If all you’ve heard about mulch is bad puns like “mulch ado about nothing,” you’ve been missing out on one of the most important parts of gardening. People who know how to use mulch effectively spend less money watering and have more productive and attractive vegetable and ornamental gardens with healthier soil.
This is a case that Bodie Pennisi, a professor in the Department of Horticulture at the University of Georgia’s Griffin campus, is happy to make any chance she gets. “I am a huge advocate of mulch because of what it saves and how good it is for the health of your garden and your plants,” Pennisi said, citing three benefits of mulch. “Mulch conserves moisture, meaning it helps you reduce the amount of water you apply, which is a huge environmental benefit; it keeps weeds down so that you don’t have to apply herbicides; and it improves soil health.”
In making her case that people should maintain a minimum 3-inch layer of mulch, Pennisi explains that one reason it is so beneficial is that mulching replicates what occurs in nature. “Think about what happens in the forest. Leaves fall and we call it leaf litter. But it’s really mulch. The leaves keep decomposing and become part of the topsoil layer by breaking down into compounds that are used by plants and micro-organisms. So, it’s a very, very important part of the life cycle of plants, and we should not underestimate the important role mulch plays in our home landscapes.”
To help home gardeners understand how to use mulch, Pennisi described various kinds of mulches, both organic and inorganic.
What is mulch?
The first thing to know about mulch is to understand what it is. Pennisi defines mulch this way: “Mulch is any non-living layer that accumulates on or is applied to the surface of the soil that is not growing plants.” That layer, added Pennisi, can be organic (natural materials that will degrade or decompose and eventually disappear into the soil) or inorganic (materials that either don’t degrade or decompose, or will do so very slowly).
One of the great things about mulch is that you can grow your own organic mulch for free. It comes in the form of needles from pine trees, or leaves or stems from hardwoods and shrubs, or from grass clippings. Instead of bagging landscape “debris,” just recycle it onto vegetable or ornamental beds. Other organic mulch, such as pine or hardwood bark, is available at garden centers. Examples of inorganic mulch include gravel, stone, lava rock, plastic sheets or pieces of rubber.
If you have a small lot and need to supplement with mulch from garden centers, the type of mulch for sale will depend on where you live, said Pennisi. The South, for example, has a strong lumber industry because there are abundant pine trees. Throughout the region, pine straw and pine bark are readily available, popular and inexpensive mulches.
Here’s a look at some choices for organic mulch.
Organic mulches
Pine bark
Commercial pine bark sold as mulch is available in two sizes: small and large nuggets. Which you use is largely a personal choice often made to match the size of your landscape and the type of plants in the landscape — smaller nuggets for small plants in small landscapes — or simply a preference for the visual appeal of one size vs. the other. A functional difference is that the smaller nuggets tend to form smaller air pockets between the individual pieces than do the spaces that will occur between the larger nuggets. If you’re having trouble visualizing that, Pennisi said to think of large gravel versus small gravel and the difference in the spaces between those individual pieces of gravel. An advantage of the smaller nuggets is that they are more efficient than larger nuggets in trapping air and insulating roots from freezing temperatures. The smaller air pockets present fewer avenues for soil warmth to escape into cold winter air than the larger spaces between the bigger nuggets. On the negative side, rain can move small nuggets out of your landscape faster than it can move the large nuggets. And smaller nuggets will decompose faster than larger nuggets. As with all mulches, don’t place bark mulch next to plant stems or tree trunks. Leave at least an inch of space.
Pine straw
These are the needle-like leaves of pine trees that have been collected after they have turned brown and fallen to the ground. Pine straw is a popular mulch in the South, a region that was once a giant pine forest stretching from Virginia into East Texas. Pine straw-filled trucks next to the landscape sections of box stores are a familiar sight throughout the region. Pine straw has advantages of being a readily available, inexpensive and efficient mulch, but it also has some negatives. It tends to break down very quickly and wind and rain can move it out of place, especially on slopes, said Pennisi. However, she quickly added, that doesn’t mean it’s not a good mulch.
