PROPER WATERING
OF PLANTS
by Jerry Parsons, Ph.D.
Horticulture Specialist, Texas Agricultural Extension Service in San Antonio

Has your water bill reached astronomical proportions? Maybe an evaluation of your watering methods will result in more efficient use of the available water and at the same time reduce the amount you are using. First, check your methods of application. There is no use in applying water any faster than the soil will soak it up. If applied faster than this, the surplus will either flow into the street or else flood your neighbor. Sandy soil will usually absorb water almost as fast as water can be applied but tight clay soils will absorb water very slowly. Select the method of application that best fits your soil type. Drip irrigation systems for shrubs makes the most efficient use of water for beds.

Secondly, use a mulch wherever possible. A good mulch of grass clippings, pine bark or dry leaves conserves moisture, prevents compaction, keeps soil temperature lower, reduces weed population and in case weeds and grass do get a start, they are much easier to pull if a mulch has been used. Periodically check the depth of the mulch material. Organic mulches tend to decompose or sometimes wash away, so frequent checks and replacement where necessary will help conserve moisture.

While night-time watering can be conductive to development of plant diseases, one does get more efficient use of the water in the early morning and late evening when evaporation rates are lowest.

Last, but by no means least, is the practice of doing a thorough job of watering each time it becomes necessary to irrigate. A thorough watering at 7- to 10-day intervals encourages deep root penetration and full utilization of the available soil moisture. Just because plants happen to wilt during the heat of the day doesn't mean the soil is dry. If plants remain wilted until morning then the addition of water is warranted.

Don't stop watering completely as plants can die due to moisture shortage during periods of high temperature and high wind movement. Just remember to use the precious resource as efficiently as possible.
One of the best uses of a valuable resource such as water is to insure the survival of recently established plants. Summer is a critical period for recently planted shade trees and ornamental plants. Not yet completely recovered from the effects of transplanting, they often need special care to withstand dry weather conditions.

Water is a recently established plant's prime need. Hot summer days and drying winds take a great deal of moisture from the leaves and stems. This water must be replaced through root absorption of soil water. When transplanting a balled-and-burlapped tree (roots of container grown trees and shrubs are not removed), most of its wide-spreading, deep-growing roots are cut. In its new location, the only water which a newly established tree can utilize is that which is in the soil close to the tree. If this soil area becomes dry, the leaves wilt, turn brown and drop. Death of many branches or even the entire tree or shrub can follow.

It is wise to water at regular intervals all trees planted within the past year. IF the soil is sandy, water once a week; if it is clay or loam, thorough watering every ten days or two weeks should suffice. Using an open-end hose, regulate the water flow so there is no run-off and let it flow until the soil around the tree is saturated.

If the tree or shrub was worth planting, it is certainly worth the little extra effort required to keep it in good growing condition.
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WATERING VEGETABLES AND FLOWERS

The only fate worse than thirst for a plant is death. In fact, they often accompany one another! Even if some folks are wise enough to know when to water a thirsty plant just seconds before it crosses death's threshold, these procrastinators are still losers. When a plant thirsts and is severely stressed, overall vigor and production is decreased. This is especially true of plants which are expected to produce fruit. Not only will total yield be decreased, but fruit quality will also decline. Carrots, onions and tomatoes start cracking! Eggplant fruit gets bitter! Shrubs mysteriously die! Flowers bloom with mediocrity! Trees do not grow rapidly! The house's foundation cracks! What is causing all of these calamities? Lack of and improper application of water is the culprit of all of these problems.

Most people don't intentionally make their plants suffer. There are three categories in which the thirst inflictors fall - - those who don't know when to water, those who don't know how to water and those who are so absent-minded they forget. Some may qualify for more than one of these categories.

How do you know when to water? Some people say water when the plant wilts. When a plant wilts, it is obviously in a stressed condition and the damage (foliage burn, foul-tasting fruit, production loss) has already occurred. Besides, does the wilting of a plant indicate drought? I have seen wilted plants standing in water, i.e., they were water-logged. Plants wilt with root rot diseases. So wilt is the worst possible indication. Soil moisture is the best criterion for watering. If the soil moisture is adequate, don't water, even if the plant is wilted. To test for soil moisture, probe around plants with your finger. If the soil is moist several inches deep, your plant is alright.

