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.

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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.

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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.