Steele: Things to ponder before deciding to use rock mulch

Rock mulch is a good choice in fire sensitive areas where wood chip and bark mulch are a fire hazard.

As water bills escalate, many people are considering converting lawn areas to xeriscape.

Often, the first thought is to cover the area with rocks, believing this will cut water use and be low/no maintenance.

Disadvantages:

Rock mulches reflect heat back onto plants intensifying water needs and heat stress.

Like paved surfaces, they give off absorbed heat, raising temperatures around buildings, people, and the city, creating increased air conditioning use/costs. Even with landscape fabric underneath, weeds grow in rocks. Seeds, soil and plant debris accumulate on top of the fabric.

Unlike compost mulches (e.g. manure and Glengrow) rocks do not feed soil micro-organisms and plants.

Solutions:

Choose plants such as lavender, yucca, cotoneaster, Okanagan sagebrush and rabbitbrush, blue fescue, Sedum ‘Autumn Joy’, Nepeta ‘Walker’s Low’, and Armeria ‘Dusseldorf Pride’ that thrive in hot, drought conditions in their native habitats.

Plant so that vegetation will cover most of the rock once plants have grown to mature size. This reduces weed growth and cuts down on reflected heat.

Choose plants that are evergreen, or with minimal leaf drop. This includes varieties of mugo pine, juniper, and Colorado blue spruce in addition to the above mentioned plants.

Where lots of weed seeds will blow in from adjacent areas or where there is major leaf drop from trees, don’t use rock mulch.

In my experience, the only rock mulch that is easy to weed out of is screened crusher chips. It acts like soil. The fine sand must be screened out or it will go hard like a compacted gravel path.

For texture add some large feature rocks.

Plants may be fertilized with manure tea made from compost, worm castings, etc.

Advantages:

Rock mulch is a good choice in fire sensitive areas where wood chip and bark mulch are a fire hazard. Compost-type mulches may also catch fire.

Okanagan soils and compost mulches can become hydrophobic (repel water) when dry. Water/rain sinks through crushed rock chips into the soil below.

Unlike wood waste or compost-type mulches, rocks don’t need to be replenished every one to three years as they do not decompose.

Recommendations:

Install drip irrigation or hand water until plants are established.

Before putting down any mulch material, be sure to remove all weeds, including roots, or they’ll grow through the mulch. Soak the soil well.

To dramatically reduce water use and weed growth, put mulch of some kind on all exposed garden soil.

If you use rock mulch, be sure to choose screened crusher chips and plants that thrive in extreme heat and drought conditions.

Planting in fall gives plants a longer time (than spring planting) to get established before the next summer’s drought.

Be sure to soak plant and planting holes very well and loosen roots before planting.

Avoid using landscape fabric, especially in areas where plants may be moved or added.

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