Sometime in the past 20 years, there arose vast fleets of garden waste bins emptied by trucks smogging their way around suburbs. Auckland landscape gardener Mark van Kaathoven reckons we pay someone to take away next year’s garden fertiliser and buy it back as dead compost in plastic bags.
Van Kaathoven, whose business is called Urban Jungles, treasures his garden waste and takes other people’s, too. His small property is a top destination for central Auckland’s native birds because he lays that waste on the ground to fuel a lush tropical forest that surrounds every inch of his property that isn’t house or paths. His berm, too, is a forest emerging from layers of garden waste. “It all starts from the ground up,” he says. “None of this would be happening if it wasn’t for the ground.”
With the support of the local residents’ association, the local board and Auckland’s deputy mayor Desley Simpson, his handiwork has also spread to nearby parks. A team of vetted landscape gardeners drop waste there for free, including cabbage tree leaves and flax. Most commercial composters can’t process such stringy vegetation, which is one reason 88,800 tonnes of garden waste ends up yearly in Auckland landfills, generating 12% of their emissions.
Park trees that were struggling amid mown grass are flourishing as their roots are newly nourished by spongy plant matter about a metre deep. The layers decompose into nutrient-rich soil and are repeatedly replenished by unmulched garden waste.
“I’m doing what happens in the bush. Does Mother Nature deliver everything mulched up?” asks van Kaathoven. In the parks, he covers the waste with woodchips for aesthetic reasons.
The layers effectively comprise a mulch, however. Matt Norris, a soil scientist at crown research institute Plant and Food Research, says covering soil with mulch improves soil health. “If you have bare soil, you lose a lot of moisture, and that soil heats up. And there’s nutrients in mulch, so those will be broken down and released into the soil and potentially be available for plants to take up.
“You’re putting organic matter into the soil, and the more organic matter you’ve got in any soil, the healthier it is. It maintains soil structure and feeds a whole bunch of micro-organisms and bugs that help cycle nutrients through the soil.” Soil organisms range from big creatures like earthworms to myriad species of micro-organisms and fungi. Different organisms are responsible for digesting and releasing nutrients from different types of plant matter.
“Weed suppression is another thing that mulching is often used for,” says Norris. “For example, in organic cropping systems, mulch is commonly used as a substitute for herbicides to control weeds.”
Plus, there’s the sponginess. “One of the issues you’ve got in cities is water attenuation after heavy rainfall events. There’s a lot of hard surface, and when you get big rainfall, it all runs off. Urban designers will be thinking about how they can create structures to help attenuate water flow. Mulch is in that category. It’s like a sponge. You’re building up organic matter in the soil, and you’ve got a nice layer of mulch on top. Well-mulched soil will absorb a lot more water than a hard surface and release this water over time in a slower way.”
That played out during Cyclone Gabrielle, when van Kaathoven’s neighbours were flooded and he wasn’t. Downhill neighbours have now lost their flood insurance.
“Green infrastructure is a capacitor for huge rainfalls. Grey infrastructure alone will never cope,” van Kaathoven says. “Being ‘tidy’, like many people perceive it, is the most dangerous thing you can do for the environment.
“You’ve got to work with your environment, not condition it to your rules and regulations. If more people did that, they’d be pleasantly surprised by how much fun they’d have.”
