You might be thinking, “Mulch? Really? An entire tip about mulch??” Well, while it may not be the most exciting topic at the dinner table, mulch remains one of the most essential and foundational elements in both landscape architecture and home gardening. And mulching is not just about making a garden look tidy. When used properly, mulch dramatically improves plant health, reduces maintenance, and supports soil ecology. When used incorrectly, it can suffocate roots, harbor pests, and even kill plants outright.
At its most basic, mulch is a layer of material spread across the soil surface. It is often used around trees, shrubs, and perennials, serving functional, aesthetic, or dual purposes. The word likely stems from the German “molsch,” meaning soft or decaying. That origin is fitting, because mulch works best when it is quietly doing work: decaying, stabilizing, enriching, and protecting soil as it breaks down.
Why We Mulch
Professional designers and home gardeners alike rely on mulch because it offers a wide range of benefits. Research continues to validate what gardeners have observed for centuries. A well-applied mulch layer makes growing easier, particularly in hot, dry climates like Southern California.
Moisture Conservation
Mulch slows surface evaporation and keeps the root zone moist for longer periods. In a region where water is increasingly scarce, this is one of mulch’s most important functions. Several studies have shown that mulched soils lose up to 50 to 60 percent less water to evaporation than bare soils. That translates to less frequent irrigation and longer intervals between watering, which is especially helpful during the establishment phase of new plantings.
Weed Suppression
A consistent layer of mulch blocks sunlight from reaching the soil surface. This prevents many weed seeds from germinating in the first place and makes any weeds that do emerge easier to remove. A depth of two to three inches is typically sufficient to prevent most unwanted seedlings from becoming established.
Soil Temperature Regulation
Mulch functions as a natural insulator for soil. During summer, it shades the ground and keeps roots cooler. In winter, it reduces heat loss and helps protect roots from minor temperature fluctuations and frost. While frost is not a frequent concern in most of Los Angeles, mulch’s primary role in this climate is to reduce peak summer soil temperatures. This helps limit plant stress and prevents wilting.
Improved Soil Health
Organic mulches such as bark, compost, leaves, and wood chips gradually decompose and contribute organic matter to the soil. As they break down, they feed beneficial microbial life, improve nutrient cycling, and increase the soil’s capacity to retain moisture. Mulch also encourages the presence of earthworms, which naturally aerate and restructure soil over time. The result is a looser, more biologically active, and nutrient-rich soil profile.
Erosion Control and Surface Protection
In sloped areas or gardens with irrigation, mulch helps prevent surface erosion by absorbing the impact of raindrops and diffusing the energy of moving water. It also protects the soil from crusting or compaction caused by foot traffic, hoses, or dry cycles.
All of these benefits are particularly important when establishing new trees and shrubs. A well-mulched root zone stays cooler, retains more moisture, and resists weed growth, giving young plants a stronger chance of successful establishment.
Plant scientists often say that mulching is one of the best things you can do for your plants. But when applied incorrectly, it can just as easily become one of the worst.
Common Mulching Mistakes
Misusing mulch is surprisingly easy. Some of the most common errors, such as applying too much or using the wrong type, can cause serious harm to plants over time.
Crown Rot from Over-Mulching
A common and damaging mistake is piling mulch directly against the stems or trunks of plants. This practice is often called “volcano mulching.” It creates a ring of constantly damp material around the plant’s crown, reducing airflow and trapping moisture. Over time, this environment fosters fungal pathogens such as Phytophthora, which infect the inner bark and can kill the plant from the base upward. To avoid this, always keep mulch pulled several inches away from the crown or trunk. This ensures proper air circulation and helps prevent chronic rot conditions.
Hydrophobic Mulch Layers
Some types of mulch, especially finely shredded wood products, can become water-repellent if they dry out and form a crust on the surface. This issue tends to arise when mulch is applied too thickly or left undisturbed for long periods. Fungal mats may develop, and these can repel water, causing irrigation or rainfall to run off instead of soaking into the soil. As a result, even plants that appear well-mulched may suffer from drought stress if the soil below remains dry. Occasionally raking or fluffing the mulch helps restore porosity and allows water to penetrate again.
