Have you ever spread your blanket or towel on a pebble beach and lain on those stones for a couple hours? The warm sun and the sound of waves crashing on the shore might have been appealing, but lying on rocks you probably felt like you were on fire. Your feet burned. So did your legs. Your back. Those pebbles reflected the sun and made walking or lying down on them sheer torture. Well, don’t think your flowers and shrubs won’t also feel the heat if you use stones or river pebbles as mulch.
It may be tempting to use gravel, pebbles or volcanic rock as mulch. These hard mulches, sometimes referred to as inorganic mulches, do have some benefits. They hold water in the ground well. Also, unlike wood mulch, gravel or pebbles will pretty much last indefinitely, making it a cheaper long-term mulching solution than other types of mulch which have to be replaced every two or three years. Heavy rains won’t generally wash pebbles away as often happens with bark mulch. Another advantage of using stone mulch next to a house or shed, termites can make themselves at home in a wood mulch. I have yet to see termites munching on stones. Finally, stone mulches are both effective and attractive in a xeriscape, one with cacti, grasses and other plants that can take extreme heat.
That being said, I am not a fan of inorganic mulches because of the damage they cause plants, especially during those brutally hot summer days. When the air temperature hits 90-degrees, the reflective heat coming off the gravel or pebbles can easily reach 110, 120 degrees, even higher. I learned that lesson firsthand, some 30 years ago, when I used river pebbles as a mulch in a small garden in front of my living room. A couple yuccas thrived. Two junipers burned at the base. Clearly, the heat radiating from the pebbles scorched the needles. Also, one of the primary benefits of mulch is that it improves biological activity and mixes organic materials into soils. Stone mulch won’t accomplish that.