There are so many claims nowadays about various lawn and landscape products that it’s difficult to know what works and what doesn’t. The hubbub swirls in particular around the use of mulch in landscape settings. It is generally recognized that mulch can be beneficial, but what is the science behind it?

One of the issues to be addressed is the use of mulching mowers. Are grass clippings recycled into a lawn really doing any good for the grass? The experts say “yes.”

Mulching with mowers

“Mulching is good for the turf because mulching returns grass clippings to the turf bed and nourishes the soil,” says Patsy Penner, market coordinator with The Grasshopper Company. The science is known: Clippings in general are 90 percent water, so they are a diluted nutrient, but they also are comprised of up to 4 percent nitrogen, .5 percent phosphorus and 2 percent potassium. The exact proportions may vary by grass species.

Because of the high water content, Penner says, clippings decompose readily within a few weeks and do not contribute to thatch buildup. In fact, clippings and other organic matter on the soil surface can help prevent thatch buildup by promoting the growth of beneficial microbes. Nor is there a carryover from year to year under normal conditions in healthy soil with active decomposing organisms, such as bacteria and fungi.

Rain or irrigation after a mulching mower has passed over the lawn can actually help settle clippings down into the soil surface, Penner says, but moisture deposited on the grass before mowing can be a problem. Thus, time of day can be important to the quality of mulch; even dew on grass can reduce mulching efficiency. It may be more beneficial to collect clippings or side-discharge them than to attempt to mulch in the rain. The wetter the clippings are, the more likely they are to clump, and the underside of the deck may need to be cleaned more frequently.

Jamie Palmer, a product manager with John Deere, says that the science of using clippings as mulch is well studied. Clippings can provide between 10 and 25 percent of the grass’ nutritional needs if mulching is done correctly. Grasses that do not need to be immaculate may not need any fertilizer at all, especially during the off-season. However, the mower should be a professional model with good mulching design features.

“The mulching mower actually cuts the clippings multiple times,” Palmer says. The universal rule of mowing only one-third of the grass blade should apply, for the health and appearance of the turf, but in addition, small clippings provide better mulch than large clippings. Professional mowers specifically designed for mulching have features such as multiple chambers and baffles that direct the clippings into the blades more than once. For example, John Deere has a mower with moveable baffles that take into account elements such as amount of moisture in the grass.

Palmer also points out that turfgrass managers can modify mowing behavior to increase efficiency. For example, when mulching clippings, a mower should be slowed down a bit. If mulch mowing must be done at high speed, the horsepower of the machine should be increased to provide more power. The mower operator can also look behind the mower to see if clippings are lying on top of the grass or are windrowing. If hey are windrowing, that indicates the clippings are too large to filter down through the grass. Mowing more frequently forces the grass into a more horizontal growth pattern, which can create a more lush lawn as mulch is added.


A lawn care operator engages the Mulch-on-Demand Deck on this John Deere mower.
PHOTO COURTESY OF JOHN DEERE.

The biology of mulch

One of the first scientists to tackle the job of evaluating the scientific literature on the topic, as well as evaluating mulch itself, is Linda Chalker-Scott, an associate professor and extension urban horticulturist at Washington State University. A plant physiologist by trade, she began looking at mulch a dozen years ago during a study on the use of wood chips to help stabilize restoration projects. She is a firm proponent.

Chalker-Scott explains that it is only in the last half-century or so that mulch has found widespread use, but indications are that they have been used without fanfare much longer than that.

Because organic mulch has nutritional value, one of their primary benefits is to plant nutrition. Whether applied to turfgrass or to the ground around horticultural plants, organic mulch – such as wood chips and grass clippings, as opposed to inorganic ones like gravel and plastic – can have significant nutrients that will be passed on to the soil as they decompose. They can be particularly rich in nitrogen and phosphates.


Mulched clippings are good for lawns. Professional mulching mowers are designed to chop clippings multiple times and won’t leave clumps or windrows.
PHOTO COURTESY OF THE GRASSHOPPER COMPANY.

“The greener the mulch, the richer it will be in nitrogen and other nutrients,” Chalker-Scott says. Thus, grass clippings and freshly ground wood chips can be very beneficial, while others such as straw and sawdust will have a lesser effect. The use of composted organic waste, especially when combined with high-nitrogen chicken manure, gives even more of a nutritional benefit. A separate category of the industry is the widespread use of mulch products in hydroseeding.

Of course, this leads to more efficient propagation, establishment and growth of plants, whether the mulch/compost is used as a topdressing on lawns or as a ground cover around trees and shrubs. This is particularly true when used around transplanted plants, which are susceptible to environmental conditions that mulch can alleviate. It is as much about the protection of plants as it is about nutrition, which makes thinly layered mulch an excellent method of enhancing seed germination and survival. Once the plants start growing, mulch depth can be increased.

