Over the last few years landholders across Central and North East Victoria have often been surprised, even excited, by their find of a new orchid on their property or local reserve, only to realize and then bemoan that they have a new weed – the South African Weed Orchid (Disa bracteata).
Exactly one year ago I posted a story about the the spread of this weed into north east Victoria, including the Strathbogie Ranges region. The insidious nature of this threat was brought home to me when a Committee of Management that I’m part of (for the Burge Family Reserve at Gobur) realized it needed a coordinated control program to tackle this problem. This became even clearer after correspondence with Richard Hartland, someone with a long history of dealing with SAWO:
“Surfacing plants produce mostly one and occasionally two or maybe more offset bulbs (whilst exhausting the current one) to produce plants for the next time they surface. They also tend to surface quite quickly if suitable conditions occur. I have picked over areas and thought them done and returned several weeks later to find more large plants that could not have been missed.”
It’s not clear how or exactly when this weed reached the Burge Family Reserve, but undoubtedly we humans were the vector.
Last year saw the Burge Family Reserve CoM dug out about 20 kg of SAWO plants and chemically poison perhaps just as many, on the 38 ha Trust for Nature grassy-woodland reserve. Even if we’d dug out and killed every flowering SAWO on the reserve, we knew there’d be plenty this year, as it takes a plant 2-3 years of growth and food storage (in the below-ground tuber) before a flower spike is produced and young, non-flowering plants are virtually undetectable.
Given this year’s wet and productive growing season, we were ready for a bumper crop. Whether it’s luck or our management, the infestation on the reserve and neighbouring Billy Goat Hill property is probably only half it was last year, according to the CoM Chair, Jonathan Hayman. But, a word of warning: “only a minority [of SAWO plants] actually flower. This year I have found around 220 at the White Elephant Reserve and only about half a dozen of these were going to produce flowers and then seed.” (Richard Hartland pers. comm. 26.11.2021).
The good Winter-Spring rains in 2020 and again this year produced good growing conditions across much of Victoria and SAWO populations seemed to crop up across the countryside. Of course, what we know officially about the distribution of SAWO (documented records in databases) lags behind what’s actually happening. The Atlas of Living Australia shows where the species has been documented in Victoria (below), but landholder records we’ve collected in the past few months suggest it has already moved much further, for example:
- Chesney Hills (Mt Meg) and the Warby Ranges (Pangarang Lookout Walk) (Karan Balfour pers. comm. 8.11.2021) near Wangaratta
- Surrounds of Lake Mokoan near Benalla (Karen Brisbane pers. comm. 16.11.2021)
- Reedy Creek, near Broadford (Eric Smith 8.1.2021)
- Hughes Creek, Dropmore (Sam Strong 22.11.2021)
- Fernhills Rd, Shean’s Creek near Euroa (28.11.2020)
- Weibye Track, Ruffy (Janet Hagen pers. comm. 28.11.2020)
- Greens Pinch Reserve, Kilmore (17.12.2020)
The well-established populations in the Warby Ranges suggests SAWO has already spread widely in the district – has it reached the NSW border yet? More information from these areas would be useful.
Recent research has concluded “most of the accessible areas [in South Australia, Victoria & Tasmania] are already occupied by [SAWO] but future expansion will continue based on (different) climate change scenarios. Further expansion is predicted especially in eastern Australia and eastern Tasmania.” The current, observed spread of SAWO in North East Victoria appears to confirm this prediction.
Richard Hartland again, regarding the White Elephant Reserve near Bacchus Marsh:
“The areas where the [SAWO] is found have a large amount of orchids and lilies, e.g. Microtis unifolia, Diuris sulphurea, Thelymitra pauciflora, Wurmbea latifolia, Arthropodium stricta etc. There is probably a likelihood that the SAWO is using similar [mycorrhizal] fungi to these species. Most of the areas are also somewhat degraded by overgrazing and have bare or mossy areas between grasses which seems to suit [SAWO]. Moisture also plays a role as plants have disappeared from areas now dominated by eucalypts. They are likely to colonize Themeda triandra grassland [perhaps] due to Themeda being a C4 species and summer-active, leaving the site wetter during the active growing period.”
SAWO patterns at Burge Family Reserve
As well as having a fragmented Box-Gum Grassy Woodland, the Burge Family Reserve has extensive areas of Kangaroo Grass, smaller areas of Wallaby and Spear grass and patches of exotic grasses (eg Wild Oat, Sweet Vernal, Yorkshire Fog). At Gobur, SAWO has been found growing in association with all these grass species except Wild Oat (perhaps because of its dense growth, or that SAWO is hard to observe among the tall thick sword). At a finer scale, we have often found it growing with the Common Onion Orchid, which tends to occur in patches of up to several dozen plants per square meter (image above). This correlation is strong enough for us to use the onion orchid as an indicator species when searching for SAWO in the reserve. Looking at the distributions of Common Onion Orchid and Kangaroo Grass, it would appear that the distribution of SAWO may be crudely (and not surprisingly) correlated with these common grassland and grassy woodland species.
What is now clear to us is that until (if?) we get this weed under control, there will need to be strict bio-security measures to limit the spread of Disa bracteata to new, uninfected sites. This may well mean restricting public access to the reserve during seed ripening and shedding (e.g. Dec-Feb).
Two short videos recently produced by the Euroa Arboretum and native plant nursery:
- SAWO -A weed in waiting (Viridans)
- Climatic niche shift and possible future spread of the invasive South African Orchid Disa bracteata in Australia and adjacent areas (research article)
- SAWO in the Nillumbik Shire
- SAWO in Victoria (Agriculture Vic.)
Control of SAWO on the Burge Family Reserve and neighboring Billy Goat Hill nature covenant is part of a Trust for Nature project to improve the health of Box Gum Grassy Woodland in the Gobur district of North East Victoria. This project is supported by the Victorian Government through the 2021 Community Volunteer Action Grants.