Rock as a Mulch Material
Rock is widely used for aesthetic purposes in the landscape. The color variations and textures of rocks offer many options in finishing touches. Rock also has many benefits as a mulch. There are pros and cons to using rock mulch instead of organic mulch.
Combined with a landscape fabric, rock suppresses weeds by smothering their growth and blocking light on the surface to keep weed seeds from germinating.
A layer of fabric and mulch over bare dirt will stop the splattering of the dirt onto walls and walks, as well as the splattering of soil borne diseases onto lower foliage.
Rock will deflect the compacting effect of rain on certain soils.
Rock does not decompose like organic mulches do, so there is no benefit of improving the soil, but it also means the need to replenish frequently is small.
The need for landscape fabric underneath is certain, otherwise the rock will sink, especially on sandy soil, and the result will be a mix of rock and soil on the surface – a prime spot for weed seed to germinate.
Temperature can be a problem with rock on hot, sunny days. It can get very warm and raise the soil temperature as well as the air temperature in the immediate vicinity. You’ll have to make allowance for that in the choice of plant material for that area. Some designers get around that by using rock in areas that don’t have plant material close and organic mulch in the planting beds.
Cleanliness of leaf and other plant debris is more critical in rock because of the contrasting appearances and if not cleaned up, the risk of weed seed germination in the decaying plant material is possible. With larger rock it is easier to blow fallen leaves out of the area.
Rock can be a better choice for areas near the house because it doesn’t attract insects as easily as organic mulches.
Future replanting of areas with rock and landscape fabric can be a little more awkward than organic mulch without fabric. If a piece of organic mulch makes its way into the planting hole, it will decompose.