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

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.