A recent paper in the journal Nature Cities¹ created a bit of a stir with its headline that the Greenhouse Gas emissions (GHG) produced by growing a portion of fruit and vegetables in small scale urban horticulture are six times that of conventional horticulture.

This has led to incredulity in some circles: for many, urban horticulture and city gardens represent an alternative model for small-scale local food production that rallies against our ‘industrial farming methods’ – so surely it must score better.

However, when you examine the article further, you find the authors are actually very supportive of urban horticulture and the paper provides a lot of helpful practical advice on how small-scale urban gardens can reduce their carbon footprint. They highlight how short-lived infrastructure can massively bump up the greenhouse gas emissions from food production. They also acknowledge the many other benefits from urban agriculture that should be considered.

How the footprint of food production was assessed

Evaluating the carbon footprint of anything is complex, but it becomes especially difficult when you try to compare very different ways of producing food. This study collected data from 250 conventional farms and 73 urban horticultural sites in the US, France, Germany, Poland and the UK.

The urban horticulture group was divided into three further categories of seven urban farms, nine collective gardens, 55 private gardens and two others. The researchers recorded the inputs and infrastructure and kept a diary of yields, then calculated the GHG that could be apportioned to a standard serving of fruit and vegetables from each of these methods of food production.

What the research demonstrated

Although the overall finding was that urban horticulture produced six times the GHG of horticulture per serving, we need to delve a bit deeper to find out what this really means.

To start with, collective gardens may have other priorities beyond efficient food production. The collective gardens had a very high GHG per serving, almost 10 times that of conventional horticulture, which would have skewed the overall urban horticulture results.

But we must bear in mind, that many collective gardens have important social and therapeutic functions, so efficient food production may not be a priority. If these gardens are producing low yields, then the number of resources used and therefore the GHG per serving of food produced will be disproportionately high. (Note that when apportioning resources, they took account of the area of the garden devoted to food growing.)

The biggest culprit contributing to the high GHG emissions of urban horticulture was all the energy used to create the infrastructure.

On average this contributed to 63% of the GHG emissions. The insecure tenure of many urban sites and short-lived nature of project funding increases the risk of infrastructure only being used for a brief period of time.

Urban farms can compare favourably to large scale horticulture

The urban farms showed up much more favourably in this analysis. Once one outlier (with a very high GHG output) was removed from the urban farm dataset, the GHG output per serving was almost identical to that of conventional farms.

Urban farms are often run as small businesses prioritising efficient food production, so the resources used per amount of food compares favourably. Other studies have also highlighted the potential for high productivity of urban farms².

Choice of crops

The study presented the GHGs for a number of different crops. The variability in GHGs per kg of vegetable produced was extremely wide between sites. This may have been a reflection of the difficulty in accurately estimating yields from small plots. Some crops such as tomatoes grown in lowtech facilities at urban farm sites produced less GHG than those grown in heated glasshouses in commercial horticulture.

Benefits of urban horticulture beyond GHG emissions

• Food security. In the UK, we import around half of our vegetables and 85% of our fruit⁴ and we have long, complex supply chains that are very vulnerable to being disrupted by unexpected events. Direct food supplies from urban farms and other alternative schemes have an important role in food security and food sovereignty⁵. This was demonstrated in the Covid pandemic in 2020 where food sales from some vegetable box schemes rose by 111%⁶.

• Social benefits. Urban horticulture also provides many social and therapeutic benefits to society⁷, including reducing social isolation, providing access to green spaces and improving community cohesion. These benefits can be far-reaching but difficult to quantify. This is acknowledged in the study.

• Diet. It’s been shown that people participating at urban horticulture sites often change their diet to eat more fruit and vegetables and less meat, which reduced the GHG impact of their diet by 12%⁸.

• Biodiversity. The study doesn’t mention the impact on biodiversity, but this is an important component. Urban horticulture sites often have diverse planting providing valuable habitats and nature corridors in cities⁹. This is likely to compare more favourably than the intensively managed monoculture of conventional horticulture.

