A cashier at the Home Depot store in Chaska told me this summer that, at the rate that bags of wood mulch were moving out of the store, the company would have to start its own forest to keep up with demand.

It would be interesting to follow all the customers home to discover how and where the bags of mulch get used.

Wood mulch is used in landscapes to achieve a variety of objectives: suppress weeds, conserve soil moisture, decrease soil compaction, reduce the spread of soil-borne diseases and buffer plants from the damaging effects of traffic and lawn equipment.

Many woody landscape plants evolved in forests where the soil is typically covered by a moist layer of decaying leaves, twigs, and branches. Mulching trees and shrubs can recreate some aspects of a forest’s soil environment, even in sun-baked landscapes far from the forest.

From a horticultural standpoint the term mulch actually refers to more than just wood chips.  Iowa State University categorizes mulch as inorganic or organic. Inorganic mulches, which decay slowly if at all, offer the advantage of low maintenance. Stones and crushed rock are probably the most commonly used inorganic mulches in landscapes but stones and other inorganic mulches do not deliver some of the benefits of organic mulches. Another problem with stones is that they can absorb enough sunlight to alter soil temperature and stress plants.

On the organic side, a tremendous variety of materials derived from plants or animals are used as mulch. These include tree bark, wood chips, grass clippings, pine needles, shredded leaves, sawdust, straw, peanut and cocoa bean hulls, peat moss, ground corn cobs, animal manure, recycled wood and paper, and composted sewage sludge.

Historically, tree bark has been the most popular material for use as mulch in ornamental landscapes. Mulch derived from the bark of mature softwood trees, including pine and cypress, is very resistant to decay. Bark from young softwood trees decomposes more quickly because it does not contain the high levels of lignin, waxes, and tannins characteristic of bark from mature trees. Hardwood bark, because of its high cellulose content, also decomposes rapidly. Many so-called bark mulches also contain wood, which hastens decomposition. Wood chips from pruning and removal of trees decompose faster than bark, but they contain higher levels of nitrogen, especially if foliage is present.

If have purchased 20-yard truckloads of hardwood mulch several times during the past decade and have been happy with the product. Buying by the truckload is more economical than buying individual bags of mulch but spreading 20 cubic yards (equivalent to 270 bags) requires advance planning and some serious physical labor to distribute the pile.

All mulches have advantages and disadvantages; here is a quick overview:

Partially decomposed compost -- Adds plant nutrients and builds soil structure. Excellent mulch material. Highly recommended for use on annual, vegetable and perennial gardens.

Shredded leaves -- Provides food for earthworms and builds soil structure. Excellent mulch material. Highly recommended for use on annual and perennial gardens. May mat and block rainfall if shredded too fine.

Cocoa bean hulls -- Chocolate aroma when wet. Recommended for annual and perennial beds. May mat and interfere with water penetration. Can blow away in exposed areas. May develop an unsightly but harmless white mold.

Grass clippings -- Completely dry clippings can be applied at 1-2 inches. Can be used on annual, vegetable and perennial beds. Herbicide residues in clippings may injure mulched plants. Use clippings from non-treated lawns or wait three mowings before using clippings.

Pine needles -- Will not cause a measurable change in soil pH. Best used on perennial beds as they will last for two or three years. Removing needles annually from underneath evergreens may eventually cause nutrient deficiency in evergreens.

Wood chips -- Appearance of mushrooms or other fungal growths is common, but not harmful. Has little effect on soil nitrogen when on the soil surface. Best for permanent landscape plantings. Can be used on perennial beds. Once worked into the soil, chips will soak up high amounts of available nitrogen. Compensate by adding a high nitrogen fertilizer when you incorporate it.

Shredded bark -- Additional bark must be added every two or three years. Has little effect on soil nitrogen when on the soil surface. Best for perennial and landscape plantings. Will soak up large amounts of nitrogen if worked into the soil. Compensate by adding high nitrogen fertilizer when you incorporate it.

Straw -- Keeps soil splash off vegetables, which may decrease soil-borne diseases. Chopped straw is easier to handle and turn in at end of the season. Must be free of weeds so it doesn't introduce weed seeds into the garden.

Landscape fabric -- Highly effective with low maintenance for several years after installation. Needs top dressing with more attractive material. Deteriorates after several years.