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To Mulch or Not to Mulch: What is best for my pecan tree?

SPRING! It is quickly approaching, but what is it. It is one of the four conventional seasons, coming after Winter, but preceding Summer. Astronomically speaking, it starts on the Vernal Equinox and ends at the Summer Solstice. If you ask folks what they associate with this season, they’ll say Spring is a time of renewal, rejuvenation, rebirth, or regrowth. For homeowners or commercial orchardists, it means you have probably planted some new trees. One of the most important practices to successfully grow a newly planted pecan tree is to provide adequate water during the growing season. Generally, water will have more of an influence on pecan tree growth than any other environmental factor. Insufficient water or drought stress will affect leaf and shoot growth and can result in tree death.

Weed competition often results in poor growth or even the death of young pecan trees. Bermudagrass and Johnsongrass are especially severe competitors, but numerous other grasses and weeds can result in very poor tree growth. Pecan trees will make the best growth in the first few years after planting if a circle, 6 feet in diameter around the tree, is kept weed-free with glyphosate herbicide, hand cultivation or mulch. Chemical weed killers (herbicides) should be used with caution and in complete accordance with label directions. Organic or inorganic mulches are the more sensible choice for residential landscapes but can be used in commercial pecan orchards as well.

Let’s begin with some basics about mulches. The practice of mulching pecan trees isn’t a unique concept developed by modern society. We’re actually borrowing it from Mother Nature. Think of native pecan trees growing in their natural environment along the numerous tributaries in Oklahoma, Louisiana, or Texas. Trees growing in a natural forest environment have their roots anchored in rich, well-aerated soil full of essential nutrients and soil mi­croorganisms. Nature produces a natural, continuous mulch consisting of fallen leaves, dead grass, twigs, pieces of bark, catkins, shucks, and other organic material. This blanket of organic material replenishes nutrients and provides an optimal environment for root growth and mineral uptake. Mulching can be an important management practice to maintain healthy trees in your landscape or orchard. This is especially true for newly planted trees. Research has shown that the growth rate and tree health is improved when competition for water and nutrients by weeds is minimized.

Mulches are used primarily to increase water infiltration, reduce evaporation, modify soil temperatures, control weeds, and increase crop yields. They also increase the biological activities in the soil, modify the level of available nutrients, and help to maintain or increase the level of soil organic matter. Under the conditions of no-tillage or minimum tillage of fine-textured soils, there is also likely to be a favorable effect on the physical conditions in the soil.

Mulch Characteristics to Consider

What makes good mulch? Several factors should be considered when choosing mulch:

Texture. Particle size does make a difference in how a mulch performs. Medium-textured mulch generally works best. Fine particles tend to pack down and retain moisture, which can evaporate before reaching plant roots. Coarse-textured materials may be too porous to hold adequate amounts of water, allowing excessive soil drying.

Nutrient value. Mulches will differ in their nutrient content and how quickly they decompose. For example, wood chips will degrade more slowly and contain fewer nutrients than manure. So, over a six-month period, it will release fewer nutrients than manure but control weeds better. All organic mulches contain some of the macronutrients necessary for plant growth including nitrogen, phosphorus, potassium, calcium, and magnesium. Additionally, they also contain most micronutrients. All organic mulches will ultimately decompose to humus. Humus is an organic component of soil that forms when leaves and other plant materials decompose, providing slow release nutrients essential for plant growth. The humus will improve the bulk density of the soil allowing better retention of added moisture and nutrients.

Availability. Anyone who considers using mulch must evaluate the availability of different mulch materials. Transportation costs, the actual cost of the mulch, and whether you have to haul it yourself can be prohibitive factors for its use in an orchard. Bulk materials may be available free in your community at Solid Waste Facilities, Rural Waste Service Centers, or Environmental Waste Facility. These facilities divert wood waste, brush, yard trimmings, and leaves from landfill disposal. The materials are chipped, mixed, and ground into mulch.

Processing. Mulches are generally available in 3 forms: fresh, aged, or composted. Commonly available fresh materials include green grass clippings or recently chipped green bark mulch. Most mulches are piled for storage and aged for a period of time before being used but are not fully composted. Aging reduces problems associated with fresh organic materials. Composting, on the other hand, utilizes controlled moisture levels, regular pile turning, and closely monitored pile temperatures to produce a biologically stabilized organic material. Ideally, organic mulch should be composted or otherwise treated before use so that weed seeds, insects, and disease microorganisms are killed. Composted mulch generally has more uniform texture than mulch that is not composted.