Rooting depth is the most limiting plant characteristic in relation to water uptake. Crops with shallow root systems, such as onions and celery, require more frequent irrigations than deep-rooted crops such as pumpkins, tomatoes, and watermelons. Very shallow rooted crops include celery, lettuce, onion, radish; shallow-rooted crops include broccoli, cabbage, cauliflower, Chinese cabbage, cucumber, muskmelon, pepper (transplanted), spinach, and tomato (transplanted); intermediate rooted crops include bean (snap), beet, carrot, eggplant, pea, pepper (seeded), rutabaga, and summer squash; deep-rooted crops include asparagus, beans (lima), parsnip, pumpkin, winter squash, tomato (seeded), and watermelon. The most critical stages of growth of vegetable crops when moisture stress is the most damaging to yields is asparagus (fern growth), snap bean (pod-filling), broccoli (establishment, head development), cabbage (establishment, head development), carrot (establishment, head development), cauliflower (establishment, head development), celery (establishment, rapid growth during hot periods), sweet corn (tasseling, silking, and ear filling), cucumber (flowering, fruit enlargement), eggplant (flowering, fruit development), lettuce (head development), muskmelon (flowering, fruit enlargement), onion (bulb enlargement), pea (flowering, pod-filling), pepper (transplanting, fruit set and development), squash pumpkin (flowering, fruit development), radish (root enlargement), summer squash (flowering, fruit development), tomato (flowering, fruit set and enlargement) and turnip (root enlargement).

Now that you know when to water, you may not know how. The "how" may be the most important part. First of all, plants need to be deep watered to stimulate a larger root system. This larger root system will be advantageous in utilizing every drop of available moisture during periods of severe drought. Deep watering involves soaking.

Floor or furrow irrigation, a technique used by some farmers, involves mounding the soil into raised beds (mounds on which to plant) and running the water down the furrows between the beds. The bed centers or rows are spaced at least 30 inches apart. The bed soil is firmed to enable it to conduct water to the top of the bed. In this way, moisture will reach the roots of young plants. The ditches or furrows between these rows are "flooded" or filled with water. The roots of the plants growing in rows on the beds have direct contact with the water supplied. This technique results in deep soaking which encourages vigorous development of root systems.

The most obvious disadvantages of this type of irrigation is water use. The system involves wetting the entire garden rather than just the area where plants are growing. A sprinkle system has the added disadvantage of wetting the plant foliage which can encourage plant diseases.

There is an easier way to deep water. It is called drip irrigation. "Drip or trickle" irrigation is a unique method of irrigation. It allows precise applications of water in the immediate vicinity of plant roots. Soil moisture in the area around the plant is maintained at a uniformly high level throughout the growing period. Small amounts of water are applied frequently, perhaps daily, to replace that withdrawn by soil evaporation and plant transpiration. Growth and production of a plant is greater when they are not subjected to wet and dry cycles which normally occur with other irrigation methods. This is very apparent in situations where plants are growing near a leaking faucet. Such plants always out perform neighboring plants. This has often been verified in tests throughout the country. Data indicates that tomatoes can yield 30-40 percent more fruit and peppers as much as 30 percent more fruit when grown with drip irrigation. All of this increase in production occurs even though much less water is used. Distribution and evaporation losses are minimized. Less of the total soil area is fully wetted than with sprinkler and furrow systems. Normally, only 25 percent of the soil surface is wetted with drip. This significantly reduces the amount of water required for irrigation.

Drip irrigation also simplifies your irrigation procedures and reduces labor requirements. This is imperative if you have a family which is allergic to dirt and sweat who will let plants die of thirst when you are away if an easy watering technique is not available. Drip systems can be easily activated from one faucet. A drip irrigation system also takes care of the third category of thirst inflictors who "forget" to water certain plants. Once the drip hose is installed around shrubs and vegetables, it never "forgets" to water - - it specifically waters each and every plant. Of course, you must remember to activate the drip system for three hours a day every other day by turning the water faucet on. If you can't remember that, then you are too derelict to be entrusted with plants anyway! In case you can remember to turn the water on BUT NOT to turn it off, simply purchase a water timer device which will "remember" for you.

So before the dry weather destroys your plants, spread a little happiness around - - install a drip system.

