The Murky Side of Mulch

A plant pathologist reveals that these popular materials can sometimes prove more harmful than helpful


Olaf Ribeiro’s Last Chance Garden on Bainbridge Island, Washington, is filled with plants so lusty you’d never imagine they were on their last leaves when they arrived. A gregarious plant pathologist who runs a busy backyard laboratory, Ribeiro is also a gardener who can’t resist the challenge of restoring a sickly plant to blooming health. Growers around the country ship disease-ridden roses, perennials, and shrubs to Ribeiro’s private lab, where each receives study, diagnosis, and, finally, nurturing by Ribeiro and his wife, Nancy Allison. When he isn’t testing sick roses shipped up from California, consulting with fruit growers, or rescuing heritage trees from decline, Ribeiro is testing mulches, potting soils, and fertilizers to get at the root cause of plant disease.

‘Olaf is one of the world’s experts on soilborne disease,” says Sue Nicol, director of outreach at the University of Washington’s Center for Urban Horticulture, where Ribeiro has taught numerous classes to professional horticulturists. What fires up Ribeiro’s research these days is his realization that mulch sometimes damages plants rather than helps them. “I realized that I should be looking at the mulch itself,” he explains, “and that opened a can of worms.rsquo;


Recently, Ribeiro was called to consult on two expensive landscapes that had gone into decline for no apparent reason. The azaleas, rhododendrons, ornamental trees, and even the annuals were close to death. Since the problem was uniform throughout the gardens, Ribeiro suspected mulch as the culprit. His lab tests showed that salinity levels in the recently applied mulch were high enough to cause serious damage. The only solution for such high salinity is to flush the soil with large quantities of water—not an easy task, given the Northwest’s predominantly clay soils.


Could a nutritious liquid brew substitute for worrisome mulch? I was curious to hear Ribeiro’s take on compost tea. “Claims have been made that compost teas will suppress or control a wide variety of plant problems. Thus far I haven’t seen any of this in the landscapes where compost teas have been applied,” says Ribeiro, who took part in a compost tea panel at a recent meeting of the Washington Florists Assocation. “The big drawback is that compost tea has to be applied frequently, so it’s costly for homeowners,” he says. Adding microbes to the soil can be done more inex-pensively with aged manure or well-composted mulches. Also, the teas need to be kept aerated or you end up with more anaerobic than aerobic microorganisms in the brew. And for maximum microbial effect, it needs to be applied as soon as possible after brewing. “This is a short-lived product,” he concludes. And while problems of inconsistency have been reduced by the use of commercially produced compost brewers, his clients who have purchased brewers have not seen enough difference to warrant the extra cost.

The dirty bottom line is to be cautious about what you spread or pour on your garden. If this sounds a bit daunting, you can take comfort from the fact that big manufacturers, in an effort to offer a more consistent product, are sending batches of mulch to Ribeiro for testing—and, it is to be hoped, acting on his findings. And Ribeiro, despite his wariness, isn’t a naysayer. He remains optimistic about mulch. He’s even been using some on a project to restore venerable heritage trees on the state government campus in Olympia, Washington. “Our success in saving large mature trees by monitoring the mulches applied in their driplines has been most encouraging,” he says. He 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 (

Considering most plant pathologists aren’t gardeners (and vice versa), it’s instructive to step outside the lab for a close look at Ribeiro’s own flowery garden. Filled with once-struggling roses and vibrantly blooming camellias, and contained by a 60-foot-long Puff the Magic Dragon hedge, it appears to be thriving on its all-organic regimen. Other than labels with full botanical names, the glossy-leaved, once-diseased plants give no indication that this is the garden of a scientist. And what does this concerned pathologist consider safe enough to use on his own garden? “I get it sprayed twice a year with liquid seaweed [a service offered by commercial companies], which greens up the leaves and helps the roses to bloom,” says Riberio. “It’s pretty consistent in quality, although it does smell a little.” H

Easing Mulch Worries

So, what can you do if you’re worried about the quality of the mulch you’re using? Start by figuring out just what it is you’re actually putting on your garden. Inexpensive pH kits can be helpful—if the mulch tests outside the 5 to 75 range it won’t do much good for plants. Another trick is to toss some quick-germinating bean or radish seeds on the mulch. “If they keel over, there’s a problem with the mulch,” says Ribeiro. If you’re buying a big batch of mulch from a garden center, be sure to ask for a nutrient analysis. Reliable sources test their mulches. To be assured of microbial activity, Ribeiro advises choosing mulch that contains aged manure. According to Ribeiro, chicken manure is the most effective for plants, followed by cow, goat, and horse manures.—V.E.

Related Posts:

  • No Related Posts

Leave a Reply