Invasion of the house plants

2026-03-15 - 16:06

It’s the monstera that Margaret Stanley has been noticing lately. That’s the tropical houseplant that was all the rage a few years ago – the one with massive, glossy leaves perforated with holes. Now, Stanley has spotted monsteras growing in shady corners of parks across Auckland. The most likely source: people liberating a plant they no longer want to care for, or one they think is dead. Monstera is just the latest in a succession of plants that have escaped and started to grow out-of-bounds: about 20 new species do this every year, and some of them end up damaging entire landscapes and ecosystems. You’d be forgiven for thinking that if a plant is for sale in a garden centre, then it’s safe for the environment. Not so. Plant species are usually only banned long after they’ve caused environmental damage, at which point it’s practically impossible to eradicate them. That’s why Stanley and other academics are focused on predicting future weeds – stopping invasive plants before they’ve committed any environmental crimes. “It’s too late by the time you detect it,” says Stanley. Two of New Zealand’s worst weeds, tradescantia and climbing asparagus, were originally houseplants. Now, countless hours of volunteer time around the country are dedicated to getting rid of them. When a plant isn’t native to New Zealand, and when it spikes in popularity – as monstera did during the Covid-19 pandemic – then it’s in danger of it taking off as a weed. Stanley, a professor of biological sciences at the University of Auckland, estimates that around a quarter of the 25,000-odd introduced plant species in the country have the potential to go rogue. “There are constantly new ones from that pool being sold or marketed,” she says. “The more they’re sold and planted, the more likely they are to become a weed.” How gardens are the key to preventing weeds It starts at home. “People are putting things in their gardens that don’t stay in their gardens,” says Stanley. New research by University of Auckland PhD candidate Diana Borse found that more gardens translated into more weeds. She studied areas of native planting in Auckland, and found that weeds increased according to the number of adjacent gardens. Greater population density, on the other hand, meant fewer weeds, since housing intensification reduces the number of gardens. When it comes to known weeds, it’s not that people are intentionally spreading them. “There are a lot of ways that plants get in and out of people’s gardens,” says Borse. “A lot of the plants that I found growing together were bird-dispersed – so birds are eating the fruits and then pooping [the seeds] out.” New weeds, on the other hand, are emerging all the time. Ecologists like Stanley aim to pinpoint which garden-centre favourites might escape into the wild. There are a few traits that increase the chances of a plant taking off: tolerating shade, for instance, or producing small fruit which birds eat. “You only need a flock of silvereyes coming along, and they’re going to take it straight to the bush or the wetland or whatever.” Mostly, though, the risk of new weeds comes down to trends: what people are decorating with, indoors and out. “A lot of ornamental species are the ones that end up getting out of hand,” says Borse. “That’s a major source, actually, of weeds: pretty species.” Isn’t a weed just a flower growing in the wrong place? The problem is balance, says Stanley: invasive weeds cannot co-exist with other species. Instead, they take over, or they ruin the landscape for everything else. Russell lupins change the level of nutrients in the soil, making it more favourable to other weeds and more difficult for native plants to grow there. Woolly nightshade produces its own herbicide, which it uses to poison the soil and stop other plants from getting a foothold. Agapanthus loves coastal landscapes, where it overwhelms and destroys fragile sand-dune ecosystems. Privet kills adjacent mānuka and fills the understorey of the forest, stopping other seedlings from establishing. The result is a monoculture: a forest where all the trees are privets. Moreover, weeds are an expensive problem. Compared to animal pests, it’s extremely difficult to eradicate plant pests: only a few have ever been completely removed. The government spends $10 million a year killing wilding pines. If left alone, pines would spread quickly and soak up enough water to threaten hydro lake levels, among other issues. And that’s just one invasive plant. To preserve what makes this country unique, says Stanley, we have to keep fighting weeds. “The problem is that 80 percent of our biodiversity is found nowhere else in the world,” she says. “Wherever you’ve got something that’s taking up all the space, then it’s replaced the plants that were supposed to be there. And because of the evolutionary relationships those plants had with invertebrates and fungi and lichen and birds, then you miss all of that biodiversity as well. So they can completely restructure the environment.” What to plant at home – and what to watch out for If people know what’s growing in their own gardens, and avoid cultivating species with future weed potential, Stanley reckons the invasive-plant pipeline can be plugged. Native species are always a safe bet, because they evolved here in balance with other plant species that keep them in check. However, Stanley cautions shoppers to double-check that plants in the ‘native’ section of the garden centre are in fact natives. (She’s more than once discovered exotics mislabelled as local species.) The national guide Plant Me Instead suggests alternatives to risky plants, both native and non-native, and the Department of Conservation also has a guide to invasive plants, some of which are banned, some not. (Banned plant lists are regional and revisited only once every 10 years). Stanley cautions that just because a species has existed in New Zealand for decades doesn’t mean it’s safe. Some plants act a bit like deep-cover spies in the environment, waiting for the right signal to activate – or the right wasp, says Stanley. “Fig trees are becoming really weedy now because their pollinator just arrived. Before that, they couldn’t produce viable seeds.” Climate change may also act as an agent turning plants from benign to weedy: Auckland and Northland are the weediest areas in the country mainly because they’re the warmest. Stanley recently began worrying about the spread of another Mediterranean species there: “Olive seedlings are starting to pop up on the Hauraki Gulf islands.” In future, Stanley would love to see an accreditation system for safe plants, like the one that operates in Australia. “It’s a bit like free-range eggs, right? You put a couple of dollars extra on the plant, and it gets a tag saying, ‘This is a safe plant for biodiversity’.