Chalker-Scott has found that protection of plant roots, especially when materials are first transplanted, is a big benefit of mulch. Roots are not as hardy as the aboveground parts of the plant, and blanket-type mulch, such as grass clippings or even gravel, are best in this regard. Maintenance of optimal soil temperatures, and avoidance of either freezing or overheating, has a moderating influence on plants and their growth or decline.

“Most of these benefits are overlapping,” she says, citing the added benefits of reduced soil compaction and erosion. Mulch can improve soil aeration and water penetration, while at the same time reducing the cost of mechanical aerification of turfgrass and in landscape settings. As for erosion control, various types of mulch will hold the fine particulate matter that often is washed from soil, and they allow rain to percolate more uniformly into the soil. She has seen dramatic results, for example, in restored natural lands and landscapes where erosion was previously a problem.

Improved soil moisture and water conservation is getting a lot of attention nowadays, as cities try to find ways to reduce water consumption. Chalker-Scott has seen water bills decrease markedly in Washington state as the use of wood chips and other mulch on landscapes has increased. It has been demonstrated that even a thin layer of straw can reduce evaporation by as much as 35 percent over bare ground.

Other benefits are not much talked about as much, but are important to plant growth and environmental protection. One is that mulch can bind up heavy metals, salts and other chemicals from pollutants to pesticides, and can be used as a barrier between landscapes and surrounding areas in the reduction or prevention of runoff. Another is the reduction of plant diseases, such as Phytophthora root rot, in areas where mulch has been applied, and some mulch, such as red cedar bark, contain chemicals that have an antimicrobial activity and reduce rot.

“The other part of this is that if you have aerated, uncompacted soils, they won’t be colonized by microbes that like anaerobic conditions,” Chalker-Scott says.

Weed control is a big part of a mulch program, she points out. Mulch not only presents a physical barrier to weed growth if the materials are applied deep enough, but some also have chemicals that can suppress seed germination. Particularly in small areas as in tree wells, weed growth can be reduced or prevented altogether.

Some problems can occur, and should be considered when applying mulch to lawns or landscape settings. One is that fine-textured mulch materials, such as grass clippings, sawdust and compost, can reduce oxygen flow to the soil and induce rot if applied too thickly. Artificial fabrics can clog up and reduce the flow of air and water, which can be followed by the growth of weeds on top of the fabric. Mulch can also increase soil acidity, hinder seed germination under certain conditions and cause diseases if the mulching materials are diseased. Contamination from treated construction materials is a possibility if they are mulched. Mulch can also compete with plants for water and air if applied inappropriately.

However, the economic benefits are obvious. Maintenance costs can be reduced, as the need for weeding and fertilizing is reduced. Water usage may drop significantly. The survival rate of landscaping plants is boosted, reducing the need for replacements. Pesticides may not be needed as often, as weeds and disease are averted. And, the environmental benefits can also be substantial, says Chalker-Scott, not the least of which is the recycling of organic materials that would otherwise be taken to a landfill.


Landscape mulch has been found to benefit both plants and the soil, and also reduces the amount of organic waste sent to landfills. Because organic mulch has nutritional value, one of their primary benefits is to plant nutrition.
PHOTOS BY LINDA CHALKER-SCOTT.

Industry trends

Robert LaGasse, executive director of the Mulch & Soil Council in Manassas, Va., says that the mulch industry is so strong that it actually improved during the Great Recession. The trend for the use of various kinds of mulch – there are about 130 corporate manufacturers with about 230 production plants certified with the council – has grown about 8 percent in the last five years alone. The growth has been dramatic since LaGasse began working for the national organization 28 years ago.

“We have over 300 registered products in our registration program. Most are mulches,” LaGasse says of the industry that once consisted mostly of sawmill byproducts. Business is actually better in a poor economy because people are staying home and improving their lawns and landscapes, and vegetable gardens are the hottest new growth trend in mulch usage. And, new types of mulch are coming on the market all the time.

“There has been significant growth in recent years for colorized products,” he notes. Pallet and construction wood recycling are also becoming more popular, though contaminants can be a problem. The council has a certification program that verifies an absence of pollutants in the mulch.

LaGasse says that there is also a movement toward the use of coir coconut husk byproducts to replace peat moss. The recycling of organic waste products has created such competition from other users that the cost is rising to landscapers, but the usage will always be there. “There are a lot of good environmental benefits from mulching,” he concludes.