A White Paper on Mulch: A Crucial Component to Landscape Health
By: Paul Clute and Tim Sloan, Woodlake Outdoor
Over 1500 years ago, the Scots began using seaweed as mulch to improve soil of the machair (the Scottish name for a raised beach common along the sea lochs and inlets). Machair were created to sustain crops, but as sandy soil does not hold nutrients well, seaweed mulch was used to stabilize the soil and add nutrients. Even earlier, some 5000 years ago, ancient agrarians used rocks as mulch to control weeds.
Mulch today still fulfills these critical soil needs – and with modern research, even more effectively.
Nutrients – Food for Plants
Anything from pecan hulls, pine needles, ground cedar, bark chunks to crushed gravel, rocks, and recycled rubber can be used as mulch. However, there are substantial benefits to using a well composted mulch, particularly for North and Central Texas soils. A composted, double-shredded hardwood bark mulch is ideal for our soil for several reasons.
First, the decomposition of organic material over a one-year period will amend the soil with nitrogen, phosphorus and potassium. These macronutrients support new growth and vivid color, support root development and seed production, and strengthen stems and fight disease, respectively. Mulching in the late winter/early spring every year creates a cycle that benefits plant health year-round.
Second, it will add stability to the sandy soils (Cross Timbers region), allowing soil to retain nutrients. This hardwood bark mulch also benefits the clay soils (North and Central Texas regions). Although positively, clay soil’s cation exchange capacity (the soil’s ability to hold particular nutrients) is quite high, negatively it remains heavy, sticky, and too compact to allow roots to “breathe.” As mulch decomposes, it creates pore spaces, allowing the roots to thrive.
Soil Temperature – Need for Regulating
It can be a challenge for plants in Texas to maintain a stable soil temperature – and necessary for plant health. As temperatures rise in the summer months, exposed soils will dry out and crack. This causes serious damage and stunted growth to the fine root hairs on all growing plants. In summer, mulch absorbs the sun’s ultraviolet rays and transforms them into less-powerful, long-wave rays. Heat transfers to the soil through mulch more slowly than if the sun hit the soil directly.
As temperatures fall in the winter months, mulch help protect tender roots from freezing. Mulch insulates soil and regulates its temperature. In the fall and winter, mulched soil loses heat more slowly.
Moisture Level – Conservation is Critical
Runoff from clay soil or excessive drainage in sandy soil – neither is beneficial for the landscape nor the environment. Mulch helps create a balanced soil barrier which will retain the correct amount of water – reducing waste while correctly fortifying the landscape.
In addition, mulch helps eliminate erosion and the saved water usage can result in lower water bills. Mulch creates stability in soil moisture, giving plants a stabile microclimate where they will flourish.
Microbes – Life of the Soil
Healthy soil – living soil – houses many microorganisms, with varied purposes. Some of these organisms consume and digest the soils and minerals. They then break down the nutrients into a useable form, readily available for plant absorption. Some microbes perform this process by attaching directly to the roots of a plant in an area called the rhizosphere.
While soil testing is imperative, it is not the complete story. It will tell you what is in your soil, but not necessarily what is useable in the soil. For example, your soil test can show high iron or sufficient magnesium (or any other hard element in soil), but it cannot identify the bioavailability.
This is where mulch comes in to play.
The simplest microbe is one that creates decomposition. They feed on the organic matter – much like an earthworm does – resulting in aerobic decomposition. Mulching increases the organic content of a landscape, encouraging healthy colonization of microbes.
Weed Issues – Nutrient Theft
As with any plant, weeds and weed seeds need proper sunlight and soil contact. Mulch reduces both, making great strides toward weed-free plant beds. Adding a pre-emergent herbicide also aids in prohibiting weed seeds to germinate. Pre-emergent is a particularly important and complementary step when adding mulch, as mulch itself can – and usually does – come with a minimal amount of weed seeds. Weed seeds are also carried by wind, birds and other animals, therefore beds must be monitored weekly. Chemically (post-emergent) or mechanically remove germinated weed seed during these weekly checks. Mulching significantly aids in suppressing the weed population, but it can’t eradicate it – “Nature always finds a way!”
Final Thoughts – Points to Consider
Other factors to consider when using mulch are aesthetics, quantity, and tree protection. A late winter/early spring application not only prepares the soil, it revitalizes the landscape, bringing vibrancy to the dormant plants.
More is not better – too much mulch can be detrimental to plants. It reduces the required oxygen/carbon dioxide exchange, which can rot the base of the stem or trunk. This can cause the roots to rise, seeking that exchange, not unlike the effect of overwatering a plant. The natural surfacing of roots looking for an equilibrium state causes a weak root system, allowing the plant to be easily blown over in the high winds common in Texas. Therefore, a good standard for mulch levels is 3” in newly constructed beds and 1.5-2” in existing beds.
Lastly, it is a good idea to mulch around your trees or tree rings. In addition to the above benefits, it also protects the tree from unnecessary scarring from possible mechanical damage during mowing operations. Remember not to install mulch up against the tree trunk as that has the same detrimental effect as over-mulching.
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