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Is Your Mulch Magnificent? Or Miserable?

Make Sure that the mulch you choose helps your plants instead of harming them!

Question. Dear Mike:  We have used black shredded mulch in the flower and shrub garden in front of our house for several years. After listening to a recent show, I'm now wondering if it might be part of the reason many of our seedlings don't take and my wife's perennials don't come back. If the mulch is to blame, what are our alternatives for preventing weeds while also dressing 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 used Licorice Root type mulches, but now I have black spots on my patio that look 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 people for years that wood mulches—wood chips, shredded bark, sawdust, and those increasingly popular 'root mulches'—can breed 'shotgun' or 'artillery' fungi that shoot tar-like spores as far as 30 feet towards light colored objects, like the side of your house or car. These spores can be removed pretty quickly if you get to them right away, says Dr.Dan Herms from Ohio State University: Soak them thoroughly with soapy water for a few minutes to loosen the natural 'glue' they exude, then scrub them off vigorously. But as we have always warned, once the spores dry they are virtually impossible to remove without destroying the 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 is carbon; carbon always looks for nitrogen to bond with so it can breakdown into new soil—that's the principle behind composting. Wood mulches take that nitrogen right out of the soil, out-competing your nitrogen-needy plants. And dyed mulches are the absolute WORST offenders; the wood in these old pallets—chipped up and sprayed with dye—is the worst type for use around plants. Our favorite mulch expert, Ohio State Professor Emeritus Dr. Harry Hoitink, warns that dyed mulch is especially deadly when used around young plants or in brand new landscapes.

There's also another problem that occurs around this time of year, when sap filled trees are chipped and shredded and the mulch sits around all piled up. Dr. Hoitink explains that this sap becomes a high-strength vinegar, with a pH as low as 2.5; no plant can survive such an acidic attack. So doubly beware of wood mulch with a sour, vinegary smell.
Heard enough bad things about wood mulches yet? (We'll post links to Ohio State and Iowa State horticultural bulletins about these and others dangers with this Q o' the week.)

So what SHOULD you use?  Our new mulch maven Dr. Herms (Harry is retired and wants to pass his well-mulched torch) warns against using one of my old favorites, straw. He says that straw is carbon-rich enough 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 will then look for other trouble to get into on your landscape (which I hadn't thought of before).

He does think highly of my personal mulch of choice, shredded Fall leaves—but doesn't think it's the absolute #1 choice. Both he and Harry feel confident that, after many years of active research, they have uncovered the BEST all-around mulching alternative.

You ready? It's compost.


"In a recent study at Ohio State, we kept track of 'weeding hours' for plots that were mulched with either 2 inches of compost or ground wood,and there was no difference between the two," he reports. "Both mulches reduced weeding time to 1/20th of that required to weed an un-mulched 'control' plot." So, solid University research now shows that two inches of compost controls weeds as well as a 'conventional' wood mulch!

And Dr. Herms—who is not an organic researcher by any means; he used the nasty chemical herbicide Round-Up to kill the existing weeds in his plots—adds that compost greatly enhances plant growth, while wood mulches slow it down or just plain kill the plants. He also feels strongly that the look is just as attractive as dyed wood. "I use compost to mulch everything in my home landscape", he told me. "The rich black compost really sets off the green of the plants and the colors of the flowers beautifully. In fact, it looks just like a dyed black mulch—but without all of wood's downsides."

Unlike wood mulches, you do have to apply to apply a fresh inch or two every year to keep weeds at bay. But Dr. Herms adds that this compost will also greatly limit disease and insect problems in the plants it mulches and improve their overall vigor and root growth; wood mulches,he notes, often have the opposite effect. And, he adds that, "adding fertilizer to plants mulched with compost had no effect at all; the plants simply didn't need any more food." Plants mulched with wood needed lots of added fertilizer.  

So there's absolutely no excuse for risking your landscape, your home's siding, and your car's paint job with wood mulches. Every large garden center has big piles of compost they'd be happy to deliver, just like wood and bark mulches.  Just remember to keep ALL mulches six inches away from the trunk or stalk of any plant; any mulch will rot a plant it's piled against. Keep all mulches six inches away from your home as well; termites will use ANY moisture-conserving cover—even stones—to reach your framing.      

For more information:

Dr. Harry Hoitink's classic bulletin on wood mulch problems from Ohio State; includes photos of shotgun fungus damage and other nuisance molds:

Dr. Hoitink's home page, with links to lots of research articles onmulches:

You Bet Your Garden   Question of the Week ©2005 Mike McGrath.


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