• Soil health. In this study, urban horticulture used 95% fewer synthetic inputs than conventional horticulture. Additionally, there are studies showing that the health of soils in urban horticulture is often better than that of nearby equivalent conventionally farmed soils, having better soil structure and storing more soil organic carbon¹⁰.

In an era when we tend to just look at headlines then scroll onto the next item – it’s easy to think this study must be on a mission to attack urban horticulture. In fact, it has provided useful guidance as to where urban food producers can reduce their environmental impact. It’s also not the only study to highlight that small scale horticulture can use disproportionate amounts of resources for the food it produces, if not managed carefully².

It’s useful to treat these articles as warnings not to be complacent. We must not assume that just because urban food production is small, local and friendly, it has a smaller impact than larger scale horticulture.

However, as with any growing space, resources need judicious management. The good news is this can be easily achieved by adopting the many sustainable practices that any good organic grower would use.

How can GHG emissions from urban farms be reduced?

TIP 1: Make sure your infrastructure gets good use for a long time. For example,
a raised bed used for just five years will have four times the environmental
impact of one that lasts 20 years. Question whether raised beds are really
necessary, and if so try and choose a more durable wood such as cedar or use
recycled materials (see below).

TIP 2: Use upcycled materials (e.g. hazel poles rather than bought canes).
Farms that used recycled wood for their raised beds managed to reduce their
emissions by 52%.

TIP 3: Composting is integral to many urban sites and can have a positive
impact. It reuses waste materials and reduces the need for synthetic nutrients,
but it comes with risks. The embedded energy in making plastic bins is
significant and there’s also the danger that small scale piles containing a
high proportion of wet, green materials can turn anaerobic resulting in high
methane emissions³. Use upcycled materials to build composting infrastructure
and consider larger scale community composting with a plan to turn and aerate
it properly.

TIP 4: Water use can also have a significant impact especially if a large
proportion of mains drinking water is used. At a few sites, it contributed to
more than 50% of the carbon footprint. Use collected water where possible
and add plenty of compost to the soil to build up its capacity to retain water.
Grow more deep-rooted perennial crops that make better use of water.

TIP 5: Crops grown at urban sites such as beans, sugar snap peas and berry
fruits will compare favourably to their air-freighted equivalents so would make
a good choice if you wanted to reduce your GHG emissions.

References

1. Hawes, J. K. et al. Comparing the carbon footprints of urban and conventional agriculture. Nat. Cities 1, 164–173 (2024).

2. McDougall, R., Kristiansen, P. & Rader, R. Small-scale urban agriculture results in high yields but requires judicious management of inputs to achieve sustainability. Proc. Natl. Acad. Sci. U. S. A. 116, 129–134 (2019).

3. Colón, J. et al. Environmental assessment of home composting. Resour. Conserv. Recycl. 54, 893–904 (2010).

4. United Kingdom Food Security Report 2021: Theme 2: UK Food Supply Sources. GOV.UK https://www.gov.uk/government/....

5. Edmondson, J. L. et al. The hidden potential of urban horticulture. Nat. Food 1, 155–159 (2020).

6. Wheeler, Amber. Covid-19 UK Veg Box Scheme Report | Food Foundation. https://foodfoundation.org.uk/... (2020).

7. Ilieva, R. T. et al. The Socio-Cultural Benefits of Urban Agriculture: A Review of the Literature. Land 11, 622 (2022).

8. Puigdueta, I., Aguilera, E., Cruz, J. L., Iglesias, A. & Sanz-Cobena, A. Urban agriculture may change food consumption towards low carbon diets. Glob. Food Secur. 28, 100507 (2021).

9. Goddard, M. A., Dougill, A. J. & Benton, T. G. Scaling up from gardens: biodiversity conservation in urban environments. Trends Ecol. Evol. 25, 90–98 (2010).

10. Dobson, M. C., Crispo, M., Blevins, R. S., Warren, P. H. & Edmondson, J. L. An assessment of urban horticultural soil quality in the United Kingdom and its contribution to carbon storage. Sci. Total Environ. 777, (2021).