Source of Raw Materials. Wood mulch is sometimes made from recycled wood that can contain objectionable additives—including arsenic from pressure-treated wood—and is not always 100 percent hardwood. Pressure-treated wood containing arsenic was phased out of residential use in 2003 by the EPA, but old crates and pallets may still be entering the recycling stream. Additionally, chromated copper arsenate is still used to treat wood intended for highway construction, fence posts, and marine construction. Currently, the most common chemicals used to treat wood are Alkaline Copper Quaternary (ACQ), Copper Azole (CA), Copper Dimethyldithiocarbamate (CDDC), Ammoniacal Copper Citrate (CC), and Micronized Copper Azole (MCA). If you’re considering colored mulch, be sure the manufacturer uses raw lumber rather than recycled wood. The dyes used for colored mulches are considered safe.

Types of Mulch

Mulches are available in many forms but are divided into two major types: inorganic and organic. Inorganic mulches include various types of stone, lava rock, pulverized rubber, geotextile fabrics, and other materials. Inorganic mulches do not decompose and do not need to be replenished often. On the other hand, they do not improve soil structure, add organic materials, or provide nutrients. Aside from the geotextile fabrics, most inorganic mulches would be limited to individual trees in the homescape. Rarely used alone, landscape fabric or weed barrier is usually covered with other organic mulches for aesthetic reasons. While the double-barrier is excellent for stopping weeds, using a fabric barrier with mulch often impedes the movement of organic material into the soil. Eventually, this creates a layer of “dirt” on top of the fabric which needs to be removed periodically. Weed seed can settle through the mulch to this layer and germinate. These weeds can be difficult to remove if they root through the fabric into the soil below.

Organic mulches include wood chips, pine needles, hardwood and softwood bark, leaves, manures, and a variety of other products usually derived from plants. Unlike their counterparts, organic mulches decompose in the landscape at different rates depending on the material, climate, and soil microorganisms present. Those that decompose faster must be replenished more often. Because the decomposition process improves soil quality and fertility, organic mulches are preferred, despite the added maintenance.

Types of Organic Mulch 

Grass Clippings. Dry, composted grass will produce a better mulch than using fresh cuttings. Ideally, dry clippings should be mixed with other materials to increase porosity and reduce matting. A two-inch layer of grass clippings provides weed control, while thicker layers tend to compact and rot. Matted layers restrict water and air supply to plant roots. Major considerations are weed seeds contained in the grass and if the grass has been treated with any herbicides. A source for some nitrogen but also higher alkalinity, which may compromise nutrition.

Hardwood Bark. This type of mulch is durable, slow to decompose, and available chipped or shredded. Shredded bark does not wash away easily and provides good suppression of weed growth. Smaller chips are easier to spread, especially around small trees. When spreading mulch around trees, keep the mulch an inch or two away from the trunk. A couple inches of mulch is adequate. Avoid chips or shredded bark from pressure-treated wood. Hardwood mulch can compact over time and can block rain and nutrients from reaching the soil when that happens.

Hardwood Chips. Many municipalities have large, available supplies of wood chips from trees trimmed along utility rights of way and storm damaged trees. It generally contains both bark and wood pieces of various sizes. A 2- to 3-inch layer of wood chips provides good weed control and like bark mulch, shouldn’t be piled against the tree trunk. Fresh or green wood chips should be avoided because their decomposition can tie up available nitrogen in the soil, depriving the growing tree of nitrogen needed for growth.

Composted Leaf Litter (leaf mold). Just like grass clippings, leaf mold used by itself will often form a layer which restricts water movement into the soil. Therefore, it is best used as an amendment to other mulches, or in a mulch mix. While it is a good source of nutrients, leaf mold can result in weed problems if not thoroughly composted. Chopping or shredding, followed by composting will provide the most useful material. If using dry leaves, apply about 6 inches deep.

Animal Manure. Cow manure is the most common, but horse manure is used as well. It is a good source of nutrients but should be composted to reduce the salt content. Ideally, should only be used as a mulch when mixed with a coarse-textured material. Knowing the source of feed consumed by the animals can be important, as some grass herbicides are still active in the manure and can result in young tree damage.

Mushroom compost. Since I get asked this question almost every year, I have included mushroom compost here. Yes, it is a good source of nutrients, but usually contains large amounts of alkalinity due to the preparation process prior to its use in growing mushrooms. It is probably best to use it as an amendment to other types of mulches.

