June 24 2020 06:00 AM

Mulch builds a solid growing environment to support featured plants.

Photo: University of Georgia

There’s no question that mulch increases the aesthetics of any landscape by adding contrast between plant foliage and flowers, outlining beds, providing structure and giving the visual of a well-maintained landscape. But when used correctly, the benefits of mulch go far beyond what the eye can see.

“You can get a 50% reduction in weeds by mulching properly,” says Barbara Fair, PhD, certified arborist and landscape extension specialist at North Carolina State University.

Clients might appreciate the healthier and more attractive landscape, but your crew will be grateful for fewer weeds, meaning less time dealing with unwanted growth and less need to treat weeds using chemicals, explains Bodie Pennisi, PhD, professor of horticulture and statewide extension landscape specialist at the University of Georgia’s Griffin Campus.

To properly lay mulch in a landscape, no more than 3 inches of bark mulch should be used around woody plants, trees and shrubs, Fair says. She also recommends that no more than 1 inch of mulch be used around herbaceous plants. For many clients, that rule of thumb contradicts the vision of a perfectly manicured landscape.

“Many times, we use mulch incorrectly,” Fair says. “It is often put down too thick at initial application. Then, people don’t like how mulch looks after a season because it can get crusty and lose color, so instead of raking it and fluffing up the existing mulch, they’ll add more mulch. You end up with many inches of mulch around plants, which affects the health of the plants.”

It might seem like a minor issue, but taking the time to explain why mulch is maintained at a particular depth might get ahead of some client concerns.

Using mulch right

Landscape professionals who are asked to continuously add fresh mulch season after season have a couple good options for refreshing a landscape’s visual appearance while maintaining the best growing environment, Fair says. Old mulch can be raked out and removed from the bed before adding fresh mulch, or a small amount of fresh mulch can be added to the top.

Three inches of mulch is enough to cut off the light and moisture that many weed species need to germinate, while still supporting the health of plant material. While additional mulch does inhibit weed growth, it also prohibits movement of water and oxygen in and out of the soil.

Photo: Purdue

“One big thing is that clients love mulch piled around the tree,” says Kyle Daniel, a nursery and landscape specialist at Purdue University’s Department of Horticulture and Landscape Architecture. “That’s what we call volcano mulch and it is detrimental to plants.”

According to Daniel, mulch should begin where you find the first roots or where the roots start to flare. When mulch is placed up the trunk of a tree, it can lead to root rot and girdling roots, which choke off the flow of water and nutrients between the roots and branches.

“Bad things happen when you have the mulch piled up around the plant,” he says.

Fair adds that mulching up to the crown of the plant will kill herbaceous plants “in no time, because drainage is the biggest issue.”

As landscape professionals discuss mulch with clients, Daniel says the first choice is whether to use organic or inorganic mulch. For optimal plant health, he strongly recommends the use of an organic mulch — mulch that comes from a plant source — like shredded hardwood bark, pine bark nuggets or pine straw. The type of organic mulch that is widely available varies based on geography: Pine bark and pine straw is plentiful in the southern regions of the U.S. while the northern regions tend to have more access to hardwood mulch.

Daniel says that larger pieces of individual mulch tend to control weeds a bit better than a triple shredded bark, but he acknowledges that aesthetically, most clients prefer a shredded mulch to large pieces like pine nuggets. An application of preemergent herbicide prior to mulching can add an extra layer of protection from weed growth, as well as a potential upsell.

Colored mulches, while a matter of personal taste, are also safe and effective choices, Fair says. The dyes used to color the shredded wood are not toxic or harmful to plants.

Photo: University of Georgia

“When you add any organic matter to the landscape, it will break down as the season goes on and provide a richer soil for the plants,” Daniel explains. “The extra organic matter increases cation exchange capacity, which in turn provides for more nutrient exchange for the plants.”

