Inorganic Mulch

Inorganic Mulch

Inorganic mulches provide many benefits to the garden and landscape. Inorganic mulches are crushed gravel and granite, river rock or small stones, lava or granite rock, decorative and colored stones, sand, crushed brick, crushed graded recycled glass, landscape fabric sometimes referred to as geo-textiles, plastic mulch, and aluminized mulch. 

Benefits of Inorganic Mulch

Inorganic mulches provide some of the same benefits, as do organic mulches.

• Conserves moisture by slowing evaporation.
• Retains moisture in the root zone.
• Controls weeds by preventing and inhibiting germination.
• Protects and increases the root zone of the plant. With the mulch's protection and temperature moderation, roots of plants come up to the surface to the mulch line, where the soil is most fertile. The first few inches of un-mulched soil become too hot and crusted for roots to survive.
• Maintains even soil temperature, cooler in the summer, warmer in the winter and a more even temperature from dawn to dusk. For a quicker warm-up for spring planted vegetable beds, pull back the mulch to let the soil warm by the sun. On the other hand, keeping a thick mulch cover on fruit trees and tender perennials will delay their break from winter dormancy, helping to protect them from damage caused by our climate's erratic temperature swings.
• Prevents soil crusting and increases water in-soak and aeration.
• Helps stops soil erosion. Helps prevent rainwater runoff.
• Prevents heavy rain from splashing soil on the lower leaves of plants, keeps the pores open.
• Helps prevent compaction on walkways throughout the garden and beds.
• Minimizes cultivation. The less disturbance of beneficial microorganisms the better. Microorganisms secrete a sticky substance that glues the soil into little crumbs, promoting better soil structure.
• Saves you time and money by reducing water, pesticide and herbicide use and plant replacement.
• Landscapes and beds with mulch look better and help create a sense of place when using mulch that is similar in appearance to our surroundings.

(The Gardenville Method, Lessons in Nature, Malcolm Beck, 1991)

Use of Inorganic Mulch

Similar to organic mulch, a 2-3 inch layer of inorganic mulch is usually sufficient.

If you are anticipating reseeding or volunteer seedlings, a rock, stone or gravel type much is better. Organic mulches tend to be a better weed barrier than inorganic mulch. Seeds find it easier to slip to the soil level through river rock, stone, gravel, and the crushed products easier than through wood chips.

With gravel, stone and the crushed mulches, irrigation and rainfall penetrates easier to the soil then it does with a wood chip or wood bark mulch. In well-amended medium water-use beds without an under mulch drip system, inorganic mulch may be preferred.

Crushed gravel and small stone mulch is sometimes preferred for xeric beds or strips or rock gardens. The crushed gravel or stones will not cause root rot in plants requiring dry conditions -- they prefer it lean and mean. Crushed granite and gravel emphasize the southwest type of landscape, helping to create the sense of place for our area.

Inorganic mulch should be used to mulch containers of drought tolerant plants. One inch to 1 1/2 inch layer is sufficient cover. Cost is usually not prohibitive when applying a more decorative and expensive mulch to such a small area. Matching or contrasting the color or texture of the plant to the mulch and container can display a fair amount of creativity.

Inorganic mulches still need to be replenished every few years, as they tend to work down into the soil.

For aesthetic purposes, it is best not to combine different mulch types in the same bed.

Plastic Mulches

My main objections to using plastic mulches in the home landscape are because they do not allow for a normal exchange of air and water and because they lack the natural look in the landscape I desire. In the home landscape, the primary use for plastic mulch is to prevent weeds. Any hole in plastic mulch invites weeds to grow through it.

But there are many uses for plastic mulch since their introduction in the 1960’s. If using landscape fabric or geotextile mulch cover in beds or borders for weed prevention, use a type that allows for water and air permeability.