Oxygen Deprivation
Mulch that is applied too deeply, particularly in layers thicker than four inches, can limit oxygen exchange at the soil surface. In overly wet or compacted conditions, oxygen cannot reach the root zone effectively. This can lead to anaerobic soil conditions that stunt growth or kill fine roots, especially in heavy soils or poorly drained areas.
Pests and Pathogens
Mulch that is placed directly against plant stems can provide shelter for slugs, snails, and other pests that feed on young plants or carry disease. Thick mulch also promotes fungal growth when it is made from unaged wood chips or sawdust with high carbon and low nitrogen content. Using well-aged mulch materials can reduce these risks. This is why choosing the right type of mulch and sourcing it from a reliable supplier are both important steps.
Choosing the Right Mulch: Organic and Mineral
Mulches generally fall into two categories: organic, which comes from plant or animal sources, and mineral, which includes rock or stone materials. Each type offers specific advantages and is best suited for particular scenarios.
Organic Mulches
These include shredded bark, wood chips, straw, leaf litter, compost, and other plant-based materials. Organic mulches decompose over time, which improves soil health while also regulating moisture and temperature. For most general planting beds, vegetable gardens, or any areas where long-term soil improvement is a priority, organic mulch is the better option. It does require periodic replenishment, typically every 12 to 24 months, depending on how quickly it breaks down. Some types, such as straw or dry leaves, may also blow away or degrade more quickly in exposed or windy locations.
Two products we regularly specify on our projects are Agromin’s Organic Harvest Cover Mulch (formerly ES-2) and or LGM Soil’s Organic Shredded Cedar Bark. Both offer a consistent texture, strong moisture retention, and long-lasting performance in Los Angeles’ climate.
Mineral Mulches
Inorganic mulches, including gravel, lava rock, decomposed granite, and crushed stone, do not decompose and therefore last much longer with less maintenance. These are especially useful in arid landscapes, desert-style gardens, or areas planted with succulents and other drought-adapted species. Mineral mulches may also help reduce fire risk in specific conditions. However, they do not improve soil fertility, and they tend to raise soil temperatures. It’s also extremely labor-intensive to remove gravel mulch if you ever want to switch things out. When choosing an inorganic mulch, note that lighter-colored stones may reflect sunlight upward, potentially scorching foliage. Darker rocks retain more heat and can accelerate moisture loss in the soil. If you plan to use stone, it is best to select a local material in a moderate tone. The mineral mulch at Downtown Palm Springs Park is a good example of this material working well with the desert-adapted plant palette selected for that project.
California Mulch Requirements (MWELO)
In California, mulch is not just a best practice; it is a legal requirement. The Model Water Efficient Landscape Ordinance (MWELO) mandates that a minimum three-inch layer of mulch must be applied on all exposed soil in planting areas. This excludes turf, spreading groundcovers, or direct-seeded areas where mulch would interfere with germination. Up to five percent of a landscaped area can remain unmulched in order to support habitat for beneficial insects, but this must be explicitly called out in the planting plan.
This regulation is more than a compliance checklist. It reflects an increasing awareness of mulch’s importance in conserving water, reducing maintenance inputs, and supporting overall plant and soil health. A thoughtfully selected mulch helps projects meet water-use targets, enhances performance across climate zones, and contributes to a more sustainable outcome.
Mulch won't solve everything, but it remains one of the most effective, low-cost, and adaptable tools available to designers and gardeners. When applied with care, it improves growing conditions above and below ground. However, how it is used matters as much as whether it is used at all. Match the mulch type to the planting palette, climate conditions, and design intent. Avoid burying plants and keep the layer at a breathable depth. Consider mulch not simply as a cosmetic finishing touch, but as an integral part of the living system you are helping to shape.
Too mulch!