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America's Gardeners Prove They Can Get Too Mulch of a Good Thing

By Adrian Higgins
Thursday, June 5, 2008

Spring is in the final stretch, which means that 99 percent of the commercial and residential landscapes hereabouts are now smothered in mulch. Where soil once dared to bare itself, blankets of chewed-up tree trunks safely shield our eyes from dirt.

A lot of people in my neighborhood have mulch laid by landscape crews, either in bags or with a bulk delivery. Otherwise, it's a weekend-long job for the do-it-yourselfer. A fairly modest 1,000 square feet of garden beds would consume more than 80 bags of bark or wood mulch.

As I see people hauling and ripping these bags, I wonder, what's driving them? Is it a clear understanding of the benefits of mulch, or is it some civic impulse to conform in what has become Mulch Nation?

"We seem to have gone nuts for mulch," said Marylee Orr, an environmentalist who says our addiction to mulch is costing Louisiana its coastal cypress forests.

Without doubt, we decorate our lives with mulch in a way our ancestors would have found strange and extravagant.

In 1916, Robert Cridland wrote a book on how to construct and care for the stylish suburban garden. But scan the index of his "Practical Landscape Gardening" for mulch, and you will find it missing. It is also absent from the book's black-and-white photographs of garden beds.

The use of mulch is advocated in subsequent gardening books, but either as a winter measure to protect marginally hardy shrubs (the mulch is removed in the spring) or in the vegetable garden for weed and moisture control. In both cases, the mulch is found with such things as grass clippings, sawdust or cornstalks.

One popular product from outside one's garden was hay that had become wet, no use for livestock feed but great for the cucumber patch. As word of the benefits of mulch spread, so too did a demand for a material that was not always handy. Prophetically, the garden writer Ruth Stout wrote in the early 1960s: "If enough people in any community demand it, I believe that someone will be eager to supply it."

Today the mulch industry has its own trade group, the Soil and Mulch Council, and the quantities of ground-up wood generated in the United States annually, while not quantified by any government agency, are palpably (or is it pulpably?) huge. The council estimates that we spread 40 million cubic yards of the stuff each year. These days, you can't go to a supermarket, mass merchandiser or even gas station without finding bagged mulch displayed for purchase.

But the economic downturn is causing shortages of mulch as fewer acres are cleared for development and the sawmills and paper mills that generate mulch as a byproduct use it instead to fuel their boilers, said Robert LaGasse, the council's executive director.

Concerned that mulch mania is leading to the clear-cutting of Louisiana's coastal baldcypress forests, an alliance of environmental groups has been formed to fight excessive logging. It is working to raise public awareness of the issue, seek government intervention, lobby retailers to stop selling Louisiana cypress mulch and urge a consumer boycott.

Most of the old-growth forest was cleared by the 1930s, and what exists now are trees that have grown since, said Orr, executive director of a group called Louisiana Environmental Action Network. Orr and others argue that Mississippi River levees have changed the silting patterns of the forest to the point that the conifers cannot reseed sufficiently and that denuded swamps are being invaded by weed trees, especially the exotic tallow tree.

LaGasse said statistics suggest that cypress growth exceeds removal in the South. He said, however, that the council is looking into whether loggers and landowners are following sustainable practices.

Horticulturists agree that natural mulch, correctly used, makes for healthier plants. A two- or three-inch layer will suppress weeds, conserve soil moisture and moderate soil temperatures. Soil that is too hot kills feeder roots, forcing plants to continuously grow new ones while in a state of chronic stress, said Linda Chalker-Scott, a horticulture professor writing in the Journal of Environmental Horticulture. Organic mulches have been shown to reduce soil temperatures in summer by as much as 50 degrees, she wrote.

Conversely, synthetic mulches such as plastic sheeting will bake the soil, raising temperatures as far down as 12 inches, she said. If rubber mulch, derived from recycled tires, has any horticultural merit, I cannot imagine what that might be. Nor will synthetic mulches, or gravel or stone mulches, build the soil, which organic mulches do as microbes convert them to humus.

Organic mulch comes in many forms. One popular kind is derived from pine or hardwood bark and is considered by many of my gardening friends to be superior to wood mulch because the bark contains tannins that resist breaking down and thus looks better longer. Cypress and cedar mulch, though wood, are believed to have the same enduring qualities as bark mulch. That notion is challenged by the Louisiana environmental groups, which say today's cypress isn't old enough to contain sufficient heartwood to be rot-resistant.

Shredded hardwood is useful in that it knits together in a way that pine nuggets do not, so it won't move so much on hillsides or during heavy rains. I have been drawn to a type called Virginia pine fines, the chaff from pinebark milling, because it is finely textured, richly colored and, with so large a surface area, easily broken down into humus.

But Chalker-Scott, of Washington State University, challenges several common notions about mulch, saying that bark mulch may contain natural waxes that form an undesirable moisture barrier and that excessive use of fine-textured mulches can prevent moisture and required gases from getting to the soil. She also debunks the belief that pine bark, pine needles and wood chips will unduly acidify the soil. "No scientific research supports this," she writes, "and in fact studies refute this perception."

She also says that the perception that fresh wood chips will lead to soil nitrogen depletion is wrong. Chalker-Scott said a thin zone of nitrogen loss exists where the chips touch the soil, but that for woody plants with subsurface roots, there is no nutrient loss. If you are worried about that, she said in an interview, you can put a thin layer of compost beneath the chips.

She favors the wood chips because they are a byproduct of arborists' treework, are locally produced and available at a low cost, and their use in the garden keeps them out of landfills.

I think one of the best mulches is your own compost, or partially rotted leaves, or leaf mold. They are not as pretty as some out of a bag, and they break down quickly, but there is a purpose and honesty about them. I'm not holding my breath in anticipation of others joining me in that belief, however.

"We have become more of a society that doesn't reuse our own yard waste," Orr said. "It seems to be easier to buy something rather than make it ourselves, so in that way we have kind of gone crazy for the mulch."

The other key aspect of mulches is not what you choose but how you use it. Automatic reapplications as soon as the sun bleaches the old mulch will inevitably lead to excessive mulch layers that will harm plants, either as a moisture barrier or in becoming an unwanted growing medium for roots.

And the weird and insane practice of piling mulch against tree trunks continues. These mulch "volcanoes" cause surface root growth and damage a tree's protective bark. "There will be fungal or bacterial infections in areas of trunk coverup," Chalker-Scott said.

So when it comes to mulch, like everything else in life, moderation is the key to success. And in these hard times, self-sufficiency is a virtue.

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