Efficient Use of Water in the Garden and Landscape
Larry Stein, Extension Horticulturist
Efficient, Responsible Water Use
The danger of exhausting valuable aquifers by excessive pumping is paralleled by the threat of polluting the groundwater with industrial, agricultural, and home landscape contaminants. Nitrates from excessive and untimely fertilization are especially threatening.
Plants, Soils, and Water
When water is applied to the soil it seeps down through the root zone very gradually. Each layer of soil must be filled to “field capacity” before water descends to the next layer. This water movement is referred to as the wetting front. Water moves downward through sandy coarse soil much faster than through fine-textured soil such as clay or silt.
If only one-half the amount of water required for healthy growth of your garden or landscape is applied at a given time, it only penetrates the top half of the root zone; the area below the point where the wetting front stops remain dry as if no irrigation has been applied at all.
Once enough water is applied to move the wetting front into 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 by 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, and the type, size, and a 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 the movement of substances through 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 with insufficient water respond by closing the stomata, leaf rolling, changing leaf orientation, and reducing leaf and stem growth and fruit yield.
Not all water is suitable for use as an irrigation source. Prior to implementing an irrigation system, the water source should be tested for water quality. The instructions for testing and the testing results may be obtained from the Texas AgriLife Extension Service or an independent water lab. The results of the test will determine if the water is suitable for irrigation or reveal if any special tactics will be required to overcome quality deficiencies.
Major factors in determining water quality are its salinity and sodium contents. Salinity levels are expressed as categories based on conductivity.
Category C-1 represents a low salinity hazard. Water in this category has a conductivity of fewer than 2.5 millimhos/cm. It can be used for most crops without any special tactics.
Category C-2 reflects salinity that results in conductivity of 2.5 – 7.5 millimhos/cm. The water in this category can be used for tolerant plants if adequate leaching occurs.
Category C-3 is high salinity water that has conductivity in the 7.5-22.5 millimhos/cm range. It can not be used effectively on poorly drained soils. On well-drained, low salt soils, the water can be used for salt-tolerant plants if it is well managed.
Category C-4 water is very high salinity and cannot be used for irrigation on a regular basis.
Sodium is a major component of the salts in most saline waters but its impact can be detrimental to soil structure and plant growth beyond its status as a component of salinity. The level of sodium (Na) in irrigation water is another important factor of quality.
There are critical growth periods when water stress is most detrimental. It is imperative that a good moisture supply be maintained during seed germination and seedling emergence from the soil. Water transplants immediately. Many shallow-rooted plants and newly planted trees and shrubs suffer water stress. Wilting followed by browning leaf tips and edges are signs of water stress.
To determine if irrigation is needed, feel the soil in the soil zone where most roots are located. Table 1 explains how to determine the soil’s moisture by feel. As you gain experience feeling the soil and observing plant symptoms, it will help you time irrigations.
Proper watering methods are seldom practiced by most gardeners. They either under or over water when irrigating.
The person who under-waters usually doesn’t realize the time needed to adequately water an area; instead, he applies light, daily sprinklings. It is actually harmful to lightly sprinkle plants every day. Frequent light applications wet the soil to a depth of less than 1 inch. Most plant roots go much deeper. Light sprinkling only settles the dust and does little to alleviate the drought stress of plants growing in hot, dry soil. Instead of light daily waterings, give plants a weekly soaking. When watering, allow the soil to become wet to a depth of 5 to 6 inches.
This type of watering allows moisture to penetrate into the soil area where roots can readily absorb it. A soil watered deeply retains moisture for several days, while one wet only an inch or so is dry within a day.
In contrast, there are those who water so often and heavily that they drown plants. Symptoms of too much water are the same as for too little. Leaves turn brown at the tips and edges, then brown all over and drop from the plant. These symptoms should be the same since they result from insufficient water in the plant tissue.
Too much water in soil causes oxygen deficiency, resulting in damage to the root system. Plant roots need oxygen to live. When soil remains soggy little oxygen is present in soil. When this condition exists roots die and no longer absorb water. Then leaves begin to show signs of insufficient water. Often gardeners think these signs signal a lack of water, so they add more. This further aggravates the situation and the plant usually dies quickly.
Thoroughly moisten the soil at each watering, and then allow plants to extract most of the available water from the soil before watering again.
A mulch is a layer of material covering the soil surface around plants. This covering befriends plants in a number of ways.
It moderates soil temperature, thus promoting greater root development. Roots prefer to be cool in summer and warm in winter. This is possible under a year-round blanket of mulch.
Mulch conserves moisture by reducing the evaporation of water vapor from the soil surface. This reduces water requirements.
Mulching prevents compaction by reducing soil crusting during natural rainfall or irrigation. Falling drops of water can pound the upper 1/4 inch of soil, especially clay soil, into a tight, brick-like mass that retards necessary air and water movement to the root zone.
Mulching also reduces disease problems. Certain types of diseases live in the soil and spread when water splashes bits of infested soil onto a plant’s lower leaves. Mulching and careful watering reduce the spread of these diseases. Mulching also keeps fruit clean while reducing rot disease by preventing soil-fruit contact.
Most weed seeds require light to germinate so a thick mulch layer shades them and reduces weed problems by 90 percent or more.
Any plant material that is free of weed seed and not diseased is suitable for mulch. 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 cure them for a week or so before placing them around young seedlings.
Mulch vegetable and flower gardens the same way. First get plants established, then mulch the entire bed with a layer 3 to 4 inches thick. Work the mulch material up around plant stems.
Organic mulches decompose or sometimes wash away, so check the depth of mulches frequently and replace them when necessary.
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 bermudagrass, which grow around new plants rob them of moisture and nutrients. Mulch the entire shrub bed and mulch new trees in a 4-foot circle.
Four distinct methods of irrigating are sprinkling, flooding, furrow-irrigation, and drip irrigation. Consider the equipment and technique involved in each method before selecting the “right” system. Select a system that will give plants sufficient moisture without wasting water.
Sprinkler irrigation, or “hose-end overhead sprinkling” as it is sometimes called, is the most popular and most common watering method. Sprinkler units can be set up and moved about quickly and easily. They are inexpensive to buy, but if used incorrectly they can be extremely wasteful of water.
Sprinkler equipment varies in cost from a few dollars for a small stationary unit to $50 or more for units that move. A solid-set sprinkler system for a small garden could cost more than $100, although it is not necessary to spend that much. The best investment is an impact-driving sprinkler that can be set to water either a full or partial circle.
Sprinkler irrigation has its advantages. The system can be used on slopes as well as level areas. Salt does not accumulate because water percolates downward from the surface carrying salts with it. Different amounts of water can be applied to separate plantings to match plant requirements.
However, there are some drawbacks. Use sprinkler irrigation early in the day to allow time for the soil surface to dry before nightfall. Irrigation in a wind of more than 5 miles per hour distributes the water unevenly. If you have poor quality water, the mist which dries on leaves may deposit enough salt to injure them. Strong winds may carry the water away to neighbors’ yards. Some water also is wasted by attempting to cover a square or rectangular area with a circular pattern. Move the sprinkler unit at regular intervals if the garden is larger than the sprinkler pattern. With caged tomatoes or trellised crops, set the sprinkler on a stand to allow the spray to arch up and over the top of the leaf canopy. Improper timing and operating in wind or at night can damage plants and wastewater.