Some people use plastic and aluminized mulches for vegetable gardens where it can easily be laid between the rows. The use of plastic mulches in horticulture and agriculture is called plasticulture. Plastic mulches provide many benefits, including water conservation, better pest and weed management, increased yields and earlier maturing crops. The use of plastic mulches is usually coupled with drip irrigation for maximum benefits. Because of all these benefits, plastic mulch is a viable alternative to pesticides and herbicides, especially for ecologically friendly home vegetable production.

Recent studies by the USDA Agricultural Research Service have proven that using ”red selective reflective mulch" increases the fruit yield of strawberries and tomatoes up to 20% compared to black plastic mulch. Red plastic mulch reflects a spectrum of light back to the plants that triggers the release of phytochrome, a protein that stimulates rapid growth and development. Lee Valley Tools, Ltd. is one supplier of the red plastic mulch (www.LeeValley.com or 1-800-871-8158).

The Center for Plasticulture at Penn State University studies many applications for plastic mulch of various thickness, components, color and crops. For much more information on plasticulture, go to http://plasticulture.cas.psu.edu/P-Mulch.html.

Clear plastic inordinately heats up the ground. But this is not always a negative factor. One useful purpose for clear plastic is in solarization – the process of killing soil pathogens and weed seeds through high temperatures generated by sunlight. Soil microbes also die during this process to a depth of 6 – 8 inches, as well as the pathogens and weed seeds. Solarization is normally effective for 2 – 3 years and is an excellent method of ridding the soil of abundant weed seeds and soil pathogens. Clear plastic mulch is also used in more northern climes to heat up the soil earlier in the season.

Crushed Glass

Crushed, graded, tumbled recycled glass can be used as mulch (for more information go to www.cwc.org/gl_bp/gbp4-0601.htm) or for other creative landscape design purposes. This is not the same as breaking panes of glass and using it for mulch. The glass should be washed and tumbled. Different types of crushed tumbled glass create different looks.

Crushed glass can be purchased in a range of colors. It is available in different grades. Crushed glass dust finer than 100 mesh can become airborne and is not advised. This fine grade of crushed glass dust can also inhibit the flow of rain or water in a landscape bed. Crushed, tumbled glass from 1/8 “ to ¼” is more reflective and ideal for mimicking the shimmering effect of a stream.

There are a number of suppliers of crushed glass (two sites to consider are www.trivitro.com and www.americanspecialityglass.com). I have not seen crushed glass sold locally, but I have seen a bed at the Lady Bird Johnson Wildflower Center mulched with it. If you need aesthetically unusual mulch for color or design, consider crushed, graded, tumbled glass.

Recycled Rubber Products

I’m including this paragraph with inorganic mulch as well as organic mulch because of its importance. Even though certain substances advertised as “recycled material” might appear good and beneficial to use in the landscape, they should be approached with caution and research. Just because a product is called “organic” doesn’t mean it should always be used or trusted as safe for our environment.

Recycled rubber products are such products to avoid as mulch. Although rubber mulch is hailed as an effective weed barrier and safe alternative to use that doesn’t need replenishment, research has proved otherwise. A study by Linda Chalker-Scott, Ph. D., Extension Horticulturist and Associate Professor at Puyallup Research and Extension Center, Washington State University, (Horticultural Myths about Rubber Mulch) cites that

• “Rubber mulch is not as effective as other organic mulch choices in controlling weeds
• Rubber mulch is highly flammable and difficult to extinguish once it is burning
• Rubber mulch is not permanent; like other organic substances it decomposes
• Rubber mulch is not non toxic; it contains a number of metal and organic contaminants with known environmental and/or human health effects.”

Rubber mulch leaches, in particular and in addition to other metals, high levels of zinc that kill plants and smells bad, actually “stinks” in hot weather. Rubber mulch is sometimes included in bagged compost products to bulk up the bag, without disclosing on the bag cover its inclusion. Buyer Beware! (www.paghat.com/rubbermulch.html)

Angie Hanna, July, 2006