Bark mulch is available in natural and synthetic form, to perform three basic functions; to suppress weeds, reduce soil water losses, and to protect against temperature extremes. In the summer months mulch can reduce the soil temperature by 8 to 13 degrees, and the average amount of time required to remove weeds is reduced by two-thirds.
The use of mulch provides other benefits such as reducing soil erosion and increasing the penetration of water into the soil. As natural mulch decomposes it provides organic matter called aggregates. The larger aggregates increase aeration and improve the moisture of the soil and structure.
The characteristics of a good mulch include the ability to apply and remove easily, economical, stays in place, supplies organic matter to the soil, readily available, and free of noxious weeds, insects, and diseases. Functions of mulch should include suppression of weeds, conservation of soil water, and moderation of soil temperatures.
When selecting the right mulch consider the season the mulch will be used in such as summer or winter, and the location. Black plastic can warm the soil in the spring months, as well as heat in the summer possibly being too hot for certain plant varieties. It is also effective in the prevention of evaporation of water from the soil. Straw, shredded leaves, pine needles, and wood chips are effective in insulating in the winter months, but during the spring and summer months will not allow the soil to warm as quickly. Mineral or synthetic mulches do not contribute organic aggregates to the soil like natural mulch does. Winter mulches are primarily used for the insulation of woody plants, laid down in the fall to keep the soil at an even temperature. Straw, pine needles, and shredded leaves are also effective mulches in the winter months. Summer or growing mulches are laid down after the soil begins to warm during the spring months. The role of summer mulch is to warm the soil, reduce weed growth, and retain soil moisture.
Another consideration when choosing the right mulch is the location. Black plastic and straw are used most often in vegetable gardens or areas of small fruit plantings. Wood chips, bark mulch, and pine needles are most often used around shrub beds and around trees. Fine mulches such as wood shavings, cocoa shells, bark granules, and buckwheat hulls are used in annual and perennial flowerbeds. Crushed stone, and fine gravel are used most often in rock gardens.
Other considerations are cost and availability of the mulch product. For example buckwheat hulls and cocoa hulls are attractive mulches but can only be found in certain regions, even when they are available the mulch can also be very expensive. Another factor is that cocoa hulls can be toxic to animals such as dogs if they are to ingest them.
Bark mulch are by-products of milled Douglas fir, pine, redwood, fir, and spruce trees. The standards for bark mulch used in landscaping are bark chunks or decorative bark, shredded bark, and bark granules or a soil conditioner. Certain bark mulches can be toxic to young plants; toxins can be leached from the bark by excessive watering or aeration. If there is a concern about younger plants the bark mulch should be spread evenly and thin. Most bark mulch products have weathered for long periods of time to remove any toxins that would harm young plants. The desired characteristics of bark mulch include resistance to compaction by the wind, the attractiveness and color options, as well as the availability.
Wood chips are a product of different hardwood and softwood species. They are often available from utilities companies involved in pruning or clearing of trees. Wood chips have a high carbon to nitrogen ratio meaning during the process of decomposition there can be a reduced supply of nitrogen to the plant. To combat this nitrogen fertilizer can be used for mulched plants. Wood chips compared to bark mulches lose their decorative color appearance over time because of weathering. This leads many people to renew their wood chip mulch each year, by adding additional inches. The problem of over mulching can arise, which can suffocate the roots of shallow rooted plants, and can cause cankers to develop around the bases of certain tree and shrub species. Proper practice is to renew wood chip mulches every two to three years, and to churn the existing mulch before adding a new layer.
Sawdust is recommended as mulch for blueberry bushes, rhododendrons and other acid loving evergreens. It can have an acidifying effect on the soil as it decomposes, also not allowing nitrogen such as wood chips. Sawdust can also have compaction and decomposition over a single growing season, so it is necessary to churn and renew sawdust mulch every spring.
