MSU Extension

Editor’s note: This article is from the archives of the MSU Crop Advisory Team Alerts. Check the label of any pesticide referenced to ensure your use is included. 

Every season it seems as if I get the same questions about mulch. What is the best kind to use? How much should I use? Which mulch will best out-compete weeds?

Certainly, as gardeners we have had our own experiences and developed opinions on the subject. With some recent research done at Michigan State University, it looks like all mulch products are not created equal.

According to Dr. Bert Cregg, MSU professor of horticulture, new varieties of mulch material available from local landscape supply stores are always increasing. He groups these into two broad categories; organic and inorganic. Both types of mulch provide some benefit to plants in the landscape. However, organic mulch such as ground tree bark, pine needles and recycled pallets have distinct advantages, such as directly contributing nutrients and organic matter to the soil. Cregg says that additional considerations in choosing mulch are cost and appearance.

During a three-year organic mulch study at Michigan State University, several things were evaluated: how effectively mulch conserves moisture and whether or not mulch has good weed suppression.

Organic mulches work two different ways to suppress weeds, Cregg noted. When mulch is placed in a landscape bed at approximately a three-inch depth, annual and some perennial weeds may be simply smothered. Lightly re-mulching on an annual basis prevents newly deposited seeds from germinating. However, as with anything organic, compounds may seep out that by their very nature retard weed growth. To weed scientists, this is called allelopathy. Many plants have compounds in their tissues that are allelopathic to surrounding plants. This is commonly observed with black walnut. The plant’s strong production of the compound juglone, kills dozens of types of plants. To a lesser degree, as mulch products decompose they do much of the same thing.

Organic mulch

Mulch available from most garden centers can be purchased in bulk or in bags. A popular type of bagged mulch is ground cypress bark. Cregg likes the warm, natural appearance of Cypress mulch, and it is more rot resistant because of its naturally occurring compounds known as phenolics. These compounds may also be responsible for the excellent weed suppression of this product. A side effect found in this study, was the growth of the shrubs was retarded, which Cregg noted, could be a beneficial thing.

Recycled pallets are also a popular source of mulch and readily available in a myriad of colors. In the study, this product showed good weed suppression and like the other organic mulches had good moisture conservation. Cregg is baffled by the public acceptance of the colored wood mulches. He said, “My personal belief is that in a few years we will look at this material like we do polyester leisure suits and say, what were we thinking?” The down side to the pallet material is that it readily moves off-site. Downpours, lawn mowers and foot traffic tend to kick it out of the bed and into a lawn.

My personal favorite is a by-product of the lumber industry, commonly sold as hardwood bark or shredded bark. This product performed well in the trial for weed suppression and moisture conservation. The down side of it may or may not be a disadvantage, because it tends to decompose more quickly. I find this a positive thing in our home landscape because plants are benefiting from the nutrients released from the mulch.

The best performer overall was ground red pine bark. In Michigan, ground red pine bark is readily available as a by-product of wood processing businesses. It has excellent weed retardation and if you think about it, how often do you see heavy weeds growing under pine trees? Allelopathy certainly plays a role in the properties of this product.

Although no studies were performed using a product known as pine straw, Cregg said that his experience with this product has been very positive. The pine straw (needles from the long-leaf pine or southern pine) is a uniform and natural looking product. (view image) It is a by-product of forest management practices done in southern states that help reduce wild fires. The easy-to-handle pine needles literally knit together and stay in the landscape bed. They have excellent weed suppression and become a good source of nutrients in the landscape. At the current time, however, only a few garden centers are carrying it.

Too much is too much!

Cregg says that mulch should be applied in a “doughnut” shape in the drip zone of trees and shrubs not a “volcano.” (view image) Too often we see big piles of the stuff dumped at the base of the tree trunk. This practice becomes a detriment to the tree by encouraging surface roots in the mulch as well as rot organisms at the trunk. I often think we need some local “mulch police” to go around crusading for tree health!

Five great reasons to use mulch

One of the most important practices we can implement for tree and shrub health in the landscape is to provide mulch. During the dry summer months mulch helps cool the soil in the root zone and conserve moisture. Mulch suppresses weeds, which will compete with plants for nutrients and water. Organic mulches such as shredded bark, compost and pine needle provides a naturally slow-release form of nitrogen for plants as it breaks down. Lastly, used in the right amount, mulch can provide a layer of insulation, which protects tree roots from early onset of freezing temperatures in winter and keeps them cool and happy in the summer. Dr. Bert Cregg of Michigan State University says that his favorite reason to use mulch is that it keeps the lawn mower and weed-wacker away from the trunk of the tree.