Almost every gardening expert will recommend using mulch around new and existing plantings, but why? Mulch has many benefits in the garden, but it can also cause problems if used improperly.
In nature, mulch forms naturally from fallen leaves, dead grasses and other materials that collect on the ground. This layer of decomposed, or decomposing, material provides a home for beneficial organisms that break the organic materials in the mulch into simpler components that can be used by plants. In addition, this natural mulch provides a cushioning layer that lessens the impact of rain falling on the soil, thus reducing soil compaction and erosion, and improving water absorption. Mulch forms a barrier that prevents evaporation — helping to mitigate against drought conditions. It also acts as a thermal insulator to moderate extreme changes in soil temperature. Mulch shades the soil surface and blocks seeds that require exposure to light for germination, as is the case with many garden weeds.
Mulches used in the home landscape perform many of these natural functions, some better than others. In addition, mulch also serves as a design element, adding color and texture to the ground plane. Determining the best mulch to use depends on where you are, the type of landscape you are working with and your aesthetic goals.
Types of Mulch
Listed here are a number of materials used as mulch, along with what they do well and where they fall short. They are broadly grouped into organic, living or once-living materials, and inorganic materials, which include minerals and synthetic products.
Organic Materials
Living mulch. Living mulch consists of low-growing plants that, once established, form a dense layer of ground cover. It may be a single species or a variety of plants that form an interlocking matrix. This is most like what you would encounter in a meadow or woodland setting. Examples of living mulch plants include sedum, lady’s mantle (Alchemilla mollis), moss phlox (Phlox subulata), golden ragwort (Packera aurea) and green and gold (Chrysogonum virginianum).
Pros: Retains moisture, reduces erosion, allows water permeation, supports beneficial organisms, and offers good temperature control and excellent aesthetics. Weed suppression is dependent on the density of planting.
Cons: Takes time to develop thick growth and requires more maintenance during establishment. It can be difficult to tell the difference between desired and undesired plants (weeds).
Shredded bark. This is made from the outer layer of trees and is a byproduct of the lumber industry. Properties and appearance vary depending on the type of tree. Trees used for shredded bark mulch include hardwoods, cedar, hemlock, pine and redwood. The types available depend on your location. Coarsely shredded grades last longer but don’t look as nice in some settings as finer grades. Over time, shredded bark breaks down to enrich the soil with organic matter. Read product information carefully since sometimes chipped or shredded wood is dyed and resembles shredded bark mulch.
Pros: Suppresses weeds very well, offers good moisture and temperature control, allows water permeation, and breaks down over time to help build soil and support soil organisms. Aesthetics vary depending on the type and grade used.
Cons: Needs to be replenished on a regular basis (one to three years) and fades over time. Coarser grades can float away, and finer grades can become compacted if applied too thickly.
Pine bark nuggets. These are actual chunks of pine bark and are available in a variety of sizes. Larger pieces last longer but have a coarser appearance and a greater tendency to float away or otherwise migrate around the garden.
Pros: Similar to shredded bark, but pine bark nuggets are slower to break down than other wood and bark products, and can hold their color for several years.
Cons: Larger pieces tend to move around in the landscape, it’s more difficult to walk on than shredded products, and it has a coarser appearance.
Shredded or chipped wood. This material can come from a variety of sources, including landscape debris, sawmill waste, chips from arborists, and recycled wood from pallets and construction. It can be natural in color (shades of brown and gray) or dyed, usually black, brown or red. It pays to find out the source of the wood since some recycled wood may be contaminated with old paints, preservatives or industrial chemicals.
Shredded wood from a clean source that has been partially composted is a good all-around choice. Chips from arborists can get hot while the contained leaves and green materials are breaking down, but the resulting material makes a good, albeit coarse, mulch, which can be had for little or no cost.
Pros: Offers good moisture and temperature control, suppresses weeds well, absorbs water, controls erosion, and comes in different colors. Wood from clean sources breaks down over time to benefit soil health. It’s best used for established beds, around trees and shrubs; it’s not a good choice for vegetable gardens or where growing plants from seed.
Cons: Needs regular replenishment and may contain contaminated materials. Aesthetics vary depending on material and grade
Compost. Good-quality organic compost is the healthiest mulch to put on your garden soil. It provides lots of organic matter and nutrients that soil organisms can pass onto the plants. It is, for this reason, short-lived as a mulching material and needs more frequent replenishment to continue to do a good job of weed suppression, moisture retention and temperature control. Its best use is in a vegetable garden, where the plants will grow quickly and fill in to provide the benefits of a living mulch.
Pros: Feeds plants very well, builds soil quality, suppresses weeds well initially, and offers good moisture and temperature control. It’s dark and fine-textured in appearance.
Cons: Is short-lived and needs annual or more frequent application. Poor-quality compost may contain weed seeds. Although compost shades the soil and slows the germination of existing seeds, it may enhance the germination of new weed seeds that fall on it.
Grass and leaves. Mown grass and chopped leaves are essentially free mulch, which also saves on the cost of disposal. However, the grass used should be free of lawn chemicals and weed seeds to prevent problems down the line. Grass clippings are high in nitrogen and find their best use in the vegetable garden. Before being putting back in the garden as mulch, grass should be left to dry in the sun for a couple of days (much like making hay) so that the rapidly decomposing material won’t burn the tender vegetable plants. Leaves should be shredded to minimize matting and to allow water to move through the layer. Shredded leaves are also easier to spread, look more even and decompose more quickly to benefit the soil.
Pros: Is free, adds nutrients back to the soil, retains moisture well and offers good temperature control.
