Mulching Garden Soils

February 2017


Mulching Oklahoma garden soils may be one of the most valuable cultural practices of gardening.

The use of organic materials for mulches can provide many beneficial effects. These include: the control of annual grasses and weeds; the elimination of the need for cultivation and the resulting damage to plant roots; the reduction of moisture evaporation; the increase of water absorption and retention; the decrease in runoff and soil erosion; and the regulation of soil temperature. Other benefits are: cleaner, more easily harvested crops; the reduction of fruit rot; and easier movement through the garden during very wet periods.

Surface mulches will provide conditions for plant roots to develop throughout the soil to absorb water, nutrients, and oxygen. Luxuriant stem, leaf, flower, and fruit growth follows the development of vigorous, extensive root growth.

Most Oklahoma soils are high in mineral content, but low in organic matter. The use of organic mulches in the home garden and their incorporation into the soil at the close of each gardening season provides an opportunity to increase soil organic matter content, improve the physical condition of the soil, and add some nutrients.

Mulching Materials

Many materials are available for mulching. The selection of a particular material depends upon the cost, availability, the season of the year, and the crop to be mulched.

Since one of the more important factors in successful home gardening is the maintenance of an adequate level of organic matter in the soil, this fact sheet will consider primarily those materials that produce this result.

Suitable organic mulch materials should decompose within a season and should not contain undesirable quantities of viable seeds and harmful disease organisms or pests.

The material should be easily applied and remain in place. It should not pack down and should be effective for at least one season. Finally the mulch should be incorporated with the soil for further decomposition. It is a good practice to incorporate or compost garden refuse at the close of the gardening season. This eliminates protective quarters for insects to use in winter months.

Application of the Mulch

 Growing Season Mulches

A mulch is frequently applied soon after the emergence of the crop seedlings or following transplanting.

A delay in application of the mulch may be desirable if the soil has not warmed sufficiently during the spring season.

In the event of excessive soil moisture, crop plant roots may develop in the mulch layer where aeration is more favorable for root growth at that time.

The depth of a mulch layer will be influenced by the texture of the mulch material since a primary objective is to prevent or greatly reduce the germination and growth of annual weeds and grasses.

The amount used might vary from 1 inch for sawdust, peat moss, cotton seed hulls, ground corncobs, compost, or similar density materials to 4 to 8 inches for straw, hay, corn stalks, or other coarse materials.

Another factor in determining the amount and type of mulch material used, is the need to provide protection to foliage, flowers, and fruits from soil-borne disease organisms that could splash upon the plant. Also, to prevent the development of fruit rots or leaf diseases.

Many of the more permanent plants of the yard and garden may also be mulched to maintain a continuous soil cover under and around the plants. This might include trees and shrubs as well as hardy perennial flower, fruit, and vegetable plants.

Some plants that benefit from summer mulching include: tomato, pepper, eggplant, okra, green beans, cucumbers, cantaloupe, squash, broccoli, cabbage, cauliflower, brussel sprouts, sweet corn, asparagus, rhubarb, strawberries, blackberries, dewberries, boysenberries, blueberries, and tree fruits. Also, chrysanthemums, columbine, roses, azaleas, asters, lilies, daylilies, perennial phlox, peonies, and many kinds of annual flowers benefit.

Generally, the semi-arid plants and most wild flowers, when grown in higher rainfall regions, should not be mulched.

Sweet potatoes do not benefit from summer mulching except in very sandy soils and in dry seasons.

Figure 1. Mid-Summer Temperature Differences between Non-Mulched and Mulched Plots Measured 1 Inch Deep in the Soil.

Figure 2. Mid-Summer Temperature Differences between Non-Mulched and Mulched Plots Measured 3 Inches Deep in the Soil.

 

Winter Season Mulches

The principal reasons for winter mulching are to provide more uniform soil moisture and to protect the plant from severe temperatures during cold weather.

Winter mulches generally are applied around and over the tops of low growing plants after the plants are in a dormant or inactive stage of growth. This usually occurs in very late December.

The amount of mulch applied is influenced by the severity of winter cold, the amount of drying winds, and winter rainfall. Often one will apply a cover, but not totally screen out plants under the mulch. Loose mulch materials are more suitable (straw, pine needles, loose hay).

As growth begins to develop, the young leaves will have a very light green color due to the reduction of sunlight under the mulch. Remove enough of the mulch to allow for normal plant growth. With chrysanthemums this might require removal of at least 3/4 of the mulch while with strawberries removal of 1/2 of the mulch might be adequate.

Often the cause of death in non-mulched plants is the result of low temperature combined with excessive drying of the soil.

How to Use Sawdust As a Mulch

Where sawdust is available it is commonly used as a summer mulch. The material, preferably partially decomposed, may be applied to a depth of 1 to 11/2 inches. This would require about 5 bushels (6.25 cu. ft.) of sawdust for a 10 ft. x 10 ft. plot. This amount of sawdust would, if dry, weigh approximately 50 pounds.

