Gardening Guru: Organic or Rock Mulch?

By Nancy J. Hawkins, UCCE Master Gardener

Mulch can make a remarkable difference in the appearance and maintenance of your yard. Adding organic (derived from plant material) or rock mulch to your landscape is a quick and easy way to beautify your home landscaping. Mulches provide much more than beauty, though. They conserve water, prevent weed growth, and slow erosion. An organic mulch can also add nutrients to the soil and lower soil temperature in our hot, summer months. Mulch comes in many forms, from shredded bark and straw to decomposed granite and river rock. There are advantages and drawbacks to each type. Which one will work best in your landscape? Here is some information to help you decide.

Organic mulches include shredded bark, straw, and compost. Shredded, natural-colored bark is one of the most common and least expensive types of mulch. Trees used for shredding include cedar, pine, eucalyptus and oak. Cedar bark is the most available in local nurseries and garden stores. It is very aromatic, especially when fresh, and is a natural insect repellent. It is longer lasting than other organic mulches, thus you would not have to replace it as often. You will often find bark mulches in different sizes, from large nuggets to finely shredded. Large nuggets are great for covering large areas and will last the longest. Walk-on or finely shredded bark is used for pathways and other areas where there is foot traffic. It will be more expensive and decomposes faster than larger sized bark pieces.

Colored mulch is an alternate popular choice. It is composed of wood chips or shredded wood that has been dyed. The most common color choices are red or black. If you are particular about the appearance of your landscaping, colored mulch is the way to go. It carries the same benefits, weed control and water retention as natural colored bark, but the cost will be higher.

Pine or other wood shavings are basically the cast-off materials of milling processes. This type of mulch is not as attractive as cedar or shredded bark but could be less expensive. You might find pine shavings at a lumber yard or wood supplier.

Straw with its beautiful golden color is slower to break down than grass clippings or leaves. It comes in smaller shredded pieces or longer straws and can be found in bales at pet food supply stores. Straw is classically used in more utilitarian gardens, such as vegetable gardens or strawberry patches. Make sure straw is weed free, otherwise you may end up with more weeds than you are preventing.

Compost is decomposed organic matter and looks like soil. It breaks down quickly but adds to your soil structure the fastest. It is inexpensive and you can create your own rich compost for free. Compost is usually incorporated into the soil rather than layered on top. If you like the rich, dark color and have compost readily available, it can be the best mulch choice for you.

Organic mulches do come with some warnings. While a concentrated layer of mulch is ideal, don’t overdo it. In our hot, dry valley summers, a mulch layer of three to four inches is adequate. Always keep mulch six to twelve inches away from tree and shrub trunks to prevent root and crown rot. Wood boring and garbage eating pests, such as termites and cockroaches, find wood-based mulches a favorite feeding ground. Make sure you purchase mulch that has been properly sterilized and stored.

Bark mulch has to be replaced every couple of years as it decomposes. The original vibrant color will fade with exposure to sun. It can wash or blow away. Raking up fallen leaves or other plant clippings in bark mulch can be difficult.

Rock mulch is decorative gravel or rock ranging from pea-sized to tennis ball size. Because rocks are inorganic, they do not break down in the landscape, so will not need replacing. Unlike organic mulches, they do not add nutrients to the soil. Rock mulch is available in a wide variety of colors, shapes, sizes, and textures.

Lava rock comes from dried, hardened volcanic lava. It is porous, light weight and can be black, brown, or red. Pea gravel consists of tumbled rock fragments coming in a variety of small sizes and colors. Decomposed granite is very small, chipped rock pieces with rough edges, thus will usually stay where placed. It works well in making pathways through your garden. River rock mulch is composed of round, smooth stones in a mixture of colors. It is useful in making dry stream beds or shallow rain gardens. When using rocks or stones as mulch, cover the soil first with a sheer landscaping fabric to prevent weed growth.

Rock mulch may not be the best material to use in areas where there is a lot of leaf drop or plant debris that will accumulate on top of the rock layer. Like bark mulch, it is difficult to rake over. Rocks are heavier than bark and will be more difficult to install or remove.

A word of caution: in our area of hot summers, rocks and stones can get very hot. Light colored rocks will reflect light and radiate summer heat. Be aware of the plants, shrubs, people and pets that will be in contact with rock mulch cover.

So, which type of landscaping mulch is better to use? My answer to that question must be, “It depends.” Some factors you need to consider are your overall landscape design, cost, labor requirements, and types of plants, shrubs and trees in your yard. Hopefully, after reading this article, you will be able to make an informed decision…. the choice is yours!

The UCCE Master Gardeners will be available to answer your gardening questions in Visalia at the Farmer’s Market, 8 to 11 a.m., every Saturday in the Sears parking lot at Sequoia Mall.

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