Plant Answers

The Use of Mulch

Although 70% of the earth's surface is covered by water, only 1% of the water is fresh and 99% of the water is underground. As it becomes more expensive in water-short areas to build up supplies by digging wells, diverting rivers, desalting seawater and seeding clouds, the conservation and efficient utilization of the available resources become increasingly important.

All of the water used in Texas is derived essentially from precipitation. A portion of the precipitation flows to streams, ponds, lakes and reservoirs, and some of this eventually reaches the Gulf. Another portion infiltrates the soil to the rooting zone, and another portion percolates below the rooting zone, becoming groundwater. Surface sources of water are recharged rapidly, but groundwater reservoirs such as the Edward's Aquifer are recharged relatively slow. The proportion of the precipitation received in the United States that is returned to the atmosphere as water vapor is estimated to be 70% from non-irrigated land areas, and 2% from irrigated areas. Most of this loss represents evaporation or transpiration from plant surfaces.

When water is applied to the soil, it seeps down through the root zone very gradually. Each layer of soil must be saturated before water will descend to the next layer. This water movement is referred to as the wetting front. Water will move through a sandy, coarse soil much faster than through a fine-textured soil, such as clay or silt.

If only 1/2 the amount of water required is applied at a given time, it will only penetrate the top half of the root zone; the area below the point where the wetting front stops will remain as dry as if no irrigation had been applied at all.

Once enough water is applied to move the wetting front in the root zone, moisture is absorbed by plant roots and moves up through the stem to the leaves and fruits. Leaves have thousands of microscopic openings, called stomates, through which water vapor is lost from the plant. This continual loss of water, called transpiration, causes the plant to wilt unless a constant supply of soil water is provided for absorption through the roots.

The total water requirement is the amount of water lost from the plant plus the amount evaporated from the soil. These two processes are called evapotranspiration. Evapotranspiration rates vary and are influenced by day length, temperature, cloud cover, wind, relative humidity, mulching, type and size of the plants, and the number of plants growing in a given area.

Water is required for the normal physiological processes of all plants. It is the primary medium for chemical reactions and transport of substances among the various plant parts. Water is an essential component in photosynthesis and plant metabolism, including cell division and enlargement. It is important also in cooling the surfaces of land plants by transpiration.

Water is a primary yield-determining factor in crop production. Plants not supplied with sufficient water respond in such ways as closing of stomata, leaf rolling, changing leaf orientation and reducing leaf and stem growth and fruit yield.

There are critical periods of growth when water stress can be most detrimental. It is imperative that a good moisture supply be maintained during seed germination and seedling emergence from the soil germination and seedling emergence from the soil. Young transplants should be watered immediately.

Many shallow rooted plants and newly planted trees and shrubs suffer water stress. Wilting followed by browning of leaf tips and edges are signs of water stress in plants.

The use of mulches can reduce water use and promote growth. A good mulch conserves moisture, prevents compaction, keeps soil temperature lower, reduces weed population and in case weeds do get a start, the weeds are much easier to pull if a mulch has been used. Check the depth of the mulch material to insure that it is 3 to 4 inches deep. Organic mulches tend to decompose or sometimes wash away, so frequent checks and replacement where necessary will help conserve moisture.

Simply put, a mulch is a layer of material covering the soil surface around plants. This covering befriends your plants in a number of ways.

• It moderates soil temperatures, prompting greater root development. Just like humans, plant roots prefer to be cool in summer and warm in winter. And they can be, under their year- round blanket of mulch.

• It reduces evaporation of water vapor from the soil surface thereby yielding three important benefits. First, it moderates soil moisture levels which promotes root growth. It also means you don't have to water as often, which reduces both your workload and your water bill.

• Mulching reduces washing and crusting of the soil during natural rainfall or irrigation. Falling drops of water can pound the upper ¼- inch of soil, especially a clay soil, into a tight, brick-like mass that retards necessary air and water movement to the root zone.

It also reduces disease problems. Certain types of diseases live in the soil and are spread when drops of water splash bits of infested soil onto the lower leaves. Mulching, and being careful when you water, can reduce the spread of these diseases. Mulching will also keep fruit clean while reducing rot disease by preventing soil-fruit contact.

Most weed seeds require light to germinate, so a thick mulch layer will shade them out, reducing your weed problems by 90% or more!

What can you use for mulch? Use any plant material that's free and not diseased. Weed-free hay or straw, leaves, grass clippings, compost, etc., are all great. Fresh grass clippings are fine for use around well-established plants but they should be cured for a week or so before being placed around young seedlings. When you are spreading green grass clippings directly around plants, spread a thin layer (less than 2 inches thick), and let it dry thoroughly before adding another layer to avoid a rotting, slippery mess. Vegetable and flower gardens should be mulched in the same way. First, get the plants established, and then mulch the entire bed with a layer 3 to 4 inches thick. Work the mulch material right up around the plant stems. When you finish, you'll have a beautiful and even expanse of mulch highlighted by healthy, productive plants.

Recent research indicates that mulching does more to help newly-planted trees and shrubs become established than any other factor, except regular watering. Grasses and weeds, especially Bermuda grass, which are allowed to grow around new plants are terrible thieves of moisture and nutrients. For shrub beds and new trees, apply mulch in a 4-foot circle.

So, don't throw away grass clippings and other forms of organic material. By using them as mulch, you will be recycling the fertilizer the material still contains. You will conserve moisture while enhancing the growth of your plants.

For more information about mulch and its many beneficial uses, see:

http://aggie-horticulture.tamu.edu/plantanswers/drought/mulches.html