The Age of Pittosporum undulatum [WEED]
(c) Saahmadbulbul (Wikipedia)
The age of Pittosporum undulatum is upon us. A plant species that has come to be considered a environmental weed in the country which it is native to, Australia. This once rainforest dwelling plant has prolifically broadened its horizons naturalising in ecosystems across the country from coastal heath to Sclerophyll forests. but what is the story of undulatum, how did this once loved native come to be so hated by gardeners and environmentalists alike?
The term "weed" traditionally evokes images of invasive non-native species disrupting fragile native ecosystems or many people, grass intruding on your flower bed or popping up between your pavers. However, the concept of native plants behaving like weeds is a complex and increasingly relevant topic.
Pittosporum undulatum exemplifies this, as it has transitioned from a favoured native species with specific habitat requirements to a plant with a weed-like presence colonising altered landscapes, out competing other endemic natives for precious resources.
The definition of a weed according Australian Association of Bush Regenerators (AABR) is: 'A weed is any plant that colonises and persists in an ecosystem in which it did not previously exist. They may affect the economy, the environment, human health and amenity.’ To a common gardener a weed is a plant growing where they don’t want it to grow. By these definitions, Pittosporum undulatum fits the bill very well.
Personally, As a landscape gardener, almost every garden I work in has undulatum present and an eager gardener wanting it removed. but the reality is, we are the problem, it is our human altered landscapes that have created the perfect conditions for the ‘Age of Pittosporum undulatum’ to unfold in many of our lifetimes.
(c) Luca Boscain
Know the Enemy
Enemy may be a dramatic word to use when speaking about a plant species, but it is apt when it comes to the way Pittosporum undulatum has colonised many of our Australian native ecosystems. A Tree from 4 -15m tall with lush evergreen domed growth, heavenly scented white/cream flowers and striking orange fruits and woody seeds that are favored by native animals as a valuable food source.
Known by its common names sweet pittosporum, native daphne, Australian cheesewood, Victorian box or mock orange, these names evoke the charm and affection we once had for this plant, even idolising it, harking back to european sensibilities. It genuinely is an attractive small tree and is easy to see why it become a popular garden plant.
The Perfect Storm
Pittosporum undulatum's ability to adapt to altered environmental conditions is a testament to its ecological flexibility. Originally confined to cool, damp forest areas, undulatum has successfully colonised a range of habitats, thriving in environments that are enriched by human activities, such as nutrient runoff due to chemical fertiliser use in farming and home gardening practices. Many other native species can’t handle the high levels of phosphorus leached into soils and river systems through such practices.
Beyond these areas, undulatum has learnt to adapt to a wide range of environments, invading forests and bushland outside its natural range. Their foliage shades out vegetation growing beneath it and successfully competes for limited soil nutrients and water. Over time Pittosporum undulatum changes in soil make up, building more nutrient rich soils due to the breakdown of their rich leaf litter. This may sound like a good thing, but it isn’t when the local flora of the ecosystem has evolved to be very sensitive to high nutrient levels. species such as banksia's and other phosphorus sensitive species begin to struggle in the face of their new rivals.
Pre-colonisation, Fire was a welcome and common occurrence in the Australian landscape. Our native flora harnessed the fires power, evolving with it. Many Native species need fire for seed germination, using it as an essential and irreplaceable element in their reproductive process.
The decline in fire frequency since colonisation has disrupted the natural checks on undulatum's spread, allowing it to outcompete native species that rely on fire for seed germination. The unintended consequences of human interventions in natural processes has allowed undulatum to thrive in ecosystems they once couldn’t due to regular fire which unlike many species, they can not survive. This is another reason Australia needs to reexamine its fire management practices and implement holistic ecosystem management approaches. Something First Nations people keenly understood.
Undulatum is a species that evolved in cool, damp temperate rainforest areas that were mostly free from bushfire. The wet, damp nutrient rich gullies of South East Queensland New South Wales and Victoria has meant they have not evolved to use fire or smoke for germination nor are they adapted to survive fire.
Understanding the mechanisms driving seed dispersal is crucial for predicting and managing the spread of invasive or weed-like species. Undulatum's prolific spread across vast distances is facilitated by birds and mammals that disperse its seeds through their fecal matter making management incredibly hard.
Management
Pittosporum undulatum needs a tailored management approach based on the species' status as endemic or non-endemic in specific areas. In native habitats, controlling undulatum populations is essential to prevent them from overwhelming other native species. This may involve methods such as habitat restoration, controlled burns, or targeted removal of undulatum individuals with chemicals. In areas where undulatum is non-endemic and poses a threat to local biodiversity, complete eradication is recommended to prevent further ecological harm. When it comes to keeping them as ornamental species in gardens, unless you live in a cool damp gully it is best to replace with another native species that attracts and feeds wildlife like cultivars of banksia, gravillia, or Callistemon.
Pittosporum undulatum illustrates how human actions, natural processes, and native plant behavior interact in complex ways. Dealing with native plants that act like weeds, needs an understanding of ecology, prioritising conservation and finding the missing links in our ecological ecosystems. Australia's reluctance to use fire in landscapes is starting to have consequences. Constantly suppressing fires could harm native landscapes and wildlife in the long run in ways we can’t yet imagine.