“Waste water today, live in a desert tomorrow.”
– Popular conservationists’ motto
Even though it’s midsummer and your vegetable garden is already growing, it’s not too late to apply mulch to the soil to conserve water.
What? You’ve already applied mulch you say? Well, you can apply more, because you can never have too mulch of the stuff.
Sorry. The recent spate of pleasant weather and what looks to be a bumper crop growing in our well-mulched vegetable garden has put me in a good mood. And when I’m happy, I tend to engage in wordplay, rather than pun-tificaticing on the planet’s many ills.
Seriously though, mulch and the planet’s ills are two sides of the same coin. The former, sufficiently widespread, could help cure, or at least “manage” the latter. So let’s talk about spreading mulch – widely as well as deeply.
What is mulch? The Oxford American Dictionary defines it as “a material (such as decaying leaves, bark or compost) spread around or over a plant to enrich or insulate the soil.” I’ll discuss soil enrichment in future columns. Mulch’s “insulating” property, critical for water conservation, is what concerns us during these times of deep, and probably persistent, drought gripping America’s western states.
Mulch is typically applied in 2- to 6-inch-thick layers, and works best when the (organic) material is finely shredded and has a somewhat spongy consistency so air is trapped between its fragments. In this way, mulch resembles building insulation, which also traps tiny air pockets – between particles of shredded newspaper or fiberglass – to slow the conduction of heat through walls or attics.
What does mulch insulate the soil from? Direct sunlight and/or dry wind that heats soil and hastens evaporation, wasting precious water.
Calculating exactly how much water is lost through bare-soil surface evaporation is complicated. Many factors, including the size of your garden, soil type (clay, sandy or loam), plant type and root structure, water-table depth and climatic conditions come into play. But, as a rule of thumb, any water that is sitting on or in the top inch of soil tends to evaporate in a few hours on hot days. Then capillary action draws water from deeper in the soil to the surface, where it, too, is lost.
Another rule of thumb is that vegetable gardens require an average of 1 inch of water per week spread over the surface to keep plants happy and healthy. An inch depth of water translates to about 60 gallons per 100 square feet of garden space, with more needed in dry conditions and for raised beds. Around half that water – 30 gallons per 100 square feet per week – can be lost to surface evaporation during the (increasingly) hot, dry period of mid-June through mid-September, requiring additional watering.
There are an estimated 45 million vegetable gardens in the U.S., and their average size is 600 square feet. Multiply those figures by an evaporative water loss of 30 gallons per 100 square feet and you get a potential loss in the range of 8 billion gallons per week. (The figures are imprecise and depend on numerous factors – how many of the gardens are already mulched being one. However, the scale of the problem and the potential for water savings is impressive.)
Some “official” websites – those of university extensions and other government services in the U.S. and Australia – state that mulching can cut evaporative losses by more than 70 percent. Other garden sites claim more modest savings, so for the sake of estimating conservatively, let’s call it 60 percent. The bottom line? Somewhere around 5 billion gallons of water per week could be saved if we all mulched our vegetable gardens.
Mulch also can conserve water when it’s mixed into the soil, where it eventually becomes “humus,” or decayed organic material, that creates water-retaining “pores.” While some gardeners simply allow their surface mulch to “rot” into the soil during the winter and then cover it with fresh mulch in the spring, the process can be speeded up if you turn each year’s mulch into the soil with a garden spade at the end of the season. The soil will readily assimilate each year’s cover, so you really won’t have too much mulch at your ecological house.
Philip S. Wenz, who grew up in Durango and Boulder, now lives in Corvallis, Ore., where he teaches and writes about environmental issues. Reach him via e-mail through his website, www.your-ecological-house.com.
