The benefits of evidence-based gardening.

All things green.
Nov. 28 2008 7:01 AM

The Science of Gardening

How evidence-based growing beats relying on old wives' tales.

Confronted with huge, uncontrollable forces, we tend to fall back on magical thinking. Say a goat was sacrificed on the volcano rim last year and lava did not engulf the village. It must follow that this year some poor goat is doomed.

To garden is to encounter forces of nature less threatening than molten lava but still bewildering. Weird weather, chewing insects, trees mysteriously dying—these can make a gardener as superstitious as the villagers.

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Perhaps every April, your great-aunt Sophie poured beer on the earth around her lilacs, and in May the flowers bloomed profusely. Your family may acknowledge that a belief in beer as flower-inducer is inconsistent with the known laws of science, and yet they will follow the custom, open the bottles, and pour on the brew.

With a similar vulnerability to irrational arguments, we, too, eagerly buy pesticides and fertilizers that we probably don't need, and that may not work, from marketers pushing fast cures for bug damage or underperforming trees.

Enlightenment is available. Two smart, crusading authors are making it possible to practice evidence-based gardening. Happily, understanding what works and why almost always leads to expending less effort, spending less money, and gardening in a way that's environmentally sound. Linda Chalker-Scott, an associate professor at Washington State University, is the author of The Informed Gardener and producer of the column "Horticultural Myths." In The Truth About Garden Remedies: What Works, What Doesn't, and Why, Jeff Gillman, associate professor at the University of Minnesota, is just as rational and informative as Chalker-Scott but more freewheeling in style.

Both base their arguments on hard science, gathering information from peer-reviewed publications like the Journal of the American Society for Horticultural Science. They ferret out research a bit farther afield in places like the Journal of the Scientific Agricultural Society of Finland and the New Zealand Journal of Experimental Agriculture.

Linda Chalker-Scott
Linda Chalker-Scott

Here's an example of Chalker-Scott's myth-dispelling arguments. There's a time-honored adage that you should dig a $5 hole for a 50-cent plant. Nursery handouts and garden magazines push the out-of-date advice to dig deep and add lots of stuff the shrub or tree ought to appreciate. A pudding of more than $5 worth of manure, wood chips, leaf mold, and/or peat moss in the planting hole seems as if it has to be helpful.

As Chalker-Scott explains, this coddling is not only unnecessary, it's harmful. Roots will circle the edge of this hospitable, luxurious planting hole and never venture out into the surrounding native soil. Crowded roots mean a tree dies before its allotted time.

In rainy times, water will fill the loose mix in the hole and not move out into the more tightly packed and slowly draining native soil, creating a potentially fatal bathtub effect.

The alternative is cheaper and easier: Fill the hole (a space no deeper than the root ball, but twice as wide—much easier to dig) with the existing native soil and then improve the new tree's diet by spreading organic material on top of the soil. Let the worms do the work.

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