Tuesday, May 15, 2012

Evidence-Based Gardening

Long ago, nobody messed with Mother Nature (much). Mostly, nobody knew how, and the traditional ways were common sense. Along came the chemical age, and then the backlash. Some say anything from a lab is harmful; others say Mother Nature alone is not enough.

Of course, I'm talking about gardening. In general, gardeners seem to be on one side or the other: chemicals should be avoided entirely, or the only good bug is a dead bug. (Conversion is possible, though: a friend of mine was fervently anti-chemical until her apartment got a bedbug infestation.)

Jeff Gillman, a professor of horticulture at the University of Minnesota has chosen a different side: looking at the evidence. His book The Truth About Organic Gardening tips over some of the sacred cows of both camps: For instance, natural pesticides aren't necessarily safer than synthetic ones (good to know if you eat organic fruit, since natural pesticides are used on organically grown fruit). And bug sprays can make an infestation worse by killing the bugs' natural predators. Does chronic exposure to pesticides raise your risk of cancer, and are organic foods more nutritious? Nobody really knows. The epidemiological evidence on pesticides is mixed and has confounding variables (possibly, higher vitamin D levels from farmers working in the sun), and produce has a variety of nutrients that can't be summed up in one score.

In a similar vein, Decoding Gardening Advice by Jeff Gillman and Meleah Maynard examine common gardening advice, and their analysis concurs with some things I've found. I don't mulch my front yard, for instance (they consider mulching "debatable" advice), and just as they say, it's full of volunteer plants (and more weeds grow there, too). They also suggest digging a big hole for your plants, selecting plants that are suited to your climate, aspect and soil, and avoiding landscape fabric that's supposed to suppress weeds. (It seems to be used by people who don't want to do yardwork, since it starts looking shabby within a few years. A better idea: get an apartment.) However, some of my results differ from theirs: I've found ladybugs very effective at controlling aphids (they don't fly away from a feast), and here in the semi-arid Denver area, I've found no need for fall cleanup.For the debatable advice, N=1 carries the day.

The adherer effect is at work in some organic (and non-organic) practices, just as it is in nutrition. People who swear by lunar cycles or manure tea are likely doing a lot of things right in their gardens. This is where experiments that Gillman cites come in handy. They admit they don't have the answer to everything, but they don't condemn any hints unless they know they don't work.

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