Mulching Trees and Shrubs

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Mulching plants is both functional and decorative. Mulch typically is an organic material spread on the soil surface to protect roots from heat, cold, and drought, and to provide nutrients to plants as it decomposes. Once you have chosen the right plant for a given site and followed the proper planting procedures, you should mulch the plant and create a stable environment for root growth.

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

Texture. Medium-textured mulch is best. Fine particles tend to pack down and retain moisture, which then evaporates before reaching plant roots. Coarse-textured materials may be too porous to hold adequate amounts of water.

Nutrient value. Organic mulch provides nutrient- rich humus as it decomposes. This also improves soil structure.

Availability. Consider the availability of different mulch material and whether you have to haul it yourself. Bulk materials may be available free from your community.

Aesthetics. The type of mulch used is a personal preference. Choose for yourself the look you desire. The Morton Arboretum uses organic mulches because of their many plant benefits. 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. Composting is probably not needed for disease and insect control if the mulch is derived from healthy plants; however, if it has been sitting outside indefinitely it is likely that weed seeds are present.

Grass clippings. Dry or compost before using. Mix them with other materials to increase porosity and reduce matting. Grass is a source for some nitrogen but also has higher alkalinity, which may compromise nutrition.

Hardwood bark. Pine bark or shredded bark can be purchased as bags of small or large chips. It’s long-lasting.

Hardwood chips. These are readily available and often free from municipal sources.

Composted leaf litter (leaf mold). While a good source of nutrients, it may increase weeds if not thoroughly composted.

Animal manure. A good source of nutrients. Compost before applying or plant damage (burn) may result due to high salt content. Ideally, it should be mixed with a coarse-textured material.

Mushroom compost. A good source of nutrients when mixed with soil or other materials. Alone it has high alkalinity and sometimes salts.

Peat moss. Compacts easily due to fine texture and dries out quickly; it’s best mixed with soil and other materials. It’s not recommended as a top dressing because water will not penetrate when it’s dry.

Pine boughs. This is a good covering for perennials in the winter.

Pine needles. Not widely available; needles should be mixed with other materials unless soil acidity is desired.

Sawdust. Compost first or mix with a nitrogen source (manure and/or fertilizer) before applying. Oak sawdust helps acidify soil and is good for azaleas, rhododendrons, and blueberries. Do not use sawdust from treated lumber.

Sewage sludge. A good source of nutrients. Composted sludge is available commercially (for example, Milorganite or Nutricomp) and should be incorporated with soil or mixed with other composted material.

Shredded leaves. Leaves are variable in texture and can be collected and shredded at home. Mix them into the soil in the fall and allow them to break down naturally during the winter for improved soil quality.

Straw. Coarse-textured, straw persists a long time, but it can blow away easily unless mixed with other materials. Although generally not suitable as a landscape mulch, it provides winter protection and is a good cover for grass seed.

Spread mulch under trees, shrubs, and throughout planting beds to a recommended depth of 3 to 4 inches for medium- to coarse-textured materials.

Pull mulch away from the bases of tree and shrub trunks, creating a donut-hole (image on left). Do not pile it 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 (image on right).

Ideally, the mulched area around a tree should extend to the drip line of the branches, or at least cover a 4- to 5-foot diameter area around the trunk. The larger the mulched area, the more beneficial.

Check the mulch depth annually and replenish as necessary.

 

Correct mulching on left, incorrect mulching on right

Provides an insulation layer. Mulched soils are warmer in winter and cooler in summer than bare soils. Roots are protected from temperature extremes, creating less freezing and thawing of the soil in winter, which can heave and injure plants.

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 preventing sunlight from reaching and heating the soil. Mulch also insulates the soil moisture from evaporation by wind. Less watering is required during high summer temperatures.

Improves the soil’s physical structure and fertility. As mulch breaks down it adds humus to the soil, increasing organic matter in the surface of heavy clay soils, improving the water-holding capacity of light, sandy soils, and slowly releasing nitrogen and phosphorous into the soil.

Prevents erosion and water runoff. 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 slows and absorbs water.

Reduces root competition. In the Midwest, most of a tree’s fine roots are in the upper 12 to 18 inches of soil. Applying mulch under trees and shrubs eliminates competition from other plants for water and nutrients. Turf roots are especially aggressive and pose the largest threat of competition to trees and shrubs. Create a “living” mulch by using plants that are more compatible with tree roots: bulbs, wildflowers, ferns, ground covers, and other herbaceous perennials.

Additional benefits of mulch. Mulch creates a safety zone around trunks, protecting them from lawnmower damage; recycles yard and landscape waste; offers a more natural appearance to the landscape; and provides a favorable environment for earthworms and other organisms that benefit soil structure and fertility.

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. Plastic mulch or weed barriers prevent oxygen and water from penetrating the soil and should not be used unless they are porous.

Excessive moisture. Fine-textured mulch, such as peat moss, grass clippings, and sawdust, holds a lot of moisture and should be used only in mixtures with coarser materials.

Heat injury. Dark-colored mulches absorb heat during the day and lose heat at night as surrounding air temperatures fall. This heat may sometimes injure succulent plant tissue.

Root collar rot. 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 plant.

Soil temperatures. If applying mulch as winter protection, avoid applying it too early in the fall, since mulch can delay the soil freezing process by retaining heat in the soil. Furthermore, if applied too early in the spring, mulch can inhibit soil warming and delay root growth. As a general rule, wait until after a hard frost in the fall to apply winter mulch and after the last frost in spring to apply summer mulch.

Weed seeds. Some types of organic mulch, including straw, hay, manure, and some leaf-litter mold, may harbor weed seeds and should be composted or otherwise treated before use so that weed seeds are eliminated.