Mulch: Everything Gardeners Need to Know

Mulch: a protective covering (as of sawdust, compost, or paper) spread or left on the ground to reduce evaporation, maintain even soil temperature, prevent erosion, control weeds, enrich the soil, or keep fruit clean2.

Why you should mulch

Mulches can be applied to bare soil or to cover the surface of compost in containers and can serve as a decorative finish to any garden project. The benefits of mulch are numerous including moisture retention in soil which results in the reduced need for watering. This is due to the mulch reducing water evaporation by protecting the soil from direct sun and moving air. Mulch also prevents rain from compacting the soil surface and decreases soil erosion.

Suppression of weeds is an important aspect of successful gardening that prevents water and nutrient consumption by competitive vegetation. Mulches are best used for weed suppression when applied from mid-to-late spring when annual weeds have not yet germinated1 or are newly emerged3. When choosing an organic mulch for the suppression of weeds and water retention, the type of material is not as important as the thickness of the mulch layer. Most mulches should be 1-2 inches thick in order to ensure the blocking of sunlight from the soil surface ensuring consistent soil temperature and moisture.

As mulch breaks down, it improves soil structure by introducing beneficial organisms, organic matter, and nutrient availability. As the breakdown occurs, it gradually releases nutrients into the ground which provides a good environment for earthworms and other organisms that improve soil turnover4. Clay and sandy soils benefit the most from organic mulching as it improves soil structure, drainage, and aeration which creates a better growing environment.

In areas with short growing seasons, using dark-colored or inorganic mulch can increase soil warming in the spring allowing for earlier planting of warm-season crops. Black plastic is best for increasing soil temperatures and killing off unwanted vegetation in new-growth areas. Alternately, light-colored mulch reflects light and moderates soil temperatures, protecting plant roots from heat injuries and preventing moisture evaporation.

Proper mulch application is important in preventing pests and diseases of plants’ stems and leaves. When mulch is piled against the stems of plants or the trunks of trees, it keeps this tissue too wet and sets up an environment for insects and diseases to establish themselves6. Examples include mulch toxicity or sour mulch, slime molds, insects, and fungus.

Mulches can be split into two main groups: biodegradable and non-biodegradable.

Biodegradable mulches

Organic mulch layers require consistent reapplication as the material breaks down. These mulches include compost, wood chips, bark, leaves, and straw.


Softwood (pine-bark) mulches are least likely to impact soil health and are readily available at garden centers and hardware stores. It is slow to decompose, is available in several colors, and has a pleasant appearance. Apply 1-3 inches deep.


When using wood chips and sawdust as mulch, it is important not to incorporate the material into the soil. When incorporated, wood chips and sawdust can deplete soil nitrogen in plants, stunting growth and health. These materials are best used when allowed 6 months to a year to age. Wood chips and sawdust are sometimes available at free or low cost from local tree removal and trimming companies and sawmills. If available in your area, wood chips and sawdust may be scheduled for a free dropoff through GetChipDrop.com7. Apply wood chips 1-3 inches deep and sawdust 1 inch deep.


Pine needles are an attractive option for mulch as they are unlikely to become matted, allowing for water penetration and aeration. Apply pine needles 2-3 inches deep.


Grass clippings break down quickly and are typically left in place to improve grass health. When used as garden mulch, grass clippings must be replaced on a regular basis. Do not use clippings from grass that has recently been treated with herbicides which would have a negative impact on garden plants. For best results, ensure grass clippings do not dry out as it would encourage matting and prevent water penetration. Apply 1-2 inches deep.


Leaves are the ideal mulch for all applications, especially when shredded or allowed to age for some time. Decomposing quickly, leaves rapidly improve soil conditions in even the poorest of soils. For acid-loving plants such as azaleas, camellias, and rhododendrons, oak leaves are the best option as the higher tannin in the leaves stabilizes acidity levels in the soil and improves plant growth3. Apply 2-3 inches deep.


Regular additions of compost will improve soil structure but needs replacing on a regular basis as decomposition happens quickly. When mulching with compost, complete decomposition isn’t necessary as breakdown continues when incorporated into the soil. Apply 2-3 inches deep.

Additional biodegradable mulch options include rotted manure, ground corncobs, hay, pecan hulls, newspaper, and cardboard.

Non-biodegradable mulches

Non-biodegradable mulches do not benefit soil composition and are typically used to improve ground stability, suppress weeds or improve curb appeal.


Gravel and stones are long-lasting and are often placed over landscape plastic or fabric. Apply gravel and stone 1-2 inches deep.


Landscape fabrics are either natural or synthetic materials used to conserve soil moisture, suppress weeds, and maintain soil temperature.

Mulch Problems and Solutions

Mulch Toxicity

Although mulch benefits plants, “sour” mulch can damage plant tissue or lower the soil pH causing injury or death5. Signs of mulch toxicity include yellowing of leaves, dropping of leaves, and plant death5. In compacted mulch, the absence of air allows microbes in the mulch to multiply and produce toxic substances, such as methanol, acetic acid, ammonia gas, and hydrogen sulfide gas5. Sour mulch smells like vinegar, ammonia, sulfur, or silage and will have a pH of 1.8 to 2.55.

To prevent mulch from turning sour or causing mulch toxicity, turn mulch piles once or twice a month and limit height and width to approximately 4 feet. If mulch already appears to have soured, spread it out thinly, spray it with water, and allow it to dry. When the smell has dissipated, the mulch will be sage to use6.

Slime Molds

Slime molds tend to appear during warm, damp weather. The mold is typically found in masses several inches to a foot or more in diameter and varies in color6. Slime molds are excellent at breaking down organic matter and are harmless to surrounding vegetation. If removal is desired, scoop the masses up with a shovel or pitchfork.

Matted Mulch

When thick layers of mulch are allowed to dry, some nuisance fungi can be encouraged to grow, forming a mat of mycelium (a mass of fine threadlike structures that make up the body of the fungus)6. The mycelial mat repels water and prevents moisture from reaching the soil. While mulch settles, ensure newly applied soil remains wet. If matted mulch is present, break apart the layers with a rake or cultivator.

Artillery Fungus

While artillery fungus helps the decaying process in mulch, it also produces fruiting structures that hold a spore mass6. This fungus shoots these spore masses high into the air which stick to any surface and resemble small tar spots on leaves of plants, on cars, or on the siding of homes6. Do not use mulches that contain wood chips or sawdust close to cars and homes to avoid damage from the fungus. Instead, use pure bark mulches, especially Pine or Atlantic White Cedar bark as they are the most artillery fungus resistant6.

Stinkhorn Fungi

The fruiting bodies or mushrooms of the Stinkhorn Fungi often appear in the fall and produce an unpleasant odor6. These fungi assist in the decomposition of mulch and are harmless to surrounding vegetation. If removal is desired, scoop up and dispose of the fungi as they appear.

Toadstools & Mushrooms

While harmless to surrounding vegetation, removal of toadstools and mushrooms in mulch is encouraged when small children and pets have access to the area as these fungi are known to be toxic.

Advantages of Mulching

It is important to mulch rather than cultivate around shallow-rooted plants, such as rhododendrons, azaleas, and camellias to prevent root disturbance. Mulch improves soil quality by making the soil more fertile, and resistant to disease and erosion while reducing the amount of required maintenance in gardens5.

Disadvantages of Mulching

Heavy mulching over a period of years results in a mat of mulch and soil covering the crown area of the plants and inhibits root development. Alternately, thin layers of mulch (under 2 inches in depth) do not improve growing conditions for surrounding plants.












Mulch is applied to improve soil, creating beautiful gardens. Learn more about proper mulching on the blog!

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