10 Benefits and Drawbacks of Gravel Mulch

Gravel gardens are an increasingly popular way to create waterwise sites, especially with xeric and prairie selections. They lend a clean aesthetic with seamless transitions between garden spaces, from seating areas and walkways to planting beds.

Using gravel as mulch has definite benefits, but a few disadvantages warrant consideration before installation. Its effectiveness varies by growing area and extent of use. A combination of organic mulched beds and gravel walks and borders may allow the best plant health while not compromising water conservation or the streamlined aesthetic.

Let’s dive into this ancient technique, used some 700 years ago by the Anasazi of the Southwest—especially fitting for dry climates.

Gravel Mulch and Gravel Gardens

Gravel gardens are replacing turf as xeric solutions to minimize the need for supplemental irrigation. Gravel may comprise the entire site, using rocks for planting beds and walkways or used for one or the other (just lining a bed or creating a path).

This becomes a useful technique in parks and public spaces where heavy traffic warrants a permeable surface while still allowing trees and other specimens to grow. We see this more in home gardens as we get creative with natural resources, water conservation, and lawn alternatives.

Whether organic or inorganic, mulch regulates soil temperatures, suppresses weeds, and retains moisture. Gravel mulch ranges from rocks, crushed stone, lava rock, pea gravel, decomposed granite, and fines. The size and material depend on your aesthetic, site layout, and what’s available locally.

Pieces should be less than half an inch to serve the roles of water conservation, filtration, and weed suppression. An inch or two atop soils, and three to four max, is sufficient for use as mulch around trees, shrubs, and perennials. Avoid using landscape fabric or plastic sheeting under the rocks.

Longevity

Gravel mulch is lasting. It doesn’t degrade or break down quickly like organic mulches (wood chips, bark nuggets, leaves, compost, straw). While it may move on site with heavy rain and runoff, it generally stays in place for long-term insulation and display qualities.

Using a lasting material is a sustainable solution and also a lower-maintenance one in terms of repeated applications. While labor-intensive at installation, it doesn’t need frequent reapplication. Occasional additions for leveling or refreshment will help, but gone are the days of seasonal mulching. The drawback is that it stays in place, making replanting or changing things around challenging.

Crushed stone or other materials may also help reduce erosion. With enough in place, soil washing and degradation may slow, especially with larger rocks or round cobbles.

Doesn’t Improve Soils

Just as gravel mulch doesn’t degrade easily, it also doesn’t serve to nourish soils. Organic mulches decompose to become part of living soils. The added biomatter contributes to feeding microbes that work beneath the soil to improve nutrient availability for roots. Supporting microbes means contributing to soil health.

With organic materials, we often ascribe a seasonal (or biannual, or annual) spruce-up as the existing layer breaks down. Adding a fresh layer benefits plants and continues the cycle of adding nutritive material to the soil structure. Eventually, mulches become soil. Add a layer of compost before you install the rocks to help nourish soils from the start. Preserving immediate planting areas with organic mulches helps supply continual nutrition.

As the stones themselves don’t contribute to soil nutrition, larger ones create crevices and opportunities for earthworms, arthropods, and fungi. Leave penetrable areas of the landscape, with or without rocks, for these assets that improve soil structure.

With crushed stone aggregate, particles that do erode and degrade may affect soil composition. Once you install rocks and fines, removing all of them is challenging when reconfiguring a site. Soil pH may increase or decrease depending on the material. Limestone, for example, may raise soil alkalinity.

Cost Efficient After Initial Investment

Upfront, gravel mulch costs more than traditional mulches. The material itself is more costly, added to the delivery and potential labor cost to spread it (or the value of your own labor). 

Due to its longevity, the cost upfront is counterbalanced by the reduced need for reapplication. You won’t need to make the same investment over and over again. And, if it’s subbing for a lawn, there won’t be the costs associated with regular maintenance.

