Centre for Alternative Technology

Gardens and Climate Change


The Whole Story – Zero Carbon Gardening at the Whole Home House

Chloë Ward

Find out about the climate-conscious garden of the “Whole Home House” on CAT display site.

Radio 4’s ‘Gardeners’ Question Time’ is playing again in the Whole Home, and the experts are talking climate change. Mr and Mrs Whole are listening intently. The (thought to be fictional) residents of the Whole Home on the CAT visitor circuit want to do everything they can to lower their emissions. Their environmental halos are shining brightly, and their garden is as ecological as they come, but what more can they do? The radio feature is all about how gardeners can adjust to the challenges of a changing climate. It’s about drought resistant plants and collecting rain water – i.e. how to continue having a pretty garden while the weather goes haywire. They’ve heard it all before.

‘We don’t want to adapt to it, we want to stop it,’ says Mr Whole as he peruses his latest Carbon Gym scores. ‘What’s the point of having the best Yucca display in the county, if society is collapsing under resource conflict?’
‘I think it’s crazy,’ agrees Mrs Whole, as she labels her latest batch of blackcurrant jam. ‘Just a few years ago most of the media was denying climate change – now they are saying we’ll just have to put up with it. And those who think it’ll all be fun and sunshine and growing pineapples… They’re so naive, it makes my blood boil.’

So, what should Mr and Mrs Whole be doing? They are well on the way to a climate friendly garden already:
• They don’t use a patio heater or have bonfires
• There’s no concrete in the garden (cement manufacture gives off lots of carbon dioxide)
• They don’t use peat (therefore leaving it in the ground as a carbon store)
• They don’t use chemical fertilisers (which take a lot of energy to produce)
• They’ve exchanged their petrol lawn mower for a push powered one
• They compost their weeds and food scraps (preventing methane emissions from land fill)
• Their raised bed edgings are manufactured from recycled plastic and sawdust (the sawdust acting as a carbon store)
• They grow fruit and vegetables, so reducing their food miles (and emissions from refrigeration and packaging)
• They prefer to spend a week enjoying their garden than jetting off on a foreign holiday

Although they want to prevent climate change rather than adapt to it, Mr and Mrs Whole do realise the importance of preserving resilient ecosystems – the more biodiversity, the better able humanity will be to cope with unpredictable changes.


So they also,
• offer many types of habitat for wildlife within their garden,
• support sustainably managed local woodlands by using coppice products such as pea sticks and barbecue charcoal,
• grow heritage vegetables and save their own seed thus preserving vegetable diversity.

But Mr and Mrs Whole want to know how greenhouse gasses interact with their garden. They know that carbon is stored in trees and have heard that there can be lots in the soil as well. They’ve also heard that UK agriculture is responsible for 10 per cent of climate change, and the entire food sector including imports, up to 30per cent...but what about gardens?

Carbon Dioxide (CO2) is the major greenhouse gas, responsible for 63 per cent of climate change. Remember the Carbon Cycle? It has arrows and pictures of cows and you probably had to learn it at school. CO2 is absorbed from the atmosphere by plants and ‘fixed’ in photosynthesis to make sugars and starches. So, plants are full of carbon. If the plant dies, or when leaves fall in the autumn, much of the carbon passes into the soil, so making soils a carbon sink too. When plant tissues are eaten by animals the carbon within them is used in respiration – they (we) breathe out CO2. So in a happily functioning system the carbon goes round and round, from air to plants to animals to air – all good. But humans have brought a few other things into the equation. We have reduced the amount of plant biomass in the world (by deforestation) and increased the number of animals (by livestock farming). There are now fewer living things fixing carbon and many more living things respiring it. We have also constructed huge artificial animals that respire the plant tissues of the past i.e. fossil fuel power stations that burn fossilised plant remains, so reducing ancient carbon sinks and emitting CO2. A coal or gas fired power station acts just like a very big animal within the carbon cycle. So the problem is that there is too much CO2 in the atmosphere and not enough in the plants and soil. We are all familiar with the concept of planting trees as carbon sinks.

