Understanding Mulch

Many folks take mulching for granted. They are not sure why they need to mulch, how much or when to apply it, or why type to use. However, proper mulching is one of the most beneficial things that a homeowner can do for their landscape.

Mulches are defined as materials placed over the soil surface to maintain moisture and improve soil conditions. They are intended to help reduce moisture loss from the soil, minimize weed competition, and improve soil structure (the aggregation of soil particles).

When properly applied, mulch can really complete a landscape. However, it must be selected and applied properly in order for the homeowner to receive the full benefits. If applied too deep or the wrong material is used, it can cause significant harm to trees and other landscape plants.

Benefits of Mulch
Proper mulching has many benefits:
– Helps maintain soil moisture
o Reduces evaporation and need for supplemental irrigation/watering
– 2-4” layer reduces the germination and growth of weeds
– Improves temperature regulation
o Keeps soil temperatures cooler in the summer and warmer in the winter
– Can improve soil aeration, structure (aggregation of soil particles), and drainage over time
– Can improve soil fertility
– Mulch around trees can help facilitate maintenance and reduces the likelihood of injury or damage from lawn equipment and tools
– Gives planting beds a “finished”/ polished look

Mulch mimics the natural environment. Forests create their own mulch via leaf litter and decomposing organics on the soil surface. This is critical to the overall health of the ecosystem as it improves nutrient uptake and root growth of forest plants. Mother nature, when left to her own devices, usually gets it just right.

Urban area on the other hand, often have a more hostile growing environment. Generally speaking, land development and construction in urban areas leads to poorer soil quality, less organic material in the soil profile, and increased soil temperature and moisture fluctuations. These combine to create an environment that is hostile to the establishment and health of landscape plants. Applying a 2-4” layer of the proper organic mulch in these areas can create a more natural environment for trees, shrubs, and plants.

When considering trees, it is important to remember that tree roots are not a mirror image of the top of the tree. Roots can in fact extend out a great distance from the trunk. General tree maintenance guidelines reference the “drip line” (the outermost extension of the tree canopy), however roots can extend much further than the “drip line”.

Most nutrient absorbing roots grow in shallower soil though. These shallow, fine roots are essential for the absorption of water, nutrients, and oxygen for all plants. A think layer of mulch inside the “drip line” can improve soil structure, moisture, oxygen, and temperature in this crucial layer of the soil profile.

Mulch Types
Mulches are available commercially in many forms. The two major types of available mulches are organic and inorganic.

Inorganic mulches are generally comprised of material like:
 lava rocks
 stones
 crushed rubber
 certain geotextile material
 various other materials

These materials do not decompose and do not need to be replenished over time. They also do not provide any nutritional, structural, or organic value to the soil profile.

Organics mulches are composed of material like:
 wood chips
 pine needles
 hardwood and softwood barks
 cocoa hulls
 leaves
 other organic materials

These materials decompose in the landscape at different rates depending on the material and climate. The decomposition process improves soil quality and fertility, however they must be replenished over time.

How Much Mulch to Apply
The amount of mulch applied is just as important, if not more than the type being applied. Too much mulch can be harmful. The accepted depth for most mulch application is between 2-4”, however many landscapes fall victim to over-mulching, with large mounds around the bases of trees being quite common.

Although organic mulches must be replaced over time as the parent material decays, the rate of decomposition varies depending on material and local climate. Some mulches, like Cypress, can take years to decompose.
Many homeowners will top-dress with new mulch annually (to refresh the color). This can create a buildup to depths that become unhealthy for most plants. If top-dressing old mulch, remove the ole material every so often, keeping application rates in line with decomposition rates to avoid creating a anaerobic growing environment.

Improper mulching can lead to a host of problems:
– Excess root zone moisture
o Added stress for the plant and causes root rot
– Piling against tree trunks or plant stems can cause stem tissue stress and lead to insect and disease issues
– Certain mulches can affect soil pH (proper pH is essential for any life form to grow and develop)
o Continued use over extended periods can lead to micronutrient deficiencies in the soil or nutrient toxicities (yes, there can be toxic effects from too many nutrients in the soil)
– Mulch piled high against tree trunks may encourage rodent infestations
– Thick application of fine mulches may prevent the penetration of air and water (anaerobic and hydrophobic conditions)
o This is the opposite of why we mulch in the first place
– Anaerobic soil may give off foul odors (rotten egg smells)
– Build up of alcohols and organic acids may be toxic to young plants

 

 

Proper mulch selection and application method are very important to the overall health and sustainability of landscape plants.

Guidelines for proper mulching:
– Inspect plants and soil in the area to be mulched
o Adequate drainage?
o What type of mulch is best? (when in doubt, ask a professional)
– Check depth of any mulch already present
o Do not add if there is enough in place
o Rake old mulch to refresh the appearance and break up any matted layers
– Pull back from stems of plants and trunks of trees so that the crowns are exposed
– Organics are usually preferred over inorganics b/c of their beneficial soil contributions
o Organics should be well aerated and preferably composted
 Avoid foul smelling mulch
– Composted wood chips make a great mulch
o Especially when old leaves and bark is included
o Fresh wood chips may be used around established trees and shrubs
o Avoid non-composted chips that have been piled deep without exposure to oxygen
– For well drained sites use a 2-4” layer of mulch
o If there are drainage problems, a thinner layer should be used
o Avoid placing against tree trunks
o Do apply out to the drip line of the tree and beyond if possible

Keep in mind when selecting mulch for your landscape that there are many factors that must be considered in order to get the full benefits of mulch applications. For more information about mulch varieties, benefits, and application timings, contact a landscape professional.