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Feature article - From the nursery to the field: horticulture informs translocation of Small Purple Pea Swainsona recta
Zoe Knapp (1,2) , Peter Bredell (1) , Emma Cook (3) , Bek Hyland (1) and Tom North (1)
1 Australian National Botanic Gardens, 2 Current address: Norfolk Island Botanic Garden, 3 ACT Government.
The Australian National Botanic Gardens (ANBG) maintains the most extensive collection of Australian native plant taxa worldwide, coupled with vast horticultural expertise spanning over 4,000 species. Our living plant and seed collections, combined with concentrated expertise in field botany, taxonomy and evolution, seed biology, and horticulture, enable us to provide holistic solutions for plant conservation, from wild sourced plant material collection to propagation of plants for translocation. Here, we present a case study that demonstrates how establishment of ex situ plant populations and associated horticultural knowledge can inform long term recovery in situ.
Since the 1980s, the ANBG has been increasingly involved in the propagation of threatened species as part of multi agency conservation projects. The ANBG has an important role supporting plant conservation in other Commonwealth terrestrial reserves managed by the Director of National Parks. Its location in Canberra also positions the ANBG as the primary horticultural facility for plant conservation in south east New South Wales, for example geographically (proximity to field sites for plant material collection, and delivery of propagated plants), horticultural capacity (expertise, specialised infrastructure), and local climatic conditions required for propagation and cultivation.
One of our most rewarding projects has been a long standing collaboration with the ACT Government’s Office of Nature Conservation to rebuild local populations of the Small Purple Pea Swainsona recta. The Small Purple Pea is a slender, erect perennial forb in the family Fabaceae. Once considered widespread in grassy woodlands across south eastern Australia, the species is now restricted to relatively small and fragmented populations in NSW and the ACT and is listed as Endangered under the Environment Protection and Biodiversity Conservation Act 1999 (EPBC Act), the Nature Conservation Act 2014 (Australian Capital Territory) and the Biodiversity Conservation Act 2016 (New South Wales).
In 2023–2025, the ANBG will provide 1000 Small Purple Pea plants for re wilding at crucial sites in the ACT. However, our involvement extends beyond mere plant supply (albeit a challenge on its own, as noted below); we engage in all phases of conservation projects, leveraging our expertise and learning from others. Below we outline some of the broader contributions that botanic gardens horticulture can make to in situ plant conservation.
Seed collection from wild populations
We collect seeds from across all wild populations to capture as much genetic diversity as possible. Through close coordination between our nursery and the ANBG’s National Seed Bank we are facilitating research into seed dormancy and germination cues, and conservation banking requirements for the species.
Propagation and cultivation techniques
Many species propagated in the nursery have either never previously been propagated or are notoriously difficult to propagate. Our horticulturists have developed efficient propagation methods for the Small Purple Pea, which is labour intensive to grow. Seeds sown in the nursery germinate sporadically over a period of up to nine months, requiring ongoing pricking out and potting up of new seedlings. The development of a specialised growing medium has dramatically improved cultivation success in the nursery. Flower buds are removed from plants in the nursery for genetic reasons (discussed below) and to allow plants to focus energy on root development. The species’ deep, fleshy tap root appears to be critical to its survival in the wild, allowing plants to recover after exposure to, for example, grazing, fire and drought. In the nursery, plants grown in deeper tubes have a substantially more robust and well developed root system than those grown in standard forestry tubes.
Seed production
In 2016, seed from several Small Purple Pea populations in the ACT region was germinated and planted in the ANBG’s dedicated seed production area (SPA) in a scientific design intended to maximise genetic diversity of resulting seeds. Each year, horticulturalists have bagged and collected seed from these plants, which is then passed to the National Seed Bank for cleaning, viability assessment and banking. This alleviates pressure on wild populations, provides easy access and enables research and observations on the species. Interestingly, seeds collected from the SPA appear to have much higher germination rates than seed collected from wild plants, although results are anecdotal.
Ex situ ‘insurance’ collections
Our scientifically documented seed bank and living collections, most of which are linked to herbarium specimens, serve as an invaluable resource for research, genetic preservation and as backup populations for future recovery efforts. The seed bank and living collections include over 300 threatened species listed under the EPBC Act, and many more that are protected under state legislation, and/or known to be rare or threatened.
Maintaining genetic differentiation
Concurrently with this project, the ANBG partnered with NSW Local Land Services (Central West and Central Tablelands) to assist with recovery actions for the Small Purple Pea in NSW. Since there is evidence of genetic differentiation between the northern (central NSW) and southern (ACT and southern NSW) populations (Buza et al., 2000), we remove all flower buds from plants in the nursery to prevent cross pollination between genetically distinct populations.
Cultivation informs planting requirements and post-planting care
Insights from nursery cultivation inform optimal planting practices. For example, we’ve observed the following for Small Purple Pea:
• Plant age: Plants require at least 18–24 months from germination to develop a sufficiently robust root system to survive translocation.
• Planting hole size: Growing plants in deeper tubes also means digger deeper holes for planting. However, any investment in facilitating the successful development of a deep tap root is likely to improve translocation success.
• Timing of planting: Spring and early autumn are suitable planting timeframes because plants are actively growing and more likely to successfully establish post translocation.
• Plant spacing: The species does not tolerate physical crowding, and translocated plants have been intentionally spaced, and mulched to inhibit the growth of competitors near the plant.
• Post-planting care: A fine quartz gravel mulch (less than 5 mm diameter) was applied to translocated plants to 30 cm diameter around the base of the stem. This will help to retain soil moisture and deter weeds around the base of the plant. This method has been applied to half of the translocated plants (randomly allocated) to experimentally test whether the treatment influences post translocation survival.
• Seed treatment: Seeds are physically dormant and require heat shock to alleviate dormancy prior to sowing. This was achieved by placing seeds in hot water (95 °C) for 3 minutes followed by immediate transfer to cold water (4 °C) for 1 minute, and then repeated. Seeds were then rinsed under room temperature water before soaking in room temperature water for 1 h to allow seeds to imbibe.
• Pest and disease susceptibility: Translocated plants were protected with a wire mesh plant guard because plants in the nursery were selectively susceptible to rodent predation. In the nursery, caging plants to prevent rodent herbivory resulted in powdery mildew and a scale infestation likely due to overcrowding, reduced airflow and exclusion of beneficial predators. These problems are considered unlikely in open translocation sites.
Horticultural experimentation informs translocation methods
Nursery based experimentation can inform better translocation methods. Nursery trials in this case have resulted in the development of an optimised growing medium leading to healthier, more vigorous plants, the use of deeper tubes to facilitate optimal root development, a better understanding of how and when to translocate plants, and the recognition of the importance of post translocation care.
Combining our expertise for better outcomes
ANBG is increasingly moving away from a restricted role as plant supplier, towards a model of involvement in conservation projects from planning to post translocation. This helps to combine our multi agency expertise and skills as outlined above to achieve better conservation outcomes.
Next steps
The seed stores and the plants produced in the nursery, with the resulting ecological knowledge about the species, are being used in translocation efforts to increase the size and vigour of wild populations. The Office of Nature Conservation has rigorously planned the Small Purple Pea translocations, incorporating experimentation and data collection to inform future translocation success. It will be very rewarding to see the plants go back into the wild, and hopefully result in genetically diverse, self sustaining populations.
Further reading
Buza, L, Young, A and Thrall, P. (2000). Genetic erosion, inbreeding and reduced fitness in fragmented populations of the endangered tetraploid pea Swainsona recta. Biological Conservation, 93(2): 177–186. https://doi.org/10.1016/S0006 3207(99)00150 0