But in temperate climates like the UK, a healthy soil can contain more carbon than the vegetation growing above it. So, how to get more carbon into the soil? Remember the chemical definition of organic? Organic means it’s got carbon in it. This used to seem like an irrelevant concept to the aims of organic gardening – keeping our gardens free of inorganic nasties, but now it couldn’t be more meaningful. When we add organic matter to the soil we are adding carbon.

So, adding compost, and green manures (fertility-building crops) has got to be good. A rich, dark soil is what we want for our vegetables and it contains carbon too. But it all has to be carefully managed – leaks from the system can occur too easily. For example when the soil is dug, carbon compounds can be oxidised to CO2, so gardening with minimum tillage is a good thing in this respect. Compost making also needs to be carefully managed to ensure the process doesn’t give off methane…

Methane (CH4) is a powerful greenhouse gas with 23 times the global warming potential of carbon dioxide. Agricultural methane emissions mainly come from enteric fermentation, which is part of the digestion process of ruminants (e.g. sheep and cows). This is one of the reasons why CAT’s zerocarbonbritain report is advocating a 60 per cent reduction in animal farming.
But what about plant matter? Methane emissions are a problem when food waste is landfilled, so could this be happening in our
compost bins? Methane is produced when there is a lack of oxygen, so if a heap becomes slimy, the anaerobic conditions cause methane to be produced. A compost heap needs to have plenty of dry matter, to keep air spaces in it, so allowing good aerobic composting. Of course, respiration will be occurring from the microbes, but if you have no smells and end up with a rich dark compost, then you’ve got a carbon rich compost to go into your soil.

Nitrous Oxide (N2O) is a whopping 296 times stronger than CO2. In agriculture, high N2O emissions result from spreading animal manures, or chemical nitrogen fertilisers on the land, from where denitrifying bacteria convert ammonium salts to nitrous oxide. However, this can also be a problem with green manures. Whenever you combine nitrogen rich organic matter, oxygen and warmth, the soil microbes will produce N2O. There is always going to be some leakage from the soil into the air, but what we need to do is minimise that leakage. Keen composters know it’s important to balance nitrogen-rich (green) waste with carbon-rich (brown) waste in the compost bin. Both are needed by micro-organisms to turn waste into compost. The same applies when adding green manures to the soil – a really lush green manure may result in N2O emissions, especially if there is not a nitrogen hungry crop there to take it up. The jury is still out on how best to use our green manures while keeping N2O
emissions down. In the meantime, common garden sense prevails. Green manures are great – they improve soil structure, prevent erosion and leaching, but as with any soil conditioner, use them appropriately for the crop you are growing.

So what does all this mean for Mr and Mrs Whole?
They are already stock-free gardeners, so are not reliant on the products of animal farming, and don’t have to worry about N2O emissions from animal manures. They are keen to keep their compost heap aerobic and with a good C/N ratio. Indeed their home-made compost is doing wonders for their beds. Most of the garden is never dug, as it comprises the lawn, flower borders and fruit area, so no emissions from tillage there. The raised vegetable beds are dug only to incorporate green manures. Mr and Mrs Whole are very keen on green manures, but they will pay extra attention to how they use them. And they will keep up with the latest advice.

By the look of their soil they know they are doing something right – it is getting darker and spongier each year. A good soil not only acts as a carbon sink, but also holds more water, is less easily eroded, suffers less from leaching, and grows really nice veg.

‘Well that was an education,’ says Mr Whole, ‘yet another good reason to look after our soil. And I think, another good reason to write to “Gardeners’ Question Time”.’

‘Oh here we go,’ sighs Mrs Whole.

For more information

Find out how to visit the “Whole Home House” on CAT display site.
zerocarbonbritain – www.zerocarbonbritain.org
food climate research network – www.fcrn.org.uk
‘soil not oil’ report – the Soil Association www.soilassociation.org
high fibre compost – CAT Publications: Cool Composting factsheet;
Hot Composting tipsheet; Compost tipsheet

From Clean Slate issue            20