Pacific Northwest | February 15, 2004Pacific Northwest MagazineFebruary 15, home
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Cure-All or Culprit
'Mulch' may be the mantra, but know what you're getting into
You'd never guess this flowery stretch of rhododendron, primroses and foxglove is a living laboratory to test mulches and fertilizers in the garden of plant pathologist Olaf Ribeiro on Bainbridge Island.
Wouldn't you expect someone who has an e-mail address with the word "fungispore" in it to be an expert on plant disease? Olaf Ribeiro, a gregarious plant pathologist who runs a lab in his Bainbridge Island back yard, doesn't disappoint. When he isn't testing sick roses shipped from California, consulting with fruit growers or rescuing heritage trees from decline, he's testing dozens of mulches, potting soils and fertilizers to get at the root of plant disease.

Ribeiro's Last Chance Garden is a living laboratory where he and his wife, Nancy Allison, have made a lovely home garden out of formerly disease-ridden plants. Before being released into the garden, each plant is treated and deemed fully recovered. Ribeiro shuns pesticides, saying the garden's purpose is to show that once-diseased plants can be kept healthy with organic products.

What got Ribeiro out of the lab and into the garden was a dawning realization that, despite the much-heralded value of using mulch, he was seeing lots of sick plants in mulched gardens. He noticed that plant decline often seemed linked to a mulch laid down the previous season. "I realized that I should be looking at the mulch itself," he explains, "and that opened a can of worms."

Mulch, mulch, mulch is the commonest of garden mantras these days. It has been held up as the answer to all garden ills — mulch protects plants in winter, keeps down weeds, improves the soil, prevents water evaporation and nurtures plants. The reality is that it can damage them, too. Ribeiro has found a worrisome variability in the mulches on the market. Some are devoid of any micro-nutrients, while others carry active pathogens. He's found salt levels that "go off the scales, nothing will grow in this stuff." He's tested mulches by the truckload and by the bag, and found that even from the same company the quality varies wildly among batches.
Illustration Now In Bloom
Crocus flower early, are dependable and spread willingly. The bulbs are so tiny that they're easy to tuck into pots, line walkways or throw out in handfuls to carpet the ground beneath deciduous trees and shrubs. Crocus come in shades from white through deep violet, often striped, veined or flushed with varying colors, and range from a few inches high to a statuesque half a foot. C. chrysanthus are often called snow crocuses; 'Gipsy Girl' (above) has fragrant golden flowers striped in maroon, 'E.A. Bowles' has lemon-yellow flowers feathered with purple.
So what's a gardener to do? You can start by figuring out just what it is you're actually putting on your garden. Inexpensive PH kits have little strips that turn color (yes, it sounds like a home-pregnancy test); if the mulch doesn't test between 5 and 7.5, it won't do much good. If you have a little more time, try tossing out quick-germinating bean or radish seeds. "If they keel over, there's a problem with the mulch," says Ribeiro.

Mulches increase the soil's microbial activity, thus keeping plants healthy, but Ribeiro has found some mulches to be totally inert. To make sure your brand of mulch is full of life, you can test it with a Solvita Soil Life Test sold by Woods End Lab in Mount Vernon ( or 1-800-451-0337). It is a simple kit for home gardeners that gives a reading on a mulch's vitality. And if you're buying a big batch of mulch from a garden center, be sure to ask them for a nutrient analysis. Reliable sources test their mulches.

To be assured of microbial activity, Ribeiro advises choosing mulch that contains aged manure. And he answered my burning question: Chicken manure is the best kind for plants, followed by cow, goat and horse in that order.

The dirty bottom line is to be cautious about what you put down in your garden. If all this sounds a bit daunting, be consoled that the big manufacturers, in an effort to be more consistent with their products, are sending batches of mulch to Ribeiro for testing, and (we hope) acting on his findings.

Ribeiro is working on a book that explains the complicated interactions that make potting soils, fertilizers and mulches effective or not. He's at the revising stage, with no promises on when he'll publish, but you might keep an eye on his Web page at

And what does this concerned plant pathologist consider safe enough to use on his own garden? "I get it sprayed twice a year with liquid seaweed (a service offered by Washington Tree Service and other commercial companies), which greens up the leaves and helps the roses to bloom. It's pretty consistent in quality, although it does smell a little."

Valerie Easton is a Seattle free-lance writer and contributing editor for Horticulture magazine. Her e-mail address is Jacqueline Koch is a writer and photographer living on Whidbey Island.

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