Cons: Breaks down quickly and needs regular replenishment. Grass and larger leaves may compact and slow water absorbance. Grass may be contaminated with weed seeds, needs to be “cured” to prevent burning and can get smelly if applied too thickly.
Pine needles. Pine needles form an excellent mulch most appropriate to a woodland setting with acid-loving evergreen trees and shrubs. Their high resin content makes them slow to break down, and their needlelike shape resists compaction. Pine needles make a good covering for pathways; however, because they can be slippery when wet, they aren’t so good on sloping paths.
Pros: Is long-lived, offers excellent water permeability and thermal insulation, suppresses weeds well, retains moisture; improves soil quality slowly over time, is economical in areas with a lot of pine trees and has a pleasant scent.
Cons: Comes in only one color and can get slippery when wet, especially on otherwise compacted surfaces.
Inorganic Materials
Stones and gravel. Many plants are well-adapted to growing in and around rocks. Stones are the longest-lasting natural mulch material. They are most suitable in arid or harsh conditions where organic materials could break down rapidly or be blown away. Stones tend to heat up more than organic materials, especially with the darker colors. Light-colored stones reflect a lot of light upward and can burn the undersides of the leaves of plants that are not adapted to those conditions. When installing a stone mulch, it is a good practice to put down a water-permeable geotextile to prevent soil from mixing into the stone. Also consider other inorganic materials like crushed shells, polished glass and ceramic chips.
Pros: Is long-lasting, comes in a wide variety of colors and textures, and offers good water permeability and soil protection. Weed suppression varies depending on preparation and on keeping organic materials (leaves and grass clippings) from accumulating in the bed.
Cons: Is heavy, adds only trace nutrients to soil and requires containment with edging or some border. Depending on aesthetics, the bed may need to be cleaned on occasion.
Geotextile. Also known as landscape fabric, geotextile is made from polyethylene fibers either woven or spun into a material with very small pore sizes. It allows water and dissolved gases into the soil but blocks weed growth from below. It finds its best use in the garden as a separating layer between the soil and a surface mulch, especially for stone or gravel. It is also used under pathways and in French drains to keep soil from mixing in with the aggregate. As a surface layer, it has poor aesthetics and a relatively short lifetime. One problem in garden beds is that, while it suppresses weed growth from below, roots of plants growing above the textile can penetrate it and be impossible to remove without cutting away the affected geotextile.
Pros: Retains water well, and is an excellent soil barrier and a good weed barrier.
Cons: Is unattractive and short-lived unless covered by another material. It needs to be cut away to allow additional planting, and it may resurface over time if not pinned down well when installed.
Plastic sheeting. Plastic sheet mulches are used mainly in agricultural settings for particular crops or seasons. They are rarely used in the garden landscape, though I have encountered them as underlayers in some installations. While effective for weed suppression and containing the soil underneath, they also block water and air. If additional planting is to be done, cutting through this layer can result in a mess. The best landscape use is for an area where no plants will be grown and where blocking water infiltration is not an issue.
Pros: Is long-lasting, and blocks evaporation from soil and weed growth from below.
Cons: Suffocates soil and blocks water infiltration. It’s unattractive unless covered by another material.
Rubber. Most rubber mulch comes from recycled tires that have been processed and shredded to remove the steel cord. These are available in a variety of shades, from earth tones to colors like blue, green and red. This mulch does not break down as quickly as organic materials because it is synthetic. For this reason, it also does not feed the soil and offers few additional benefits to plants besides weed suppression and moisture control. As with stone and gravel mulches, rubber mulch should be used with an underlay of geotextile to help it stay clean and to make it easier to remove in the future should your preference in materials or color change. This may be a good choice where low maintenance is more important than plant growth and where exotic colors in the ground plane are an important part of the design. Rubber mulch is being used in playground applications; however, some groups dispute that it is completely safe.
Pros: Is long-lasting, has a consistent appearance, stays in place, offers good water permeability and weed suppression, and comes in many colors and textures.
Cons: Does little to enrich soil or plant health and costs more per cubic foot than wood or bark products. There are many manufacturers, so quality may vary. There is still some concern about leaching chemical additives and residual steel cord.
Mulching Tips
Although mulch offers many benefits, improper use can result in problems.
Timing. Good times to apply mulch are in the spring, after the soil has warmed up and after you have applied any fertilizers or top dressings of compost, but before hot or dry periods set in.
Adding mulch in the fall is good for protecting tender plants that are on the edge of hardiness for your region. However, if your plants are well-adapted to the cold, then waiting until the ground is frozen and applying a winter mulch will help prevent frost heaves and give better bloom for plants that need a cold winter.
Don’t layer mulch too deep. For most mulches, a 2- to 4-inch-deep layer is recommended. Applying it too thickly limits airflow to the soil and can suffocate plant roots. A layer that is too thick can also get soggy and develop a bad smell due to growth of anaerobic bacteria. Overly thick layers of organic mulch are slower to break down and are more subject to compaction.
Keep mulch away from trunks. One of the most obvious misuses of mulch is the “mulch volcano.” This is where mulch is piled up in a cone around the base of a tree. When this occurs, the bark stays wet, and conditions for bark rot can develop. Also, some trees will put out new roots into this cone that circle the tree and strangle it, usually several years down the line. The same holds true for shrubs and perennials, where covering their base can result in crown rot. In general, mulch should be pulled back 2 to 4 inches from the trunk or crown of a tree or shrub.
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Check for quality. For each mulch listed here, there are a number of suppliers and, therefore, potentially variable quality. If you are doing a big project and have questions, get a sample of the material and check it out for yourself. Is the color and feel right for you? Does it smell funny? Do you see things in it that don’t belong? If you are not happy with it, find another supplier.