Peat Moss. Not commonly used as a mulch in orchards as it compacts and mats down easily due to fine texture and dries out quickly. When dried, peat moss becomes impervious to water and difficult to remoisten. When mixed into the soil, it will improve soil structure and improve soil fertility. It can be mixed with other coarser mulch materials.

Pine Needles. If available, pine needles or straw is moisture retentive, lightweight, durable, and slow to decompose. During heavy rain events, pine needles tend to stay put and not wash away, making them an excellent choice on slopes. However, it will acidify the soil and should be mixed with other materials unless you are applying it over an alkaline soil.

Sawdust. Should be composted or mixed with a nitrogen source such as manure or fertilizer. As discussed earlier, fresh sawdust will reduce available nitrogen in the soil and possibly induce nitrogen deficiency symptoms in young trees. Do not use sawdust from treated lumber.

Sewage Sludge. Even though sewage sludge is readily available and can be a good source of nutrients, it is typically not used in nut orchards, especially if you are following GAPs or FSMA regulations on your farm. Common issues include pathogen content, heavy metals, and environmentally persistent chemicals.

Pecan Shells. Pecan shells make a long-lasting, attractive, dark brown mulch that is effective in retaining moisture in the soil. Sharp edges may inhibit slug and snail activity. Furthermore, it is slow to decompose and very ornamental. Availability is usually limited to areas near pecan shelling facilities.

Benefits of Mulch

Although each type of mulch has its own pros and cons, mulches, in general, possess a number of benefits that can both protect your trees and enable them to prosper.

Provides an insulation layer. Two to four inches of mulch will help insulate the young tree roots, providing more uniform growing temperatures and providing protection from temperature extremes during the summer and winter. Scientists have confirmed that mulched soils are warmer in the winter and cooler in the summer than bare soils.

Conserves soil moisture. Bare soil surfaces heat-up in summer, causing water evaporation and sometimes root desiccation and death. A layer of mulch reduces moisture loss by blocking sunlight and minimizing soil moisture evaporation by wind. Since moisture evaporation is reduced, growers may need to reduce their irrigation frequency when using mulch. Additionally, the use of mulches will reduce soil crusting, thus improving water absorption and movement into the soil.

Improve the soil’s physical structure and fertility. Decomposing mulch adds humus to the soil, increasing organic matter in the surface of heavy clay soils and improving the water holding capacity of light, sandy soils. Fertility of the soil is improved as the decaying mulch slowly releases nitrogen, phosphorus, and other nutrients into the soil. Organic mulches also provide food and a favorable environment to beneficial soil organisms, such as earthworms and “good” fungi.  Organic mulches will improve soil aeration, structure (aggregation of soil particles), and drainage over time.

Prevents erosion and water runoff. Mulches reduce water runoff from rainfall or sprinklers, thereby reducing the possibility of soil erosion. Soil compaction is reduced, as well as water entry into the soil being improved by the porous surface of the mulch. Bare soil disperses or breaks apart when impacted by rain or sprinkler droplets. Mulch protects soil from being eroded and reduces water runoff by providing a “sponge” surface that absorbs water and slows it down. The layer of mulch also protects the soil from erosion, caused by wind and heavy rains.

Reduces root competition. Most of a tree’s roots are in the upper 6 – 18 inches of soil. Applying mulch under pecan trees eliminates competition from other plants for water and nutrients. Turf roots are especially aggressive and pose the largest threat of competition to young pecan trees. A 2- to 4-inch layer of mulch will reduce the germination and growth of weeds. Mulch excludes light, preventing germination of some weed seed. Small-seeded weeds do not have enough energy to emerge through a three-inch layer of mulch. Initial weed control will be good, but no organic mulch will control weed growth long term. Some perennial weeds will require hand removal or chemical control.

Protects Trunk. As discussed earlier, a properly placed circle of mulch around a tree prevents weed and grass growth near the trunk. This greatly reduces the possibility of mechanical trunk damage due to improperly used string weed trimmers, lawnmower decks, or tractor-pulled mowers.

Reduce diseases. Mulches can reduce the splashing of irrigation or rainwater, which is a prime mode of movement of disease spores to reach the stems and leaves of trees. Mulches can also alter the ratio of beneficial microbes to pathogenic organisms, thus reducing plant disease.