Hardwood mulch
This is a little more expensive than pine bark, but it lasts a little longer, unless you are in a hot and humid climate that gets a lot of rain. In that case, pine and hardwood mulch will decompose at about the same rate, Pennisi said. Fungi that break down mulch into organic, soil-like matter are triggered by temperature and moisture. Accordingly, these fungi are most active in hot and humid regions such as the South and Southeast.
Leaves
Leaves are a wonderful natural mulch. They are most effective when ground or chopped because smaller pieces will lay flatter and stay in place a little bit better than whole leaves, Pennisi said. You can use various size shears or clippers to chop up leaves, though the fastest and easiest way may be to run over them with a lawnmower.
Grass clippings
Leaving some grass clippings after mowing serves as a mulch for lawns. Collecting some of those clippings and spreading them in your ornamental beds or vegetable gardens is another environmentally sound way to take advantage of a free source of mulch. Grass clippings, for instance, are especially functional as pathways between rows of vegetables, helping to hold moisture in the ground, returning nutrients to the soil and helping to keep shoes and boots free of mud as you work among tomatoes, beans, cucumbers and other edibles. They serve similar purposes when spread in ornamental beds. Be aware that grass clippings can form mats. You may want to avoid using them depending on whether you treat your lawn organically or with chemical fertilizers, pesticides and herbicides.
Wood chips
If you have access to wood chips, either because you have a wood chipper or can acquire chips from a tree service company, you have access to one of the most natural-looking and long-lasting mulches. Be aware, though, that as wood chips decompose they will take nitrogen from the soil. Monitor your plants. You may need to adjust your fertilizer program to compensate. Depending on your landscape design and “the look” you want, wood chips also can be used to create a very natural-looking path, especially in woodland gardens.
Straw
Often called wheat straw, the stalk of grain plants is more useful as a mulch for vegetable than ornamental gardens. Other than using it to help retain moisture on freshly seeded fescue lawns in the fall, most people tend not to want “the look” of yellow straw in ornamental beds visible from the street. Fescue seedlings will quickly cover the straw as it decomposes and as the seedlings sprout and grow. Till the straw into vegetable gardens at the end of the season or the start of the next one.
Compost
If you’re lucky enough to have space for a compost bin, you likely already know that homemade compost is one of the best mulches you can apply. An alternative to making your own compost is to ask your local municipality if they make — and sell or, better yet, give away! — compost from yard trimmings that sanitation crews collect. In some places, they might even deliver it to you! This compost may not be as attractive as what you can buy or make yourself because it’s composed of different pieces of yard waste. But, if you are more practical than particular, municipal compost might be a great option for an outstanding mulch.
What about color?
Some people might wonder if colored mulch sold at garden centers could present a problem because of ingredients used in the coloring process. Pennisi doesn’t think so. “There’s no reason someone should be concerned about the color of a mulch other than their own individual color preference,” she said. Some people want a mulch with a natural color that blends into the garden and others want a mulch that will be noticed.
“The mulch you take out of the bag is moist because moisture was trapped in the mulch when the mulch was bagged,” Pennisi said. “So, the color of any mulch will always be more vibrant when you first open the bag. Once you put it in your garden, the moisture will evaporate, and within a day or two the mulch color will become a lot less vibrant. You pretty quickly will begin to see the color of the mulch fade out because the sun’s UV rays break down the chemicals in the mulch and cause the color to bleach. Think about what happens to your plastic lawn furniture, the color fades with time.”
Organic mulch contains natural chemicals from carbon polymers and inorganic mulch has chemicals that have been artificially added from a mineral source, Pennisi explained. In either case, she added, chemicals are going to end up in the soil and all of them eventually will be broken down into different components.
Inorganic mulch
A number of inorganic mulches can be used in various situations.
