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Question. Dear Mike: We haveused black shredded mulch in the flower and shrub garden in front ofour house for several years. After listening to a recent show, I'm nowwondering if it might be part of the reason many of our seedlings don'ttake and my wife's perennials don't come back. If the mulch is toblame, what are our alternatives for preventing weeds while alsodressing up the appearance of the front of our house?
---Michael in Mount Laurel, NJ.
Mike: What kind of mulch should I not use? The last 2 years I have usedLicorice Root type mulches, but now I have black spots on my patio thatlook like soot. And what can I
do to get the spots off? Thanks,
---Anna Marie, a teacher in Cherry Hill, NJ
Answer. I have been warning peoplefor years that wood mulches—wood chips, shredded bark, sawdust, andthose increasingly popular 'root mulches'—can breed 'shotgun' or'artillery' fungi that shoot tar-like spores as far as 30 feet towardslight colored objects, like the side of your house or car. These sporescan be removed pretty quickly if you get to them right away, says Dr.Dan Herms from Ohio State University: Soak them thoroughly with soapywater for a few minutes to loosen the natural 'glue' they exude, thenscrub them off vigorously. But as we have always warned, once thespores dry they are virtually impossible to remove without destroyingthe surface they're adhering to.
Wood mulches can also slow the growth of established plants—and yes,just plain starve new ones to death—by 'tying up' the available food inyour soil, a process known as "Nitrogen immobilization". Wood iscarbon; carbon always looks for nitrogen to bond with so it can breakdown into new soil—that's the principle behind composting. Wood mulchestake that nitrogen right out of the soil, out-competing yournitrogen-needy plants. And dyed mulches are the absolute WORSToffenders; the wood in these old pallets—chipped up and sprayed withdye—is the worst type for use around plants. Our favorite mulch expert,Ohio State Professor Emeritus Dr. Harry Hoitink, warns that dyed mulchis especially deadly when used around young plants or in brand newlandscapes.
There's also another problem that occurs around this time of year, whensap filled trees are chipped and shredded and the mulch sits around allpiled up. Dr. Hoitink explains that this sap becomes a high-strengthvinegar, with a pH as low as 2.5; no plant can survive such an acidicattack. So doubly beware of wood mulch with a sour, vinegary smell.
Heard enough bad things about wood mulches yet? (We'll post links toOhio State and Iowa State horticultural bulletins about these andothers dangers with this Q o' the week.)
So what SHOULD you use? Our new mulch maven Dr. Herms (Harry isretired and wants to pass his well-mulched torch) warns against usingone of my old favorites, straw. He says that straw is carbon-richenough to cause some of the same plant-food stealing problems as wood,and that it often contains seed heads that can cause weed problems(which we've warned about in the past) AND attract rodents that willthen look for other trouble to get into on your landscape (which Ihadn't thought of before).
He does think highly of my personal mulch of choice, shredded Fallleaves—but doesn't think it's the absolute #1 choice. Both he and Harryfeel confident that, after many years of active research, they haveuncovered the BEST all-around mulching alternative.
You ready? It's compost.
Now for years, I've been telling people that compost is a great soilimprover, plant feeder and disease fighter, but that it didn't qualifyas 'mulch' because it wouldn't prevent weeds or retain soil moisture aswell as shredded leaves. WRONG, says Dr. Herms.
"In a recent study at Ohio State, we kept track of 'weeding hours' forplots that were mulched with either 2 inches of compost or ground wood,and there was no difference between the two," he reports. "Both mulchesreduced weeding time to 1/20th of that required to weed an un-mulched'control' plot." So, solid University research now shows that twoinches of compost controls weeds as well as a 'conventional' woodmulch!
And Dr. Herms—who is not an organic researcher by any means; he usedthe nasty chemical herbicide Round-Up to kill the existing weeds in hisplots—adds that compost greatly enhances plant growth, while woodmulches slow it down or just plain kill the plants. He also feelsstrongly that the look is just as attractive as dyed wood. "I usecompost to mulch everything in my home landscape", he told me. "Therich black compost really sets off the green of the plants and thecolors of the flowers beautifully. In fact, it looks just like a dyedblack mulch—but without all of wood's downsides."
Unlike wood mulches, you do have to apply to apply a fresh inch or twoevery year to keep weeds at bay. But Dr. Herms adds that this compostwill also greatly limit disease and insect problems in the plants itmulches and improve their overall vigor and root growth; wood mulches,he notes, often have the opposite effect. And, he adds that, "addingfertilizer to plants mulched with compost had no effect at all; theplants simply didn't need any more food." Plants mulched with woodneeded lots of added fertilizer.
So there's absolutely no excuse for risking your landscape, your home'ssiding, and your car's paint job with wood mulches. Every large gardencenter has big piles of compost they'd be happy to deliver, just likewood and bark mulches. Just remember to keep ALL mulches sixinches away from the trunk or stalk of any plant; any mulch will rot aplant it's piled against. Keep all mulches six inches away from yourhome as well; termites will use ANY moisture-conserving cover—evenstones—to reach your framing.
For more information:
The latest research on mulches, from Iowa State & Ohio StateUniversity:
Dr. Harry Hoitink's classic bulletin on wood mulch problems from OhioState; includes photos of shotgun fungus damage and other nuisancemolds:
Dr. Hoitink's home page, with links to lots of research articles onmulches:
You Bet Your Garden Question of the Week ©2005 MikeMcGrath.