Water Retention

Gravel mulch’s ability to boost water retention is both positive and negative, depending on your growing environment. When we see it in xeric landscapes, we associate it with improved drainage and the ability of water to filter through the site. It improves moisture retention to support roots. Gravel mulch does the job of conserving water by delivering it to the soil and reducing evaporation – an ideal solution for dry climates.

Crushed aggregates are a permeable alternative to paved walkways and large stepping stones. The downside is that too much retention can potentially lead to waterlogged conditions. This is most likely to occur in winter in cold climates (see below).

Increased Heat Transfer

Research shows that gravel mulch increases temperatures between the surface and the soil. The aggregate absorbs the sun’s warmth and locks it in with that of the surrounding soil mass. The heat transfer is greater than with organic mulches. In cool climates, this extends the growing season by warming roots for a longer period of time. Plants may emerge earlier in the season or with warming soils down to a foot depth.

The drawback of this increased heat transfer is too much warmth, both at the root level and radiating from the surface. In hot, southern climates, this can be problematic, especially for young vegetation. Larger trees and shrubs may perform better than smaller specimens, as they’re less sensitive to radiant heat and surface soil warming.

Another disadvantage of increased warmth is the potential for soils to become oversaturated in winter. Natural thaw cycles intensify. In winter, we use a mulch layer to insulate roots and crowns and regulate soil temperature, hopefully staving off frost heaving from freeze-thaw cycles. 

With gravel mulch and increased warmth in cold climates, the frost layer beneath the rock melts. It creates added moisture and soggy conditions for roots, potentially leading to fungal problems like root rot. Instead of evenly cold soils that benefit plants in dormancy, they become damp at a time when roots aren’t working to absorb moisture.

Weed Suppression

A great benefit of gravel mulch is that it leaves little space for weed seeds to germinate. It also creates a barrier for those already in the soil, making it a challenge for seedlings to penetrate. 

Weeds, though, seem to love a good challenge, and it is possible your stony landscape will experience them. They’re visible among rocks, making them easy to pick before they develop seeds. 

Reduces Ongoing Maintenance

Minimal application rounds and fewer weeds mean less labor overall. Compared to turf, there’s also a reduction in management resources (time, irrigation, chemicals, equipment).

As with any garden, the material still requires some upkeep. Picking up debris, leveling areas, or infilling worn areas are part of the routine. Decomposed granite also gets dirty when removing or installing plants, as soil spills or intermingles when digging or lifting. Correct it with a light rake, rinsing, touching up with more material, or letting it dry to drift or infiltrate on its own.

Harsh Establishment

One study looked at German landscapes modeled after North American spaces that incorporate prairie-style plantings and gravel mulch. It supports the many advantages of gravel mulch, including increased vitality due to warmer roots. It also found that young, small specimens had a higher mortality rate than larger ones. They determined that aggregate mulches can create a harsh environment for establishment due to the radiant heat, need for extra water in hot months, and possible water saturation issues in cold months. 

Overall, it’s best to start with larger plants when working with a gravel garden. Instead of seedlings or plugs, start with perennials and shrubs that are further along, with larger root systems.

Supports Design

Other than functionality, rocks alter the design and aesthetics of the site. They bring a clean look that links spaces with a sweep of the same (or similar) materials. From mid-century modern to contemporary to naturalistic, incorporating stone in beds, walkways—or both!—accents varying garden styles.

In addition to a crisp aesthetic, gravel mulches offer textural interest. From chunky and coarse to smooth and fine, they bring variation and contrast among leafy greens and blooms. Choosing the right composition is important for both the plants and the overall site’s workings.

Firewise

Rock groundcovers are a fire-wise approach to help slow the spread of fire. Since it’s inflammable, using gravel in walkways and around the home is a superior fire-safe option.

Many materials contribute to fire-safe gardening practices, including organic mulches and even turf. For organic mulches, research at the University of Arizona found compost and finely shredded bark to be the most resistant, though it may smolder and ignite over time. Turf kept low and free of tall weeds or blades was retardant. The most flammable mulch material was bark chips and nuggets. Rocks and stones are still the most non-combustible and well-suited for the zones near structures.