Originally published June 28, 2025 in the Yakima Herald-Republic. Minor edits have been made for web publication and accessibility.
by Debra Kroon
Mulch Matters, a Haiku
Brown, moisture holding,
Nurtures soil with rich decay.
Suppress weeds, Hooray!
Mulch does matter, especially here in the Yakima Valley where we are experiencing hot weather, drought conditions, and water strains and restrictions. Today we look at important reasons to add mulch to our garden areas, along with examining the various options to use, or not.
What is mulch? In short, it is a covering, organic or inorganic, that is placed over soil. It may be wood chips, bark, rock, straw, lawn clippings, cardboard, leaf debris, rubber or plastic sheets. The importance for the use of mulch is multi-fold:
- It helps with water retention. The more you can increase the soil’s ability to hang on to water, the less water you need to apply. In our semi-arid desert, this is critical.
- Mulch helps moderate the soil temperature. In essence it provides for cooling the soil in the hot summer and provides additional warmth in the cold winters.
- Soil structure is improved via the decomposition of the organic mulch. As it breakdowns it provides nutrients and builds the soil to increase water retention while also increasing capacity for drainage.
- If you have a site that struggles with sliding soil, such as a hillside or slope, mulch can help stabilize the underlying soil and keep it in place.
- Weed suppression. Who does not battle weeds? You water, you get weeds. Using a good layer of mulch, after removing weeds will help to suppress them for quite some time. Additional applications over time are needed as the mulch breaks down, and weed seeds are blown in.
- Aesthetically, mulch provides a nice bow on top of all your hard work. It enhances the beauty of the garden.
What type of mulch to use? It really all depends on the area you cover and your end goal. If it is for a vegetable garden, lawn clippings (that have not been sprayed with an herbicide), straw, leaf mold are all good options. In your vegetable gardens, stay away from wood chips or bark because it will make reusing the bed more difficult the following year, plus it will take a long time to break down.
Linda Chalker-Scott, PhD, WSU Horticulturist and one of the contributors to The Garden Professors Blog recommends arborist wood chips as a solid choice for landscape mulch. Chalker-Scott writes: “In areas where trees are a dominant feature of the landscape, arborist wood chips represent one of the best mulch choices for trees and shrubs. A 1990 study evaluated the landscape mulch potential of 15 organic materials, including grass clippings, leaves, composts, yard waste, bark, and wood chips. Wood chips were one of the best performers in terms of moisture retention, temperature moderation, weed control, and sustainability.” She also recommends a deep layer of mulch, greater than 4 inches. Her Fact Sheet on arborist wood chips may be found here:
There are many other options for mulch, which can make the decision difficult. Availability, price, ease of application, ease of management post application. For example, putting down rock as a mulch may look aesthetically pleasing, but weeds will still find a way, they always do. Controlling them after the rock is down may be difficult. Rubber mulches have been in the marketplace for a long time. How does rubber break down in the environment? It does not biodegrade. It is made from recycled tires. It will break down by microbes, very slowly. It could take years, and then it will not totally decompose. Then there is the worry about chemicals leaching into your soils. Chemicals like VOCs and PAHs.
Other methods of mulch include sheet mulching, which is applying layers of newspaper or cardboard over your soil, then applying chips, bark or rocks on top to hold it in place. This may be a good choice for an area that you do not plan to grow plants in. Once it is down, you do not want to disturb it by digging holes in it and adding plants. It would defeat the original purpose.
When you mulch, there can be downsides. You might disturb important native pollinators that are ground-nesting, such as bees. If the mulch is high in carbon, it can tie up the nitrogen in the soil as it decomposes. If you like to use naturalized gardening practices, which is letting the plant go to seed and then spreading the seeds naturally, mulches can inhibit those seeds from generating.
The best time to apply your mulch is in the spring, after the soil has begun to warm, but before the weeds make an appearance. If you put the mulch on too early, it may cause a delay in the warming of the soil. However, the best time to apply mulch is when you have the time. Keep in mind that summer and fall may not be as effective as a weed suppressant. It is important to prep the area first by removing the weeds before applying the mulch. When applying mulch around trees, avoid the volcano approach. Do not dump a tower of mulch around the base of the tree. This can lead to the bark staying wet and creating opportunities for disease to grow.