Mulching is a routine practice for landscapers. However, not all much is created equal. The following is an article detailing Ohio research on different mulches and their properties and effects in the landscape.
The beneficial effects of mulching landscape trees and shrubs are universally accepted. Mulch helps to protect plant roots against extremes in moisture and temperature; it improves general landscape appearance; and it suppresses weed growth within planting beds. Although all these benefits are important, the types of mulching materials and their impact on plants are far more complicated.
The Ohio State University Landscape Mulch Studies A three-year Ohio State University mulch study was designed to determine the effects of three basic options when selecting mulching materials. These included: 1) bare soils; 2) wood pallet mulch/hardwood bark; and 3) composted yard waste mulch (grass clippings, tree trimmings, and urea). All mulches were applied over the surface of the soil at a depth of 2-3 inches in order to determine the effects that the different mulch treatments (or lack of mulch) would have on the surrounding soils and plants. The plant material used in the studies included the River Birch ‘Heritage’ and two species of Rhododendron and Taxus.
Moisture & Temperature
Moisture level readings within each mulch type were determined by seasonal differences. Between April and June, the wood pallet mulch sites contained the highest moisture levels. As expected, the bare soil sites contained the lowest moisture. Interestingly, during the months of July and August the soil moisture levels were reversed. The highest moisture was then found within the bare soil study plots with the wood pallet mulch sites now containing the driest soils. This reversal in soil moisture levels in the wood pallet mulch was attributed to the formation of a fungal mat, which created a hydrophobic layer.
Despite the differences in soil moisture caused by the various mulch types, the researchers of this study did not believe it affected observed plant growth differences. The wood pallet and composted mulch treated areas also affected soil temperatures (i.e., they provided cooler temperatures in the spring as compared to the bare soils), but again these effects on observed differences in plant growth were not considered significant.
Soil Organic Matter & Microorganisms
How the different mulches influenced the percentage of soil organic matter over time was also evaluated. After two years, there were no significant increases in the % of soil organic matter within either the bare soils or the wood pallet mulched sites. Alternatively, and as expected, there were significant increases in the % of soil organic matter within the composted yard waste treated sites. The addition of nitrogen within the composted yard waste further increased the % of soil organic matter. The greater nitrogen availability enabled the microorganisms to break down carbon materials into even finer particles. The research determined that bacteria were significantly higher in both mulched treatment sites as compared to the bare soil site, and was attributed to the availability of carbon as an energy source. It is important to note that the increased microbial rates within the wood pallet treatments were only realized when nitrogen supplements were added. The research data suggested that neither soil temperature nor moisture differences caused by the specific mulch types significantly influenced microbial growth rates.
The reduced availability of nitrogen for use by plants was most pronounced in the wood pallet mulch treatments. After each site received supplemental nitrogen, the wood pallet mulched sites actually had less nitrogen available to plants than the plants in bare soils. The microorganisms feeding within the wood pallet mulched soils were starved of nitrogen and consumed the limited nitrogen before the plants could have access. However, nitrogen immobilization was reduced as compared to existing levels prior to the nitrogen supplements. There were no differences in the reported reduced availability of nitrogen between the fertilized and the unfertilized composted yard wastes (i.e., they both contained very low nitrogen immobilization levels). This shows that nitrogen supplements to landscape plants can be significantly reduced if landscape beds are treated with composted yard waste mulch (i.e., composted yard waste mulch treatments are already providing approximately 1# of N per 1000/sq.ft. /year).
The Ohio mulch data indicated that as nitrogen availability increased, the growth rates of plants also increased. The research showed the observed plant growth within composted yard waste mulch was greater because of a nutrient effect (primarily nitrogen). The trees growing in the composted yard waste mulched areas had between a 15% to 20% increase in trunk caliper in comparison to trees grown in bare soils!
Providing an environment that optimizes the growth of beneficial microorganisms will enhance their ability to naturally compete with pathogenic microorganisms. The build-up of suppressive beneficial microorganisms within the soil is encouraged by composted yard waste (“Build it and they will come”). Ultimately, natural biologic controls can occur and in some instances, the actual suppression of diseases.
The Ohio research data reinforced previous studies that discovered some diseases of nursery plants could be at least partially suppressed with composted mulches. For example, other studies have suggested that Verticillium wilt of maple trees could be suppressed with composted mulch materials. On the other hand, field studies have shown that bare soils (no mulch) will provide an even spread of the disease Phytophthora. A similar response has also been shown to occur with wood pallet mulches. Some studies have even indicated an increased spread of Phytophthora with wood pallet mulches. Insect Influences
The Ohio research has shown that composted yard waste increases plant growth. But does plant growth necessarily equal plant health? The environment and the amount of limited resources that are available determine the allocation of energy reserves by a tree. Plants with limited nutrient resources will typically invest a larger percentage of their energy into secondary defensive compounds (e.g., leaf tannins). Conversely, plants that have a larger amount of nutrient resources will usually devote a larger percentage of available energy towards growth.
The researchers also measured the growth of insects that fed on leaves of plants growing in the different mulch types. The trees growing within the composted yard waste mulch contained leaves with higher nitrogen levels. Insect growth rates increased when they fed on leaves containing higher nitrogen levels. The research indicated that leaf-feeding pests might not be attractive to extremely stressed trees, because the quality of the food would be too low.
Studies with white marked tussock moth caterpillars indicated that insects had less weight gain and grew slower when they fed upon trees growing in the wood pallet mulch as compared to trees growing in either the bare soils or composted yard waste mulch. The reason for this difference in insect growth was believed to be from the higher food quality. A significant correlation existed between the white marked tussock moth weight gain and leaf nitrogen levels.
Studies with Japanese beetle adults also indicated the greatest feeding occurred on plants growing in sites with composted yard waste and bare soils that were fertilized. The insects were therefore selectively feeding to a lesser degree on plants growing in sites having wood pallet mulch or on plants growing in bare soils that were unfertilized. It was uncertain if the adult beetles were cueing in on leaf volatiles from other beetle feedings, or if they were randomly feeding from leaf to leaf until they found a tree containing high quality food. The data from the study clearly showed that the beetles fed most heavily upon trees growing in soils with the highest nitrogen levels.
Although the mulching of trees and shrubs is an important plant health care practice, their effects can sometimes produce unexpected consequences. Different mulching materials should influence supplemental fertilizer practices. Nitrogen fertilizers can be applied to help reduce nitrogen immobilization where wood pallet or hardwood bark is found. Alternatively, where plants are growing in composted mulches, nitrogen application rates need to be adjusted to avoid overstimulation. It is generally best to apply composted products. It is most important to use these products when trees are first planted. If raw or fresh mulches are used, they are best applied in the late fall or winter in order to reduce their initial negative effects on plant growth and health. As soon as the organic matter is partially decomposed and the competition for nutrients begins among soil microorganisms, then the beneficial effects can begin.
Extracted from "All Mulches Are Not Created Equal" by Steven K. Rettke, Ornamental IPM Program Associate, Rutgers University in the October 6, 2005 issue of the Plant and Pest Advisory; Landscape, Nursery, and Turf edition.