Climate change can seem like one endless, dispiriting injunction to stop doing things we have always enjoyed doing – but gardening is one exception. Around the world, climate-mitigation efforts are hitting on plants as a potent part of the solution.
The definition of gardening has now been widened to include “rewilding” – letting plants grow as they wish – which may be the easiest, lowest-maintenance greenhouse-gas weapon we have.
But growing plants anywhere faces new technical challenges. Our winters are not just warmer but about a month shorter than they were 80 years ago. It’s a similar pattern in the US, where evidence gathered over more than a century shows the first frost of the year is arriving more than a month later than it did 100 years ago.
The plants we choose will have to change, because climates are changing. Even native plants may struggle to adapt.
New Zealand’s primary horticultural ideas bank is our civic botanical gardens. Well-known horticulturist Jack Hobbs, for one, is using his leadership role at the Auckland Botanic Gardens to encourage more people to garden within the parameters that climate change has introduced.
As he puts it, he’s in “the wellness industry”. “Without plants, there is no life.”
The Auckland gardens aim not just to connect their million annual visitors with nature but also to use the “star” system staff there have devised to help gardeners adapt to a rapidly changing environment by choosing the most suitable plants for the new climate.
With staff monitoring a wide variety of plant types daily, Auckland, like public gardens worldwide, is building a useful data bank of how the climate is affecting flora.
Pragmatic US gardeners, perhaps having experienced weather extremes for longer, have already embraced “dry” gardens, with plantings chosen to thrive with less water. These include Mediterranean plants such as lavender that enjoy low rainfall and free-draining soil; “xeriscapes”, which group together plants with similar water requirements according to the various conditions within a garden site; cacti and succulents; and prairie plantings dominated by grasses and perennial and annual plants.
Britain’s Royal Horticultural Society is warning that much of the country could eventually become frost-free, meaning the tree someone plants today may not survive the climate in 30 years’ time. The society has called for the nation’s estimated 27 million gardeners to join “the greening of Great Britain”. People are being encouraged to grow more plants and replace fences with hedges.
Flora’s headline benefit is carbon sequestration, converting greenhouse gas to oxygen. It also fosters biodiversity by providing habitat and food for insects, animals and birds. Areas of dense planting can reduce air pollution by absorbing particulates, and can dampen urban and traffic noise.
Hobbs’s top checklist item for gardeners is mulching. New Zealand’s councils warn of growing water shortages, with increased use of metering and charging. Some have subsidised water butts or tanks to encourage residents to collect roof water for irrigation.
Covering the soil with rotted or chipped organic matter keeps weeds down and conserves water. Different grades of bark, wood chip, pea straw or even grass clippings can do the job inexpensively. It needs to be laid over – rather than dug into – well-watered soil, and ideally topped up through the year.
Over time, mulch will also keep the soil healthier by fostering billions of microorganisms. The gardener’s extra pay-off is healthier plants, as damaging rust and bacterial diseases are kept at bay and can’t disrupt beneficial insects.
An early adopter of the green gardening movement, Hobbs was for a time considered a dangerous iconoclast in the 1990s for his attitude to disease and insect sprays. Studying horticulture in Europe in 1990 hardened his resolve on cleaner practices.
“Denmark was way ahead of us, stopping the use of certain chemicals and prohibiting wastewater being allowed to flow into groundwater, instead requiring it to be held and recycled. It made a lot of sense and ultimately led us to build a series of raingardens, swales and wetlands using reeds and other plants to filter heavy metals out of the run-off from the carparks and paths.”
Serious rift
Hobbs’s water innovations were uncontroversial, but his captain’s call to stop spraying the garden’s roses caused a serious rift. Legendary rose breeder Sam McGredy was so displeased that he relocated his highly successful annual Rose of the Year show from the Auckland gardens to Hamilton. Like many growers at the time, he was worried the plants would get sick, or at least become unsightly with black spot, rust and other disfigurements.
This was, after all, the largest rose-display garden in the country. Not spraying in humid Auckland was regarded as a particularly brazen act of anarchy.
Hobbs says that some years later, although their friendship was never the same, McGredy was among many former doubters who conceded that despite a few casualties, New Zealand’s best-performing roses could thrive without spray.
“I admit it was a bit nerve-racking to see how the roses would cope, because flowers do have to look good in a public garden. In all honesty, the roses, having not been sprayed in 20 years, are healthier now and look better than they did back then. It was a relief to have your instincts backed up by good results.
‘The problem with spraying is it not only kills off diseases and pests, but also all the beneficial insects, too, which leaves a vacuum for those vigorous bad pests to reinfest.”
Hobbs also wanted to spare staff the health risk of chemical exposure. “Staff were in the gardens dressed like spacemen, with four or five different chemicals in a cocktail of spray. Sure, they excluded the public while they were spraying, but the drift was going everywhere. Then, an hour later, people were sticking their noses into a rose that probably would have glowed in the dark.”
The Auckland facility has a free online database of hundreds of plants it has trialled and that currently perform well in the region’s conditions. Aspiring growers can research cultivars for scent, wildlife benefits, dry tolerance and other attributes.
The garden has attained carbon zero status and received the 2021 Healthy Parks award for innovation, community engagement, excellence and sustainability.
Restoring hedgerows
Global research suggests even a small individual planting effort per household can have a useful carbon-reduction effect. The United States’ National Wildlife Federation estimates that if that country’s 91 million gardening households planted just one shade tree in their yards, those trees would absorb about 2.25 million tonnes of carbon dioxide each year.
And if the UK’s unkempt hedgerows are restored to pre-war dimensions, as is now being attempted, they would sequester enough carbon to accelerate the country’s emissions reduction, according to research by the Campaign to Protect Rural England.
The British Government is committed to paying £15 billion between now and 2050 to farmers and landowners who plant woodlands. And the campaign’s target of increasing hedgerows by 40% has been welcomed as an important component of reaching net zero by 2050.
Ireland is researching the exact carbon sequestration of its hedgerows.
New Zealand has no direct hedgerow equivalent, but a number of Government policy proposals have been aimed at protecting wilderness areas and encouraging the covenanting of more. However, it is not proving a simple policy-design exercise. Infill housing is displacing urban backyard flora, and councils are typically against residents even cultivating their berms.
Economic potential
Conservationists are increasingly challenging local and central government’s habit of spraying weeds, especially along motorway edges and similar wasteland areas.
A chief objection is the use of glyphosate – the agent in weedkillers such as Roundup – which many other countries have restricted or banned after some evidence of carcinogenic risk. Others question the lost value of allowing plants to grow naturally on these areas, provided traffic visibility and safety aren’t impeded.
In 2021, records released under the Official Information Act showed highway contractors sprayed about 30,000 litres of glyphosate in each of the previous three years – a defoliation project spanning the length and breadth of the country.
Conservationists say wilderness corridors merit extra consideration in high-monoculture forestry areas, especially given the boost here in carbon-farming afforestation of recent years.
The Campaign to Protect Rural England says for every £1 invested in hedgerows, up to £3.92 could be generated in the wider economy, with hedgerow restoration creating 25,000 jobs.
However, it’s not feasible to let all weeds rip, given that so many introduced plants are bad for habitats, displacing desirable flora. Weedkiller has proven an indispensable weapon against some of the most destructive.
The Department of Conservation has a bounty on “the dirty dozen”, which includes the most invasive species such as old man’s beard (Clematis vitalba), common buddleia, tradescantia (wandering willie) and English ivy. But it counts 3000 imported plants as unwanted.
Some can be knocked back by importing other species to help eliminate them. Introduction of the ragwort flea beetle has helped reduce ragwort, and the buddleia leaf weevil has been skeletonising the pest plants and the harmless hybrid cultivars alike for several years.
Home horticulture boom
In this country, the biggest spur to home horticulture for many years was the serendipitous effect of the Covid-19 lockdowns. Unable to spend money on overseas travel, and with labour and material shortages impeding home-improvement splurges, New Zealanders increasingly turned to gardening. Some stockists even rationed vegetable-seedling sales, thanks to a steep rise in people growing their own food in ground-level and raised beds and covered “pods”. Space and location seem no impediment, with a welcome trend for balcony and windowsill gardening.
Much of what’s “new” in climate-change gardening is really old wisdom adapted for new circumstances. From the 1960s, UK plantswoman and designer Beth Chatto, for instance, did pioneering work in dry gardening, little imagining how the call for it would expand with global warming.
Newer fields of research give gardeners even more potential benefits. For instance, in addition to being drought-hardy, plants with rough, hairy and small oval-shaped leaves have been determined as the most effective at capturing minute particles and improving air quality. Common garden varieties such as furry greyish lamb’s-ear (Stachys byzantina) and yew trees (Taxus baccata) get two green ticks as well as being aesthetically pleasing.
The new gardening movement will continue to throw up its heretics. British botanist James Wong, a leading light of the indoor-plant craze, has told his followers he doubted the old received wisdom that indoor plants purified the air was true.
It’s a fact that plants convert carbon dioxide to oxygen, a feature that’s sold millions of peace lilies, mother-in-law’s tongues and spider plants. But, Wong says, although a Nasa study five years ago found plants do remove toxic compounds in sealed chambers, it’s an effect unlikely to be replicated at home to the same extent.
Given the air movement through most rooms, a subsequent study found it would take up to 1000 plants per square metre, depending on the variety, to achieve an effect. Many new gardeners would be up for that challenge, but fortunately the benefits to the planet from gardening extend beyond closed doors.
This article was originally published on 16 October 2021.