High Elevation Gardening Web Log "Blog"

If an organic mulch appeals, the choice of which one to use remains. Generating local, sustainable agriculture is on many minds these days, so most seem to prefer locally-harvested mulches that require a minimum of transportation. If that preference influences a gardener's choice of organic, then it probably also rules out the use of imported agricultural waste like Cocoa Hull.

An obvious local source is the hundreds of thousands of metric tons of wood waste that are generated annually. Much of it is the by-product of forest thinning and local saw-milling and is abundant and cheap. It is also very light-weight and relatively inexpensive to transport.

But, it also has its flaws. When it's dry, it can catch fire. Also, the fresh wood's tannins leach into the soil, stunting plants and inhibiting seed germination. When the mulch layer is dry, it is impermeable to our climate's frequent, shallow rains and snows, which puts an important, harvestable water source out of reach of the plants' roots. Wood mulches are prone to blowing away in our region's seasonal winds and to washing off of slopes in our torrential downpours, leaving the weed-cloth or bare earth exposed. Last, being made of digestible cellulose, wetted wood mulches break down over time, nourishing unwanted populations of fungal decomposers. Warning: check for bark beetle larvae in mulches from the forest.

Next, there are inorganic mulches. Inorganic mulches tend to heat and dessicate the underlying soil when the sun is high in the sky, but the inorganic positives and negatives are a bit simpler.

The inorganic choices range from ground-up rubber tires, to crushed flagstone slag, screened, graded gravels, and ground-up rubber tires. This last choice and many others are often eco-friendly attempts to recycle ground industrial waste, like crushed concrete, waste gypsum, and even tire grinds, into our gardens.

Quarried gravels are our region's most commonly used inorganic mulches. These rock mulches have many things going for them. Compensating for their drying effects, rock mulches collect water from our shallow precipitation events and deliver this water to the roots. When applied thickly, it acts like a leaden comforter, anchoring seeds, seedlings and new transplants against Spring's common frost-heaves. Fresh graded gravels don't release complex compounds that might curtail the growth of your landscape's plants.

Rock mulches stay put, except in cases where the smaller, buoyant cinder-grades and sands are used. Applied thickly enough, the mass of a stack of cleaned gravel prevents the mulch layer from being scattered pell-mell by the wind, the weather, or a passing dog. Also, rock mulches rarely offend the nose, that is, they don't nurture objectionable populations of fungi.

All this said, the plants have a big say in this decision, too. Woodland plants accustomed to growing amongst layers of organic debris appear to prefer the stable moistures and temperatures provided only by organic mulches. So, with Viburnums and Hostas, Maples and Boxwoods, an organic mulch is best. But, our plants from the screes, plains, and rock outcropping of the semi-arid Southwest have opposite tolerances. They long ago traded the benefit of a cooling mulch for the additional water cached away each year by nearby exposed stones.

Still, no matter which type of mulch a gardener prefers for a given site, the cost of hauling bulk loads of material long distances always tilts the scales. If money is tight, this argues for importing only light-weight mulches in large paid loads, and for gathering the heavier, rock-based mulches as close to home as possible. In a tight economy, at the end of the day, the best mulch may simply be the one that is most readily at hand.

Steve Shields is a Master Gardener Volunteer. Dana Prom Smith, a Master Gardener volunteer and coordinating editor of the Master Gardener Column for Coconino County Cooperative Extension, can be contacted at stpauls@npgcable.com. For more information about the Master Gardener Program, call Hattie Braun, Coordinator of the Master Gardener Program, at 774-1868 ext.17 or visit our Web Site: highelevationgardening.arizona.edu.