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Indoor lettuce produces double the yields, doubly fast

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Indoor lettuce produces double the yields, doubly fast

A first-of-its-kind meta-analysis shows that lettuce thrives indoors—pointing to climate-proofing the $3 billion salad industry.
September 8, 2023

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Crops of lettuce grown indoors—shielded from the elements in vertical farms, polytunnels, and greenhouses—will produce on average 50% more leaves, up to 300% faster, than their cousins out in the field. 

This is the recent discovery from the largest study of its kind to examine the benefits of artificial growing conditions for the coveted crop. It’s also good news for one of the world’s most valuable agricultural commodities: lettuce is worth almost $3 billion annually—and yet, its fragile leaves are uniquely threatened by pests, as well as the heat and drought conditions of climate change. 

Indoor growing is seen as a way around this vulnerability, but with no attempt until now to synthesize findings across the growing number of papers, farmers and policymakers have had to do a bit of guesswork on how well this method really works.

To plug that gap, the researchers on the new meta analysis pulled together 121 studies on the topic, which looked at lettuce-growing in a variety of artificial indoor environments in 29 countries. Across these studies they compared yields and growth rates from those indoor lettuces against average growth rates for lettuce crops farmed conventionally out in the field. They also looked at the role of several conditions including lettuce type, light sources, nutrient mediums, and building types, in determining how well and how rapidly the indoor plants grew.

From this large dataset, a striking pattern emerged. On average, controlled, indoor environments yielded twice as much lettuce compared to crops grown in fields: that’s 3.68 kilograms of lettuce per square meter from indoor farms, compared to an average 1.88 kilos from conventional farms.

 

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Indoor lettuces also grew at speed—50% faster than conventional lettuces in summer time, and 300% faster in winter when compared to the slowed growing rates of lettuce dealing with frosty conditions outside. 

The varied conditions across the different studies also allowed the researchers to home in on the methods and varieties that worked best. Of all the lettuce types, iceberg lettuce was most consistently productive, yielding 7.45 kilos per square meter on average. Meanwhile, greenhouse systems yielded the highest amount of lettuce across the different types, which the researchers think could have something to do with the availability of natural light in these systems.

Growing lettuce in vertical stacking arrangements rather than horizontally similarly increased the chances of a bigger crop. The way that the crops are fed turned out to be especially important, and lettuces grown in aquaponic nutrient systems fared the best of all, producing the lushest plants when compared to other growing mediums. 

The environmental benefits of indoor growing could be huge: indoor plants require less application of pesticides, and can make efficient use of land if cultivated vertically and if they use up existing infrastructure like abandoned warehouses and marginal urban land, rather than expanding agriculture’s footprint into new territory. 

On the other hand, indoor growing is a notoriously energy-intensive endeavor in cases where plants need artificial lighting, heating, or cooling to aid their growth. There may, however, be ways around this with renewable energy, and if farmers grow crops in greenhouses that are flooded with natural light instead of relying on artificial sources. 

In fact, indoor lettuce farming would produce just a fraction—less than a tenth—of the carbon emissions of regular field-grown lettuce, if produced using renewable energy, and when factoring in the avoided carbon losses of land expansion from conventional farms, the paper shows. 

Even with this huge promise, indoor farming will be too niche and expensive an approach for most crops. But the study’s striking numbers show that it could make a real difference through at least one widely-consumed food—in the process “advancing our understanding of where and when this technology can bring the most value to society,” the researchers say.

Harris et. al. “Let-Us Investigate; A Meta-Analysis of Influencing Factors on Lettuce Crop Yields within Controlled-Environment Agriculture Systems.Plants. 2023.

Image: Jatuphon Buraphon via Pixnio

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