Before application, mix one pound of actual nitrogen per 5 bushels of sawdust to aid in further decomposition (1 lb. actual nitrogen would be 3 Ibs. of ammonium nitrate or 5 Ibs. of ammonium sulfate).

If no nitrogen were mixed with the sawdust some of the nitrogen in the soil would be used for sawdust decomposition after mulching. The result would be nitrogen starvation of the mulched crop. Such plants would have yellowish-green foliage and limited growth.

Strawberry runner plants may readily root down through such a layer of sawdust while it provides excellent control of most annual grasses and weeds. This amount of mulch would perhaps be equal to adding between 6 to 10 inches of additional irrigation water on non-mulched strawberry plants.

Using organic materials as mulches may cause an increase in certain garden pests. The mulch provides an excellent environment for sowbugs (or pillbugs) to grow. Some treatment to reduce this pest may be needed. Recommendations for this are provided in Fact Sheet EPP-7313 “Home Garden Insect Control.”

Polyethylene Film and Other Sheet Forms of Mulching

The use of air-tight sheets of plastic are less desirable as mulching materials because of the lack of air movement into the soil surface during periods of soil drying. However, this may be offset by specific advantages of the practice. When attempting to garden in an area infested with perennial weeds or grasses the use of chemicals to control them may make gardening in that area undesirable for several months.

An alternative weed control program is cultivation of the area, and application of the needed fertilizers. Then cover the surface with black polyethylene and set plants or plant seeds in the soil through slits in the film. The result of this system will be the destruction of the perennial grasses or weeds if the covering remains over the soil throughout the growing season. There may be small amounts of grass or weed growth through the slit openings and these plants may require physical removal at the close of the gardening season.

Also available to the home gardener are commercial fabric weed barrier products. Weed barriers are manufactured geotextile materials that provide a protective barrier primarily for weed control in the landscape. Fabric weed barriers are air and water permeable in most cases. However, on slopes water tends to run along the fabric surface without readily penetrating the material. In most cases gardeners use a decorative mulch on top of the fabric barrier to restrain the water from run-off and add aesthetic appeal. Nutsedge and other similar weeds have been known to grow through the fabric barrier.

The commercial vegetable industry has had degradable plastic film available to reduce their dependency of cleanup and disposal. Such films are available for home gardens. Photodegradable films disintegrate under the exposure of ultraviolet light in a time-released manner that usually lasts through one growing season. However, remnants of the plastic can remain after a growing season. Soil and foliage that may cover the plastic also blocks the Ultra Violet light, thus preventing complete breakdown.

The use of aluminum foils or laminates may provide similar effects. Also, certain kinds of insects (leaf hopper, spider mites, and aphids) may be repelled depending on the intensity of reflected light to the under side of the plant foliage.

Table 1 provides information on mulch materials and some of their characteristics.

Yard Trash – A Valuable Resource

Yard trash such as lawn clippings, leaves, pine needles are an inexpensive and valuable resource for your vegetable garden and landscape. Try recycling yard trash instead of sending it to the landfill. Yard trash can be transported to your compost pile (see Fact Sheet HLA-6014 “Making a Compost Pile”) and used later for mulch and/or soil conditioning.

 

Table 1. Some Mulch Materials and their Characteristicsa

Materials Does the Mulch Crust, Pack or SealWill the Mulch Control Grass and WeedWill the Mulch Remain Effective for a SeasonBlaze (fire) PossibilityAvailable Nutrients PresentRemarks
Peat Moss 22224May be blown or moved by wind or rain
Sawdust 2222510 to 12 lbs/bushel
Bagasse 22225• May be blown by wind
Cotton seed hulls42222Usually remains where placed
Straw 12253• Presprout seed by wetting bales
Leavesb33233May be blown by wind
Cotton burrs 22222Don’t use burrs from chemically defoliated plants
Lawn clippings 53452Compost - don’t use fresh as a mulch
Hay - prairie 13243Usually better if chooped
Hay- legume 23232May become very hot during early decay
Compost 12211 Also used as a row cover in seed planting
Wood chips 13125Excellent permanent mulch
Barkb12125Excellent permanent mulch
Wood shavingsb12155
Leaf mold 12223
Gravel pebbles 34115Frequently used over polyethylene
Aluminum foil 51115May aid in insect control
Black polyethylene51125Will destroy perennial grasses and weeds
Fabric weed barriers 51145Will not absorb water readily on slopes
Pine Straw 42214Shred before incorporating into soil
Shredded newspaper 23325Wet to keep in place
Photodegradable film51225Soil covered portions will not decompose

(a) 1 = excellent, 5 = poor

(b) Black walnut sawdust, bark, and leaves is very toxic to some kinds of plants.

• Add 1 pound actual nitrogen per 50 pounds of dry organic matter to provide for decomposition.

 

David Hillock
Extension Consumer Horticulturist

 

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