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 Nursery handouts and garden magazines push the out-of-date advice to dig deep and add lots of stuff the shrub or tree ought to appreciate.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.

Do go ahead and dig in soil improvements, Chalker-Scott advises, for vegetable gardens or annual flowerbeds, in which nutrients need replacing yearly. But there's really no need to dig organic amendments—manure and peat moss, etc.—into landscapes that are permanent. Treat those plantings of trees and shrubs as if they were forest ecosystems, not agricultural fields—wood chips and decaying leaves on top, no tilling-in of fertilizer.

I have a lot of resistance, which I should probably get over, to one of her chapters: "The Myth of the Fragile Root Ball." The common advice is to handle very, very tenderly the roots of a tree you're getting ready to plant. It's really scary seeing those pallid underground organs exposed to air and sun.

Chalker-Scott admitted, in a recent phone conversation, that her husband, also in the horticulture biz, watched in horror as she hosed away the soil from the roots of a newly purchased redbud (which cost $100—a lot for a tree). He gasped when she then trimmed away two-thirds of the root ball—near-dead woody material and circling roots that might have strangled the trunk later. She makes a good argument that her apparently harsh treatment induces vigorous new root growth. (Not too harsh; she does keep the roots in water when she's not actively trimming them.) Her redbud, along with many other trees she has treated this way, is doing very well.

Jeff Gillman
Jeff Gillman

What makes Gillman a bit more of a storyteller than Chalker-Scott is that along with culling information from the newest scientific journals, Gillman also does his own experiments at home. For example, he tested the effect of Michelob Light and Guinness on plant growth. (Beer-drinking gardeners really love to think of beer as nutritious for all living things. But it turns out that beer is actually bad for plant growth.)

In another experiment, he surrounded slugs (the bane of gardeners for their voracious leaf-chomping) on a plate with a circle of crushed eggshells, said by many to be an effective barrier. He and his daughter watched the slugs pass over the crunchy shells without being at all bothered. He also tested Scotch whisky on plant leaves, a pesticide recommended by a TV garden expert, and found it killed the pests but burned off all the leaves, too.

Gillman evaluates the various fertilizers garden centers and magazines and relatives and gardening gurus may swear you need to add—gypsum, Epsom salts, bat guano, fish meal, seaweed extract, molasses, and Coke. (Some are useful in cases of specific soil deficiencies. Note: Molasses and Coke, never good.)

Gillman's second book, The Truth About Organic Gardening: Benefits, Drawbacks, and the Bottom Line, has drawn fire from people who think he's attacking organic practices. He's completely in favor of the basis of organic gardening, which is enriching the soil. However, he writes, "People who say that because something is 'natural' it's good are oversimplifying just as much as those who say that any old chemical that kills a pest is good." Both authors point out that organic pesticides, derived from plants, are not necessarily safer than synthetics. Rotenone, an extract from the roots of a tropical plant, is much recommended in organic gardening publications but highly toxic to fish and slightly toxic to waterfowl.

It must drive both authors nuts to hear people say, "I'm an organic gardener. I never use chemicals." Everything on earth is composed of chemicals.

Gillman advises his readers, "Search for the why behind everything you do for your plants." We may never leave our sometimes irrational natures behind, but learning the why lets you see your plants and shrubs and trees as part of an interesting system, a system that can work without a lot of intervention.

Constance Casey is a former New York City Department of Parks gardener and writes the monthly "Species" column for Landscape Architecture Magazine.

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