The Hidden Threat Emerging from New Zealand Backyards

New Zealand's lush native bush areas, particularly around urban centers like Auckland, are under siege from an unexpected source: home gardens. Recent research from the University of Auckland (UoA) has illuminated how everyday ornamental plants and houseplants are escaping cultivation and invading nearby native ecosystems. This phenomenon, often referred to as the 'house plants invasion,' underscores the unintended consequences of popular gardening trends. PhD candidate Diana Borse's work reveals a direct correlation between the density of residential gardens and the proliferation of weeds in adjacent revegetation sites, challenging assumptions about the benign nature of backyard greenery.

In New Zealand, where 80 percent of biodiversity is endemic—found nowhere else on Earth—these invasions pose a profound risk. Native plants, insects, birds, and fungi have co-evolved over millennia in delicate balance. When exotic weeds take hold, they disrupt this equilibrium, forming dense monocultures that smother seedlings and alter soil chemistry. UoA's findings come at a critical time, as climate warming expands suitable habitats for tropical escapees like monstera deliciosa, once a trendy indoor plant popularized during the COVID-19 era.

Diana Borse's Pioneering PhD Research at UoA

Diana Borse, a PhD candidate in the University of Auckland's School of Biological Sciences, is at the forefront of this investigation as part of the Bioprotection Aotearoa Research Centre—a national Centre of Research Excellence dedicated to protecting Aotearoa from biological threats. Supervised by Associate Professor Margaret Stanley and Dr. Nick Waipara from Plant & Food Research, Borse's project examines the interactions among co-occurring weeds and their compounded effects on native species in restoration plantings.

Her fieldwork focused on Auckland's urban-native interfaces, surveying revegetation sites where communities plant native trees like mānuka (Leptospermum scoparium) to restore bush. Borse meticulously recorded weed presence, identifying patterns where woody invaders like woolly nightshade (Solanum mauritianum), brush wattle (Paraserianthes lophantha), and tree privet (Ligustrum lucidum) dominate. A key dataset released in late 2025 details weed co-occurrence, providing managers with evidence-based priorities for removal.

Borse's methodology combined field surveys with controlled shade-house experiments, testing how weed combinations suppress native growth compared to single-species invasions. This multi-weed approach fills a critical gap, as most prior studies targeted isolated pests. Her supervisors praise the work's timeliness, noting UoA's long-standing leadership in invasion ecology through the Applied Ecology Research Group.

Key Findings: More Gardens Mean More Weeds

The headline result from Borse's analysis is stark: the number of adjacent residential gardens directly predicts weed abundance in native plantings. Areas surrounded by sprawling suburban gardens harbor significantly higher weed densities than those near high-density housing, where fewer gardens limit seed sources. Birds, particularly silvereyes, play a pivotal role, dispersing small-fruited seeds from ornamentals into bush remnants.

Population density acts as a buffer; intensification reduces garden space, curbing escapes. Ornamental species—prized for their aesthetics—are disproportionately represented among invaders. Borse notes, 'A lot of ornamental species are the ones that end up getting out of hand. That's a major source, actually, of weeds: pretty species.' This aligns with national trends: of New Zealand's 30,000 exotic plants, over 2,600 naturalize in the wild, with 386 classified as environmental weeds by the Department of Conservation (DOC).

• Weed cover increases linearly with garden proximity in revegetation sites.
• Bird-dispersed fruits from climbers and shrubs dominate escapes.
• Multi-weed interactions amplify suppression of natives like mānuka by up to 50 percent in experiments.

From Houseplant to Invader: The Escape Pathway

Ornamental plants enter New Zealand via trade, gaining popularity through garden centers and social media. Traits like shade tolerance, prolific seeding, and bird-dispersed fruits propel them beyond fences. Monstera deliciosa, discarded during trends, now carpets Auckland parks. Tradescantia fluminensis (wandering tradescantia), once a houseplant, smothers forest floors nationwide.

The process unfolds step-by-step: (1) Planting in gardens; (2) Seed production and bird/animal/ wind/human dispersal; (3) Establishment in disturbed native edges; (4) Expansion into intact bush, outcompeting slow-growing natives. UoA's Margaret Stanley warns, 'People are putting things in their gardens that don’t stay in their gardens.' Figs exemplify emerging risks: their pollinator's arrival enables viable seeds, turning ornamentals weedy.

Annually, about 20 new species naturalize, with a quarter of introduced plants holding weed potential. Auckland and Northland, NZ's warmest regions, lead in weediness, a trend climate change will intensify.

Photo by Carnet de Voyage d'Alex on Unsplash

Case Studies: Iconic Invasives from Gardens

Tradescantia fluminensis blankets forests, preventing native regeneration by forming impenetrable mats. Woolly nightshade poisons soil with alkaloids, inhibiting seedlings. Agapanthus erodes coastal dunes, while privet dominates canopies, shading out natives. Climbing asparagus strangles trees, and russell lupins fix nitrogen, favoring exotics over impoverished native soils.

Borse's data highlights co-occurrences: woolly nightshade with brush wattle accelerates mānuka suppression. In Auckland's Waitākere Ranges, garden escapes have transformed edges into weed thickets, costing restoration efforts dearly. DOC's 2024 list flags 386 such threats, many climbers and shrubs from horticulture.

Devastating Impacts on Native Ecosystems

Weeds homogenize biodiversity, replacing diverse natives with monocultures. NZ's endemics—80 percent unique—lose partners: invertebrates starve without host plants, birds decline without fruits/nectar, fungi fail without mycorrhizal links. Soil changes: lupins enrich for invaders, nightshade toxifies.

Biodiversity loss cascades: extinct interactions ripple through food webs. Restoration fails as weeds recur from gardens. Economic toll: invasives cost NZ $69 billion (1968-2020), over $100 million yearly in damages alone; wilding pines alone $10 million/year control.

The Economic Burden and Management Challenges

Control is labor-intensive: manual pulling, herbicides, goat grazing. Eradication succeeds rarely; plants regenerate vegetatively. Regional councils spend millions; national strategy lags behind animal pests. UoA research aids prioritization, but prevention trumps cure.

Climate shifts northward viable ranges, taxing resources. Borse's co-occurrence maps guide targeted removal, enhancing efficiency.

Solutions: Informed Gardening and Policy Tools

Choose wisely: natives coevolved safely. Verify labels—exotics mis-sold as natives abound. DOC's invasive guide lists 386 weeds; check regionally.Download here.

Weedbusters' Plant Me Instead suggests alternatives: non-invasive natives/non-natives matching aesthetics.Explore the booklet. Dump waste properly; cover loads. Stanley advocates accreditation: 'safe plant' tags like free-range eggs.

Photo by Rapha Wilde on Unsplash

• Assess new plants via Landcare Research predictors.
• Join Weedbusters groups for community action.
• Report via iNaturalist.
• Prioritize multi-weed removal per Borse's data.

UoA's Leadership in Biosecurity Research

University of Auckland anchors NZ's fight via Stanley's lab, Bioprotection Aotearoa, and events like Island Invasives 2026. PhDs like Borse translate ecology to policy, collaborating with DOC, councils. UoA's datasets empower managers; future accreditation could stem the tide.

This research exemplifies higher education's role: training experts, generating data, fostering partnerships for sustainable NZ.

Future Outlook: Climate, Policy, and Community Action

Warming expands weed ranges; olives seed Hauraki islands. Policy: ban pre-emptively? UoA pushes prediction models. Communities: garden audits, native swaps. With UoA insights, NZ can safeguard taonga species.

Gardeners: audit today—replace risks. Unis like UoA drive solutions; join the biosecurity vanguard.