45 species of spyridium in Australia, and it was unusual for one to be so rare, but its context and survival in 'a very fragmented landscape' were important to note.

It is a little curious that we have not been able to find any more, but with a remaining population found only on the roadside, it was problematic.

Brad Bianco is a flora ecologist with the SA Seed Conservation Centre.

'Its opportunity to recruit after a fire is very limited because it's in a very small, linear area of native vegetation.'

Mr Bianco said the number of plants at Woods Well had declined in recent years, partly because they were not particularly long-lived.

They don't live for a hundred years, they live for decades, maybe, and over the course of their lifetime, they set seeds, and those seeds go into the soil seed bank, and they're ecologically cued to come up after a particular environmental condition is met. For some plants, this could be a flood, or a fire or something digging the soil. This process is crucial for the plant's survival.

The Threatened Plant Seed Orchard at the Adelaide Botanic Gardens.

With the plants existing only on the roadside in a small area, those ecological processes 'aren't operating'.

It's a South Australian endemic, so it's special for our state because it doesn't occur anywhere else. It's part of our natural heritage.

So far, the targeted searches being undertaken by the landscape board have not found any other populations, but the hunt will continue on local properties and conservation areas.

The board also believes there is a 'good chance' seeds remain in the ground near the remaining species, with trials to be conducted around ground disturbance and burning to see if they could help to germinate them naturally.