Cottonseed, hulls of cocoa, and buckwheat are available to only certain regions that are processing the mulch as by-products. Most often this type of mulch is decorative and can have a delicate, textured appearance. The mulch is used for flowerbeds, rose gardens, and shrub borders with high visibility. Processed hull mulches are most expensive, and prone to blowing in strong winds, as well as washing out in the case of heave rain.
Straw mulch is comprised from oats, rye, barley, wheat, and timothy and are inexpensive. Straw mulch is mostly used as winter mulch around shrubs, and trees, however in summer it can be used in vegetable gardens and around strawberries. Straw mulch can be easily blown and also lacks the attractiveness of other mulches with color and texture. Like other mulches it lowers the nitrogen supply as it decomposes and must be renewed annually. It is cheap and defective in the suppression of weeds and reduces the loss of water from the soil. As winter mulch it can protect roots from temperature injury.
Pine needles are appealing, and do well with acid-loving plants. They are available commercially but can also be raked from pine plantings. They decompose slowly, are resistant to compaction, and are easy to landscape. Pine needles provide protection around new or tender ornamental plants, and should be renewed annually.
Shredded leaves are used as summer mulch and can sometimes cause a barrier that blocks free water and oxygen movement to the soil. For the best results allowing the leaves to decompose before using them, as mulch is the best option, when the leaves are laid down and decomposing they contribute nitrogen, and other nutrients to the soil.
Crushed stone, volcanic rock and gravel are considered mineral mulches, thus organic material. They are not blown by wind, and do not harbor weeds or diseases, and do not rob the soil of nitrogen. They are most often used in shrub beds, walkways, and driveways. Depending on the location and the material used it can be finely textured or coarse. Mineral mulch can be colored to bled in with the features of the home, landscape, or patio. Unless the mulch is underlain with synthetic fabric or plastic mulch they can migrate into the soil over time.
Black plastic is very popular and has the ability to suppress weed growth, and retain soil moisture. IT is most often used in vegetable and small fruit gardens, and often used as the under layer for wood, mineral chips, and bark mulch. Although black plastic can prevent water from evaporating from the soil, it can however also prevent water from entering the soil; layering black plastic with alternating rows of bare soil can prevent this.
Geotextiles better known as landscape fabrics are woven and nonwoven fabrics of polypropylene or polyester and are used in place of black plastic. They block weeds growth, reduce surface evaporation of water, and allow water, fertilizer, and oxygen to penetrate into the soil. They are however degraded by ultraviolet rays from the sun. They are used as underline and enhance the weed suppression of the mulch which separating the soil and the mulch. Nonwoven polyester fabrics last longer and have greater resistance to temperature and chemical degradation then polypropylene, they are however more expensive. Weaving fibers together or bonding them manufactures polypropylene fabric. The nonwoven fabrics are bonded by melting with heat or chemicals, gluing, by needle punching or molding. Factors to consider are the ease of application to the landscape, water penetration, effectiveness of the material to suppress weeds growth, and the cost. The final step after application of the geotextiles is to apply a one to three inch layer of mulch on top of the geotextiles to improve the appearance, decrease deterioration, and reduce wear. They do not prevent all weed growth, weeds that germinate in bark or wood chips can growth through the fabric.
Managing mulch in the landscape consists of bringing the site to its desired grade before covering the area with mulch. If there is a problem with annual weeds consider treating the location with herbicide before mulching. Mulch applied for winter protection should be laid down in the late fall once the soil has been cooled but not frozen. Summer mulch is applied in the mid-spring, once the soil has been warmed enough for root growth. Mulch around shrubs and trees should be pulled away from the trunks in the fall to allow proper hardening of the bark for protection. Organic and mineral mulch must cover an area to uniform depth to be effective. The lower or bare spots can be prone to weed problems, and it does not insulate the soil.
The landscape, season, cost, and materials all play a part in the type of mulch chosen. With many options there is sure to be a mulch best suited from the type of landscaping job you are looking for.
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