Looks Nice. Mulching around pecan trees is recommended to improve the growing environment, but an attractive layer of mulch around trees can also add to the beauty of the landscape or orchard.

Potential Disadvantages of Mulch

Problems may arise if mulch is used incorrectly. Too much mulch can be harmful. Consider the following points to make an informed choice and avoid problems:

Creates a barrier to oxygen and water. Some people like to combine inorganic mulches with organic mulches overlaying the inorganic mulch. Nonporous plastic mulch or heavy weed barrier fabric can prevent oxygen and water from penetrating the soil. Only porous inorganic mulches should be used around trees. As discussed earlier, thick layers of finely ground mulch or leaves can become matted and form impervious layers reducing the penetration of water and air. Excessively thick layers of mulch or “volcano” mulching will limit the amount of oxygen getting to the roots of the tree. The tree crown or trunk flare at the base of the tree must be able to “breathe.”

Excessive moisture. Peat moss, grass clippings, sawdust, and other fine-textured mulches will hold excessive amounts of water, and if piled against the trunk can favor bacterial and fungal diseases that attack the bark and inner wood. Finely textured mulches should be used only in mixtures with other coarser mulch materials.  While we are on the subject, irrigation sprinklers that frequently keep the lower tree trunk saturated can promote disease and decay.

Heat injury. Dark-colored mulches can absorb large amounts of heat during the day and reach temperatures well over 100 degrees Fahrenheit. If you use green wood chips or a thick layer of green grass clippings, it can actually go through the composting process, much like a compost pile. Temperatures as high as 120 to 140 degrees can be reached in the mulch layer during decomposition, damaging roots or trunk tissue adjacent to the mulch.

Diseases, insects, and rodents. Excessive mulch mounded around the base of a tree can cause decay of the vital tissue at the root collar. Once decayed, serious disease organisms may more readily enter the tree. Additionally, mulches piled around the trunks can create problems by fostering the growth of nuisance fungi, harboring insect pests, and may create habitats for rodents that chew the bark and can girdle the trees.

Soil temperatures. In the spring, heavy layers of mulch can inhibit soil warming around the trees, resulting in delayed root growth. As a general rule: wait until after the last frost in spring to apply summer mulch.

Weed seeds. Some types of organic mulch (e.g., straw, hay, manure, leaf litter) may harbor weed seeds and should be composted or sterilized using heat or chemicals so that weed seeds are eliminated. A thick layer of fine mulch can become like potting soil and may support weed growth.

Nitrogen Availability. Looking on the internet, it is easy to find many articles stating that the use of green wood or bark mulches or sawdust will lead to nitrogen deficiency problems. It is true that mulches of this type can have a high carbon-to-nitrogen (C:N) ratio. When organic matter of this type is incorporated into the soil, it will stimulate microbial growth but be unable to provide enough nitrogen to support that growth. Therefore, the microbes must acquire additional nitrogen from the surrounding soil, thus temporarily decreasing the amount of nitrogen available to the tree. Adding nitrogen fertilizer to high C:N mulch (1–2 lbs N/1000 ft2) can alleviate competition between plants and microbes for nitrogen. However, mulches with a high C:N ratio used specifically as a mulch and not incorporated, will only interact with the soil surface and due to their slow decomposition will not affect the nitrogen status of the tree.

Applying Mulch

Now that you have a complete understanding of characteristics to consider, the types of mulch, and its advantages and disadvantages, it’s time to address the final step to using mulch: application.

You should spread mulch under trees to a recommended depth of 2 to 4 inches for medium to coarse textured materials. Pull mulch away from the base of tree trunks creating a donut hole. Do not pile the mulch up against the trunk (“volcano mulching”). Excessive mulch on the trunk causes moisture to build up, creating ideal conditions for insect pests, diseases, and decay. Mulch piled high against the trunks of young trees may also create habitats for rodents that chew the bark and can girdle the trees. Wider is better than deeper.

Over-mulching can create a moisture barrier, not allowing water in or out, encouraging either overly moist or dry conditions. Ideally, the mulched area around a tree should extend to the drip line of the branches, or at least cover a 4- to 6-foot diameter area around the trunk. The larger the mulched area, the more beneficial. Check the mulch depth annually and replenish as necessary.

Author Photo

Charlie Graham

Charles J. Graham is the Senior Pecan Specialist at the Noble Research Institute. Noble Research Institute, 2510 Sam Noble Parkway, Ardmore, OK 73401; E-MAIL: