BioCycle March/April 2016
Spring is in the air (at least in Seattle) and seed catalogs are filling my inbox and my mailbox. I have to decide between Galiano, Sunset, Merlot and New Red Fire. These are not wines or exotic travel destinations, just different varietals of red leaf lettuce. There are many bonuses to growing your own lettuce instead of buying it. In this column, I want to focus on the pleasures and benefits. The first and most obvious pleasure/benefit is instant salad. Whenever the mood strikes you just take your shears, head out back or front and in 5 minutes you have lovely lettuce.
If you want to think on a higher plane than instant gratification, let’s steer the conversation to the atmosphere. Isaac Emery (Emery, 2016) recently authored a chapter in a book on urban agriculture that does just that. Emery did a life cycle assessment of lettuce grown in a yard or community garden in Seattle in comparison to lettuce bought in a supermarket in Seattle and grown in California.
Each kilogram of lettuce bought in the supermarket costs about 0.7 kg of CO2e. That price is not reduced even if lettuce is on special and you use your special supermarket discount card. Most of this cost can be attributed to transportation related emissions and irrigation. For Emery’s study, both a high efficiency and a normal efficiency case for transport were modeled. The higher efficiency vehicle reduced total transport emissions to 0.19 kg CO2e per kg lettuce from a high of 0.42 kg CO2e. It is critical to remember that repeated trips in your car to the local supermarket will add to transport related emissions whether the lettuce was grown locally or trucked from far away. In contrast, walking to the garden to get your lettuce burns calories, not carbon.
While the transport emissions are likely pretty obvious — burning fuel while barreling up I-5 from California with the refrigerator unit keeping the lettuce chilled is a clear source of carbon emissions — the irrigation component may be less obvious. We all know that California is getting some welcome relief from the drought, but even in wet years, the electricity used to move water in California accounts for significant carbon emissions. A study examined the electricity used to provide water in the western states of the U.S. California had the highest usage — between 22 to 25 terrawatt hours (TWh)/year (Tidwell 2014). For some sense of scale, our homes use electricity in kilowatt hours, typically a few hundred to a few thousand per month. That is the equivalent of 1,000 watts. Terrawatt hours are the equivalent of 1,012 watts — or a trillion kilowatts.
A significant amount of those terrawatt hours in California are used to irrigate 5.7 million acres of farmland. California uses between 6.1 and 8.6 TWh/yr to pump groundwater for agriculture. Even in regions where reclaimed water from wastewater treatment is the primary source of water for agriculture, energy costs are still significant. For his analysis, Emery figured that each kg of California lettuce had a water associated carbon cost of 0.22 kg CO2e. Using a Seattle garden as a basis for comparison, water related emissions were negligible whether you use water from the hose or from a rainwater cistern. Rainfall is more plentiful in Seattle and temperatures are generally cooler, both reasons why less water is needed. In addition, the energy costs for water are much lower in Washington than in California because of more plentiful and closer sources of water and a lower CO2e for the electricity (the Pacific Northwest uses lots of hydropower). If you water using municipally supplied water, the cost per kg of lettuce is a measly 4 g or 0.004 kg CO2e per kg lettuce.
Biggest Bang For Carbon Buck
The big benefit of growing lettuce in your yard comes if you use compost or biosolids based soil products to fertilize the crop. This is what gives you a carbon credit for your garden. This credit has multiple components. Emery figured that each kg of lettuce has an associated fertilizer emission of 0.025 kg CO2e to provide the N and P. So you get a small credit for using a recycled source for your plant food. And a small credit for enriching your soil and helping it store more carbon. But your big credit — 0.37 kg CO2e per kg lettuce — comes from taking those compost feedstocks out of the landfill.
And that brings me to another critical point. To grow your own lettuce, you need good soil amendments. The obvious source of good feedstock to make the soil amendments for your vegetable garden are your own food scraps. So there’s the whole circle. And using your own homegrown food scraps for the compost bin means no stupid little produce stickers that appear to be glued onto everything and a pain in the neck to take off. Because the stickers are such a pain to remove, they often end up in the compost pile where they are very difficult to screen out and thus become contaminants in the compost. Those little stickers can even make it through your kitchen disposal unit into the wastewater plant and the biosolids. Homegrown food scraps typically come with minimal packaging and so the waste makes for excellent compost.
I hope that I have convinced you to plant some lettuce this season. Instant salad with exotic sounding varieties, a chance to make friends with your neighbors and the local food bank with any excess, reduced carbon emissions, and none of those sticky labels. What more could you ask for?
Sally Brown is a Research Associate Professor at the University of Washington in Seattle and a member of BioCycle’s Editorial Board.