IRA FLATOW: This is Science Friday. I’m Ira Flatow.

If you have a home garden like I do, chances are you want the food you grow to have less of a carbon footprint than the food grown on large commercial farms, right? Well, unfortunately, new research throws some cold water on that idea. A study led by the University of Michigan, which examined 73 small gardening sites across five countries, found that food grown in urban settings produces six times the carbon emissions per serving compared to commercially grown food.

So where is this extra carbon coming from? And what can we home gardeners do to reduce our footprint? And don’t worry. There is some good news here as well. Here to fill us in on this is Jason Hawes, PhD candidate in the School for Environment and Sustainability at the University of Michigan. He was the lead author on the study published in Nature Cities. Welcome to Science Friday.

JASON HAWES: Hi, Ira. Thanks for having me.

IRA FLATOW: You’re welcome. Why did you and your team want to do this study? And what was the broad takeaway here?

JASON HAWES: So this project emerged from a broader assessment of the holistic impacts of urban agriculture on people, places, and the planet. And so among the things that we didn’t know was the carbon footprint of urban farms and gardens that grew things in kind of a low-tech way that a lot of folks do in an allotment garden, community garden, or even in their backyard.

IRA FLATOW: Right. Right. So you went out and studied all these little plots of land?

JASON HAWES: Right. Right. So we worked with farmers and gardeners, actually, to try to understand what they were investing in their garden spaces, both in terms of time and resources, and then also what they were getting out of it, both in terms of food and social benefits. And we used those data to assess a variety of different kinds of impacts on cities and on people.

And one of the last projects that came out of that data set was this carbon footprint paper, which, as you say, the headline maybe is a little bit alarming. But the good news is that we were also able to use that large data set that we generated to figure out what we could do better, what we could do differently to support low-carbon gardening in the future.

IRA FLATOW: Interesting, because I was just getting my tomatoes ready for the garden and preparing my first attempts at asparagus. And you find that tomatoes and asparagus usually had less carbon footprint than commercial farms. So that’s good news. And why is that?

JASON HAWES: Yeah, that’s right. That is one of the main takeaways. So as far as we can tell, most urban fruits and vegetables have a relatively similar carbon footprint, at least on the sites we studied. But these sort of things you buy at a grocery store, for example, is much higher.

So the examples that we cited in the paper, as you say, were tomatoes and asparagus, although there are plenty of other examples. And the reason those stand out is because tomatoes in northern cities are often grown in greenhouses on their way to the supermarket. And asparagus is often actually flown in on an airplane because it spoils really quickly.

We quantify something we call embodied carbon, which captures basically everything that goes in from “cradle to grave”– so from the very beginning of the supply chain– in this case, growing the food– to the very end– in this case, at the supermarket. So if we can start to replace some of those, then that is an automatic good even before we do the sorts of best practices we identified to lower the broader carbon footprint of urban ag.

IRA FLATOW: That’s cool. Let’s talk about the major carbon footprint source of home gardening. What was that?

JASON HAWES: So the big thing was what we call infrastructure. So we divided inputs into three categories. We had first irrigation water. And then we had supplies, which were the sorts of things you put into a garden every day– so fertilizers, compost. And then we had infrastructure, which is all the stuff that we build in order to grow food. So it could be compost bins, could be raised beds.

And what we found is that when you do build those, the carbon invested in those raised beds is actually a substantial portion of the carbon footprint of urban fruits and vegetables throughout the lifecycle of a plot of land. So one of the TikTok responses that I saw to this article was someone getting really upset about the idea that they could buy tomatoes or any other crop that had a lower carbon footprint than they could grow in their backyard and saying, are you really telling me that if I go to the hardware store, I build a raised bed, and I grow the food– that that has a higher carbon footprint than shipping something in from overseas or from a distant farm?

And the short answer is yes. But the more nuanced and interesting answer is if you think about where the wood and the nails and everything that you used in that raised bed came from, it also got shipped in from somewhere distant.

IRA FLATOW: Is there an answer to this? Can I can I make it less carbon-intensive by the way I build it?

JASON HAWES: Yeah. Absolutely. So the first is if you’re listening to this and you have an urban farm or an urban garden, please don’t tear it out. This article is not intended to discourage urban agriculture. It’s just intended to highlight the ways that we can do it in a lower carbon way. And one of those ways is to keep growing food.

So if you use a raised bed for longer– for example, if you use it for 20 years versus five years– you’re going to grow about four times as much food. And so at the end of its lifecycle, then that means that more food was produced per unit of carbon invested.

Another thing that we identified which we think is a really exciting opportunity for cities is to use recycled wood or other materials as inputs to the garden. And one of the really common examples of this is food waste that becomes compost. But there are other opportunities to recycle things, like construction waste, into the garden infrastructure.

IRA FLATOW: You’ve mentioned the social benefits a couple of times. And that shouldn’t be minimized, should it? That’s an important part of having an urban garden– is meeting everybody.

JASON HAWES: Yeah, it absolutely is. We’ve found evidence for actual physical and mental health benefits associated with farming and gardening. There are lots of folks who talk about opportunities for building community resilience and creating these sorts of social networks that stand up strong neighborhoods, which, of course, has opportunities for creating more secure food access in underserved neighborhoods in many cases.

IRA FLATOW: Yeah, fresh food out of the garden instead of in a can. I know you have your own garden. You’ve said this. Has this made you reconsider any part of that to lessen your own emissions?

JASON HAWES: I actually have to move in a few months. So I’ll be building a new garden. And I’m going to do my best to try to use recycled wood to construct that new garden because I think that’s one of the really important things we can do right up front– is invest in sustainability by finding reused materials to put in the garden.

IRA FLATOW: Well, I want to wish you good luck on your new garden. And thank you for taking time to be with us.

JASON HAWES: Of course. Thank you for having me. And enjoy your garden this year, Ira.

IRA FLATOW: Thank you. Jason Hawes, PhD candidate in the School of Environment and Sustainability at the University of Michigan– he was the lead author on a study published in Nature Cities.

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