By Raakel Toppila
Each year, nature provides a blanket of mulch. While the leaves of deciduous trees are a fairly obvious ground covering each fall, there are many other natural “mulches”. Evergreen needles are also shed yearly. Grasses die back and their debris, along with that of perennials and annuals, cover the ground only to be replaced by fresh growth in the spring.
This layer of mulch has many functions. It serves to suppress weeds, retain moisture, moderate the temperature of the soil, reduce compaction, decrease competition from other plants, improve soil structure, and provide nutrients to the soil. In our home gardens we remove debris in order to maintain an aesthetically pleasing appearance. However, this layer of organic matter is an important component in our gardens and can be left in place or replaced with organic mulch.
A five to eight centimetre (two to three inch) layer of mulch will prevent light from reaching the soil surface. This will stop small weeds from emerging and prevent weed seeds from germinating. Weeds that germinate on the surface of the mulch are easily removed by hand. Some weeds, such as dandelions and thistles, have enough energy in their roots to push through a layer of mulch. Ensure these types of weeds are removed entirely before placing mulch on a bed.
Applying mulch to a bed is an effective way to conserve water. It acts as a barrier reducing evaporation from the soil surface. Research indicates that two inches of bark reduce moisture loss in summer by 21 per cent.* Due to its structure, mulch will also assist with the infiltration of water into the soil thereby helping to prevent run off and splashing of water during heavy rain.
Moderating Soil Temperature
Tiny root hairs are necessary for the absorption of moisture and nutrients. By preventing extreme daily fluctuations in soil temperature, mulch protects these sensitive root hairs from injury in the heat of the day. In the fall, a layer of mulch will help to retain ground heat that has penetrated deep into the soil, which will encourage root growth longer into the season.
Foot traffic in a garden will cause compaction around the root zones of plants. This reduces growth by limiting air exchange, water infiltration, and percolation. As the air pockets in the soil are compressed the water holding capacity of the soil is also reduced. Water will pool at the soil surface, further preventing air exchange. Compacted soil also makes root extension very difficult. A layer of mulch helps to reduce compaction by limiting the need for foot traffic due to a decreased need for weeding and by absorbing some of the compaction.
It’s beneficial to place a ring of mulch around newly planted trees or shrubs. Turf and weeds compete with plants, robbing them of nutrients and water. In order to be effective, ensure the mulch ring is between five and eight cm thick and roughly the size of the root system below it. It takes three to seven years for a plant to become fully established. Reducing the competition around the plant will accelerate this process. Continue to increase the size of the mulch ring each year until the plant is well established. A mulch ring around a tree or shrub will also prevent damage caused by lawn mowers and trimmers.
Improves Soil Structure
As organic matter decomposes it turns into humus. Humus acts like glue, holding together soil particles to create crumbs. The formation of crumbs improves soil structure. Improved soil structure allows for proper air exchange, water infiltration, and percolation.
Provide Nutrients to the Soil
When mulch is placed in the garden it continues to decompose. As microorganisms break down the organic matter, essential plant nutrients are released into the soil. However, some materials actually rob the soil of nitrogen as it decomposes. Microorganisms in the soil use up nitrogen as they consume the carbon in the organic material. As a result, nearby plants suffer from nitrogen deficiency. This deficiency is temporary. When the mulch has fully decomposed the nutrients are released back into the soil. Adding partially decomposed mulch and additional nitrogen fertilizer can prevent this condition. Grass clippings, blood meal, or poultry manure can provide the additional nitrogen required.
When placing mulch in your garden it is important to consider the texture of both the soil and the mulch. If a layer of mulch is too thick it may prevent air and water from reaching the soil. Air and water are important both to the roots of the plant and the microorganisms found within the soil. Heavy clay soils are not well aerated due to the small amount of pore space. Coarse mulch should be placed on clay soils in a layer no thicker than five cm. Sandy loam soils have larger pore spaces that allow for more aeration. A thicker layer of mulch with a fine texture can be added to a depth of eight cm.
Soil with a layer of mulch on top takes longer to warm in the spring. The layer of mulch also prevents water from evaporating. When adding mulch to a bed, wait until the soil temperature increases.
It is important not to place mulch against the base of trees, shrubs, or herbaceous plants. Mulch can provide a home for rodents that may chew on the base of plants causing severe damage and even killing the plant. Also, mulch retains moisture and heat, which will damage the base of the plant. Keep mulch 15 cm away from the base to prevent any problems.
Mulch decomposes as it ages; therefore it will need to be replenished from time to time. How often will depend on the type of mulch being used. Try to keep the layer five to eight cm thick throughout the season.
Bark provides an attractive type of mulch with good longevity. If bark is not partially decomposed it may cause a nitrogen deficiency. Some species of trees, such as hardwoods, have bark that decomposes much slower than other species. Bark is relatively inexpensive and can be purchased in bags or by the yard from most garden centres or landscape supply companies. Bark is effective in reducing compaction and is not easily blown by the wind.
Pine needles are attractive, resist compaction, and decompose slowly. They are not readily available for purchase, however they can be taken from beneath pines found on your own property at no cost. Pine needles will acidify the soil in time. However, this is a long process if the soil in your area is naturally alkaline.
Both fine and coarse textured wood chips can be purchased. Fine textured mulches decompose at a greater rate. More frequent applications will be required. Coarse textured mulches may be considered less aesthetically pleasing. Wood chips can be obtained from municipalities for little or no cost. It may also be necessary to provide additional nitrogen in order to prevent depletion in the soil.
Leaves have a pleasing appearance. They can be shredded with a composting mower or lawn mower and added to the garden. Shredded leaves decompose quickly and must be replenished regularly. They are available at no cost. Leaves may cause a nitrogen deficiency. Whole leaves should not be added. They tend to mat and block water from infiltrating the soil. Well-rotted leaf mulch is perhaps the best mulch, providing nutrients as it breaks down into beneficial humus.
Straw has a course appearance that may be undesirable for some. It will require frequent applications. Straw is inexpensive mulch that is ideal in vegetable gardens. While it provides the benefits of other mulches it also keeps the fruits and vegetables clean. Nitrogen should be added to prevent depletion of this nutrient. Straw may carry weed seeds.
In the long run, mulching your flower and vegetable beds can save you a lot of time. It can drastically reduce the amount of time you spend weeding and watering. It also looks good and prevents soil from splashing onto your flowers when it rains.
Research by the Weyerhaeuser Co. (1969)
Ohio State University Extension Fact Sheet www.growinglifestyle.com/article/s0/a12550.html