This class of mulches includes any material that was never alive. It includes rocks, gravels, plastic, stepping stones, bricks, pavers, crushed concrete, etc. In general these mulches are best used in special circumstances such as decorations, pathways or erosion control. 

Inorganic mulches are falling out of favor with experienced landscapers and horticulturists due to the problems they create and the new research that has demonstrated the benefits of organic and living mulches (will be discussed in future articles). However, they have their uses in special circumstances and will perform well in special situations or certain very dry areas of the country (West Texas, New Mexico, Arizona, etc.) or in special situations like rock gardens. 

In previous issues we talked about studying nature and observing what a plant likes in its natural habitat. If we go out West and look at our desert species, they all have a gravelly or rocky mulch around them as the wind blows away the smaller soil particles (silt and fine sand) leaving the larger material on top of the soil as a mulch. 

If one has a cactus or desert plant garden then the use of an inorganic mulch is appropriate and works best. Desert plants need the extra heat exposure and reflected sunlight created by rock or gravel mulches. Most desert plants grow on very poor soils, low microbial activity, and very dry conditions. Organic mulches increase fertility, moisture and microbial activity hence are not a good choice for these species of plants. 

When using gravel or rocky mulches special care must be given to the soil under them. Rocks and gravel are much denser (heavier) than most of our natural or improved soils, hence the rocks sink into the soil over time which ruins the look of the bed. If the soil is too soft it will allow weed seeds to germinate and grow between the rocks. To prevent this a special desert/cactus soil needs to be used that can structurally support the weight of the mulch, provide the required drainage and aeration that desert plants need, and by default serve the purpose of being a poor growing medium for most common weeds. 

These type of mulches may be igneous rocks (flint or chert) like the brown gravel, river or septic rock or bull rock, rainbow rock (quartz based), crushed limestone, or even lava rock (pumice).