LAWN WATERING

"Spring has sprung; fall has fell; summer is here; and it's hotter than...it should be!" and "drier than it should be" should also be included in this rhyme. If Texans expect to survive this dry summer, we HAVE to be water smart, i.e., utilize our most precious resource, water, wisely. Over the last seven years, researchers have made advances in understanding turfgrass water use rate (WUR). The idea is to develop grass systems that lose less water to the atmosphere and maintain more favorable soil moisture condition for plant growth.

However, people must understand that low water use does not necessarily mean less irrigation. For instance, taller mowed grasses have higher WUR than shorter mowed grass, but the amount of irrigation needed to keep turfs at acceptable levels is less for the taller grass than the shorter.

The two major aspects of mowing that influence water use are height and frequency. Grasses mowed at higher cutting heights have a reduced canopy resistance and, therefore, use more water than short mowed grass. Mowing frequently and at a short height increases turf density. Dense turf resists the upward movement of water vapor through the turf canopy. A dense turf with a tight canopy also resists air movement down into the turf canopy. The net result is less moisture lost from the turf canopy to the atmosphere. This process ultimately reduces turfgrass evapo-transpiration (ET). With higher mowed turf the leaf canopy that expands above the mowing height is less dense. This allows for more air mixing that results in higher water use rates. It is important to note that taller grass transpires more water, but has a more extensive root system that draws water from a larger soil reservoir than shorter grass. Thus, taller grasses avoid soil drought and plant wilt by expanding their roots into soil areas with enough moisture.

In contrast, lower mowing heights result in limited root systems that need more frequent irrigation to supply water to a shallower root system. To reduce irrigation needs, one should mow frequently at the highest feasible height. These management practices enhance canopy resistance and minimize detrimental rooting responses.

Proper fertilization is also important, if not critical, to effective water use on lawns. The primary goal of turf fertilization should be an increased shoot density with less emphasis on a darker green color. People should strive to meet, but not exceed, the nutritional needs of the turf.

Nitrogen is the primary nutrient used to regulate turfgrass density and color. Water use rate increases with increased nitrogen nutrition. This occurs because leaf expansion above the normal mowing height occurs at a much faster rate. Leaves forced into this upper boundary layer lose water at a faster rate because they are in an area of greater air movement and reduced canopy resistance.
Over-stimulation of growth with nitrogen is easily detected by an increased need for mowing. Under these conditions, turf will have high WUR, and rooting development will be reduced. In time, excessive leaf growth will demand a greater water supply that cannot be met by the reduced root system. Turf wilt occurs more frequently as this imbalance develops. What follows is usually a decision to increase the frequency - and sometimes the amount - of irrigation to offset the symptoms of wilt. In this case, a poor functioning root system resulted from an increased water demand. This increase was, in turn, caused by improper nitrogen fertilization and frequent irrigation. People interested in conserving water should use the lowest amount of nitrogen that gives the desired turfgrass quality and function. This was the basis for the Extension recommendation of only two fertilization applications annually -- a slow release in the spring after the second mowing has occurred and a Winterizer analysis in October.

Potassium and iron are two additional elements that may give an extra edge in managing turf exposed to drought stress. Potassium and iron have been reported to increase root growth which may account for their role in reducing wilt and improving drought avoidance. Reduced levels of nitrogen combined with iron can result in turf greening similar to normal rates of nitrogen. The implication here is that applications of iron plus nitrogen, especially in the spring, will result in a lower rate of vertical leaf growth and reduced WUR.

Current irrigation practices have probably involved partly as a result of the equipment available for irrigation rather than a complete understanding of turfgrass water needs. Manual sprinkler systems required a lot of time and trouble for a single irrigation. When using these systems, it is desirable to provide as much water as possible during a single irrigation. Irrigate as deeply and infrequently as possible to promote an extensive root system. The main purpose for irrigating deep (to the bottom of the effective root zone) and infrequently is to develop an extensive root system that continually expands into regions of available soil moisture. This prepares the plant to avoid soil drought.

The main disadvantage of irrigating too frequently has been cited as a shallow root system that becomes dependent on continued frequent irrigation.

For more comprehensive information about watering and mulching, see
"Efficient Use of Water in the Garden and Landscape" at:
http://aggie-horticulture.tamu.edu/extension/homelandscape/water/water.html

and

"Mulches for low-cost/low-maintenance landscapes"