What Mulch Is All About

By Brian Olszak 

An old growth forest

“Mulch” is a very inclusive term. A mulch can generally be any material, either organic or inorganic, which performs the following functions: reduces moisture loss in soil, suppresses weeds, and helps to regulate temperature extremes. No doubt, as well, many people mulch their beds so as to make them look neat and tidy. In reality, we’re really trying to emulate conditions in nature, and reproduce them in our own gardens. If you walk through a mature-growth forest with very tall, straight trees, especially this time of year, you will find a lot of pine needles, twigs, and leaves that have fallen to the forest floor. What you won’t find is a lot of grass or weeds, at least the types which we typically deal with in our gardens. This is because when all of this dead and decaying matter becomes matted down, it provides a natural layer which both blocks any undergrowth and retains moisture for the roots of trees. Another contributor to this is the extensive tree canopy, which monopolizes most of the sunlight, further preventing undergrowth. A big bonus of this process is that all of this dead and decaying matter provides excellent nutrition for plant life and builds the soil structure.

Bark mulch is available shredded (above) and in chips or "nuggets"

Mulches can be broken down into two main categories: organic and inorganic. The best choice of mulch for your garden or landscape, of course, depends on what you are growing.

The most obvious choices to emulate nature would be organic, wood-based mulches, which, not coincidentally, are the most common of mulches. Bark mulch is from shredded bark, while wood chip mulch is made from all the spare parts of industrially-logged trees or trees removed during construction. Bark mulch and wood chip mulches work for both vegetable and flower beds. But freshly-shredded, their nitrogen content is so low that nitrogen is leached from the surrounding soil in order to facilitate the decomposition process—which means less nitrogen is available to your plants. It is important, therefore, that these kinds of mulches have been properly composted first before use in gardens: freshly-shredded wood chips can be used on trails or walking paths instead, or left for a few months before use in gardens. In addition, it is important to not over-mulch with wood chips, as the temptation to replenish old and discolored wood chips might be too great. Over-mulching can bury root systems too deep, making water penetration much more difficult if not impossible. In the case of trees, beware of the "mulch volcano": the mounding of mulch around the trunk. This practice can cause a number of troubling conditions, including the rotting of bark and roots, places for voles and other critters to shelter and nibble on your tree, among other things.

Straw mulch is great for potatoes (above) and strawberries

Other kinds of organic mulches include straw, pine needles, and shredded leaves. Straw is a good cover for vegetable gardens and for overwintering beds, which protects tender and sensitive roots from freezing out. Pine needles can be used if particular plants need especially acidic soil, such as blueberries.

Inorganic mulches, while not providing organic material and nutrition to your landscape, do have the significant benefit of lasting much longer than most organic mulches, and therefore need less attention from you.

Landscape fabric can be cut to suit plants already in the ground

One example of an inorganic mulch is black plastic. Black plastic is very good at retaining moisture and blocking out weed growth, and is an old standby for gardeners, but has many drawbacks. Not only will water have trouble penetrating to the roots below, but the plastic can degrade in sunlight. Landscape fabrics can be a useful complement to an organic mulch in your garden. Typically made of polyester or polypropylene, these barrier fabrics are porous enough to allow water to seep through but are tight enough to prevent weed growth from beneath. You can cut holes in the fabric where you intend to plant. Placing a wood-based mulch on top of this fabric barrier can make your landscape look more natural and while also preventing the ultraviolet rays of the sun from degrading the fabric. However, these barriers can just as easily become a nuisance if weeds begin to establish above the fabric layer—this can happen when a wood-based mulch decomposes into soil where weed seeds can germinate. If you simply throw more mulch on top of this new soil layer, you will hit new levels of aggravation while trying to dig through the buried fabric when planting the next season! This can be avoided by removing old mulch and replacing it with new mulch every year. Don’t know what to do with the old mulch?—compost it! Several kinds of stones and gravels, including pea stone, can also be used for a more decorative treatment, especially in a perennial garden or for very young trees.

Pea stone can be decorative or functional

Both organic and inorganic mulches have their place in the garden, so do some experimenting, and see what works for you!