Plastic sheets
These are popular in commercial agriculture. Tomato farmers, for example, might lay plastic sheets over their fields, cut holes in them and plant their crops through the holes. You can also use plastic sheets in home vegetable gardens in the same way. They are effective in keeping moisture from evaporating from the soil and in providing a barrier that prevents most weed growth. Some think a downside might be that soil plastic sheets have a negative effect on the health of the soil because they greatly reduce air and moisture to the soil, suffocating beneficial soil microbes as a result. Some gardeners use them in their ornamental beds to prevent weed growth and “hide” the plastic by putting pine straw or other mulches on top of it. The same concerns about the effects on soil health pertain to plastic sheets used in this manner. In addition, there is always the chance that weeds will root on top of the plastic and grow up through the mulch. Homeowners who choose this type of mulch will have to monitor their gardens for this possibility.
Stone, gravel and lava rocks
All of these can be used as a mulch to prevent weed growth and to retain moisture. The decision to use these is a personal choice depending on “the look” the gardener is trying to achieve. Be aware that weeds may find their way to the soil through the gaps in the materials, and you may have to pull the weeds or treat the mulched areas with an herbicide to prevent weed growth.
Rubber mulch
Rubber mulches tend to be used more on playgrounds than in home landscapes. Their primary purpose seems to be to provide a safety cushion for running and playing children. But because everything seems to be available on the internet, homeowners willing to go to the trouble to find rubberized mulches, especially those who might want to create a play area for their children, likely can locate them and have them shipped.
Are some mulches dangerous for pets or children?
There are reports of coca mulch (Theobroma cacao) being dangerous to dogs because it contains two compounds that are toxic to them: caffeine and theobromine. As dog lovers know, their pets, especially puppies, will sometimes chew on anything. Pennisi, though, said she has not heard of any instance where a mulch sickened or killed an animal. However, just like with humans, there might be a case where an animal ingested too much or was more sensitive and it caused death. “I don’t know of any mulch that is so toxic it is going to seriously injure or kill a pet,” she said.
Cocoa mulch, which is made from cacao shells, is popular with some gardeners because it has a pleasant chocolate scent, helps repels garden pests, effectively retains moisture, and has a rich brown color that darkens rather than fades with time. Just to be on the safe side, talk with your vet if you are interested in this mulch and ask whether he or she regards it as a potential threat to dogs or cats.
3 benefits of mulch
Regardless of which mulch you use, adding it to your garden provides several important benefits.
It keeps moisture in the soil
Here’s how Pennisi said that works. “When rain comes in and it percolates through the soil profile, some of the moisture stays in the top layer of soil where most of the plant roots are. In the summer, because of the temperature difference between the cooler soil and the warmer air, some of that moisture is going to evaporate. Therefore, the plant’s roots are not going to have access to all the water from the rain. If you have mulch over those roots near the top of the soil, the mulch will slow down the evaporation and make more of the water available to the plants for a longer period of time. In winter, mulch also helps hold warmth in the soil, insulating roots from freezing temperatures.
It keeps weeds from growing
“Wind disburses weed seeds, and if those seeds find bare ground where they have access to soil, water and sunlight they are going to germinate,” Pennisi said. “Once they start growing they will compete with desirable plants for resources such as nutrients, water and sunlight — whether those plants are in ornamental beds or vegetable gardens. Mulch will prevent about 80 percent of available weed seeds from germinating if you have an adequate layer of mulch on top of the soil.”
What’s an adequate layer? About 3 to 5 inches deep, with the mulch kept at the 3-inch minimum year-round, especially in the hot and humid South and Southeast, Pennisi said. Organic mulch can decompose very quickly, she added, so be aware you may have to constantly reapply it during the growing season. If you can only apply mulch once a year, spring is the best time because that’s when many weed seeds first germinate. That’s not to diminish the importance of maintaining a 3-inch minimum layer of mulch in fall and winter. That will help reduce soil temperature loss and keep roots warm, thus helping to avoid root damage.
It leads to healthier soils
Especially organic mulches. By helping to retain moisture, preventing weeds from robbing the soil of nutrients, and adding nutrients to the soil through decomposition, mulches help micro-organisms thrive and encourage the presence of beneficial earthworms.
An added benefit is aesthetic rather than functional. Mulch gives the landscape a finished look. And who doesn’t want a good-looking — not to mention, healthy — garden that is the envy of the neighborhood?