While the specific type of organic mulch makes little difference in terms of weed control and plant health, it makes a big difference in other ways, says Joe Neal, PhD, professor of weed science at North Carolina State University. Landscape professionals should take a look at where the mulch is going and take note of potential challenges or limitations of that area. For example, he says that pine nuggets are pretty, but they float and can wash away, while pine straw is a fire hazard and should not be used around the foundation of structures with flammable exterior materials. In fact, he says that some municipalities have ordinances prohibiting the use of pine straw around commercial properties.

One of the best mulches, Fair says, is a mulch that is sometimes available from various cities or municipalities that make their own mulch of leaf compost and tree bark.

“The woody part will last longer, and the leaf part will break down and add organic matter to the soil,” Fair says. “It is one of the best mulches you can use for what it does for the soil and for weed prevention.”

Know your options

The other category of mulch is inorganic mulch, which includes rock, gravel and even pelletized rubber, a newer option that is often used on playgrounds and soccer fields. While inorganic mulch has great applications, Daniel says there are some limitations. Pelletized rubber, for example, can become very hot, which in turn makes plants very hot. Daniel and Fair agree that they prefer the use of organic mulch around plants to best support plant health.

But, as is often the case, there are exceptions to every rule. Out west, where plants are used to growing and thriving in high temperatures, low humidity and low soil moisture, plants native to the desert area will do just fine in an inorganic mulch like lava rock, Daniel says.

“A lot of homeowners like to have rock or gravel because you don’t have to remulch every year, it is low maintenance and basically permanent, and in some situations that’s great,” Daniel says. “Those materials are great to use in a place without plants.”

Photo: University of Georgia

In certain situations, gravel or rock is a very practical product, Pennisi says. While it is more costly, gravel or rock can be washed and refreshed, and it is heavy, which means it will stay in place longer. Therefore, hard to reach spots, high traffic areas and slopes where erosion is a concern are ideal places to use inorganic mulch. When working with a client, look at the living applications of the landscaped area and choose the type of mulch that best fits the job.

The proper use of mulch also provides a layer of insulation, a layer between the air and the soil. This insulation helps conserve moisture while keeping the temperature from fluctuating wildly.

“When you keep a uniform soil temperature, plant roots can withstand stress better,” Pennisi says. “For us in the South, it is especially important to reduce the fluctuation of soil temperatures.”

The layer of insulation that mulch provides is useful in both the warm and cold weather. Daniel explains that in the summer, mulch keeps the root zone cooler, and in the winter it provides a buffer from the harsh cold.

While the options for organic and inorganic mulch have been fairly standard over the years, Pennisi says she has noticed that green mulch is currently a hip trend in some parts of the country. Green mulch, also called living mulch, is any short, spreading perennial ground cover that is placed between larger garden plants. Pennisi says that green mulch is a great choice on a sloped area that might be susceptible to erosion. Much like hardwood mulch, green mulch can also be used to suppress weeds, nourish the soil between plants, moderate soil temperature and maintain soil moisture. It has the added benefit of providing a hospitable environment for birds, butterflies, bees and other pollinators.

According to Daniel, the overall trend he sees is that while inorganic mulch is often favored for hardscapes like outdoor living and kitchen areas, organic mulch is the preferred choice for landscapes.

“Most folks are getting back to organic mulch because of the benefits to plant health that people have realized,” Daniel says.

Lauren Sable Freiman is a freelance writer based in Cleveland and can be reached at laurensable@gmail.com.

Compost complications

As landscape professionals discuss mulch with clients, the topic of compost might come up. Barbara Fair, PhD, certified arborist and landscape extension specialist at North Carolina State University, says it is important to make a distinction between mulch and compost. Though they are both organic matter, but they have different applications and different purposes. Make sure that clients know the difference when talking about mulch options.

“Compost is something that has been decomposed for a long time, and it is not typically made from bark,” Fair says. “A lot of folks recommend a quarter inch of compost over their entire lawn to help improve the growth of the lawn. I’ve seen people use compost material in their herbaceous beds, but it will break down quickly. They’re doing that to improve the soil instead of for weed control.”