Why do plants need water is an interesting question. Plants are like people, plants cannot live without water! And most plants require considerable quantities.
The amounts of water needed vary with the kinds of plants, conditions under which they are growing, stages of growth, rapidity of growth and other factors.
But no plant or plant part can stand complete desiccation and live… not even “dry” seeds in storage.
Because gardeners are concerned with promoting favorable plant growth, it is important that they understand the part water plays in this and that they master the “Art of Plant Watering.”
Almost all water used by plants comes from the soil.
Because soils vary greatly in their capacities to absorb and retain moisture, and make it available, it is necessary to consider them as well as the plants when dealing with water relationships.
Why Plants Need Water?
That plants absorb water is known by almost everyone. Not as widely, understood are the uses it serves and what happens to it, nor do gardeners always recognize what great amounts of water may be employed to advantage.
Lack of sufficient moisture is a common cause of limited growth, but excessive supplies can be harmful even to the point of causing death.
The know-how of maintaining a medium-moist state favorable to plant development is basic to good gardening.
Nearly all water absorbed by plants is passed out into the atmosphere as invisible vapor.
A very small proportion is broken down into its constituent elements, hydrogen and oxygen, which are used to manufacture carbohydrates… of that more later.
The giving out of moisture vapor is called transpiration. It may occur from any plant part exposed to the atmosphere but chiefly takes place from leaves and leaf-like organs (such as petals and the green stems of cacti and some other plants that serve as leaves).
To a lesser degree regular stems lose some water by transpiration.
Rate of transpiration is largely dependent upon environment.
Higher temperatures, increased light, decreased relative humidity and air movement all increase water loss; reverse conditions conserve moisture within the plant. The character of the plant is also important.
Kinds with large, soft leaves such as sunflowers, cucumbers and hydrangeas lose moisture much more rapidly than those with smaller exposed surfaces (and those covered with a more protective epidermis) such as portulaca, asparagus and pine trees (the needles are leaves).
Plants that grow naturally where water supplies are scant and the conditions that favor rapid transpiration are ever present (as along. the seashore and on windswept hillsides) usually have special devices to check rapid loss of moisture.
In general, the more total leaf surface a plant of a given kind has, the more moisture it transpires. During periods of rapid growth transpiration is more active than when growth is slow.
If transpired moisture were visible we would have a quite different picture of plants. Each would look something like a mist sprinkler or fog nozzle discharging into the air continually a spray of moisture absorbed from the earth.
To keep this going a constant supply of moisture at the roots is necessary; if it fails the spray is reduced and finally ceases but not before plant tissues are harmed or even killed.
Why is water necessary in plant tissues? What purposes does a constant flow through the plant serve?
The most obvious need for water is to keep soft-walled cells turgid or stiff. Without sufficient moisture, non-woody parts such as leaves wilt. Water inside the cells is under pressure and keeps the cell walls distended much as air in a blown-up balloon makes the skin of the balloon tight and rigid.
Transpiration of water serves another purpose. To some degree it helps, much as perspiration of animals does, in preventing the tissues from becoming so heated under the influence of sun that they are damaged to a serious degree or killed.
Water is the medium in which nutrient elements (in solution) are absorbed from the soil; plants are not able to take in solid particles.
Water serves, too, as a transportation system within the plant.
Nutrients in solution are conveyed from roots to leaves, and elaborated food materials such as carbohydrates… manufactured in the leaves move in water to growing parts and storage organs such as tubers and fruits.
Although very little of the total water taken in by plants is chemically broken down into hydrogen and oxygen, that which is broken down is of great importance.
Both hydrogen and oxygen are essential in the formation of carbohydrates (in the process of photosynthesis) and are derived from water.
Water thus supplies two essential nutrient materials; it is a plant food. This manufacture of carbohydrates from hydrogen and oxygen, obtained from water, and carbon dioxide gas, taken from the air, is called photosynthesis.
It takes place only in parts of plants that contain the green coloring matter called chlorophyll and only in light. It is fundamental to plant growth.
The reservoir of water from which plants draw their needs is contained in the soil. The water is absorbed by fine root hairs which occur abundantly behind every root tip.
Soils consist of solid particles and spaces between. The spaces may be filled with water, air, or air and water.
When they are filled only with water, bog conditions prevail and only plants especially adapted to such environments thrive.
If air alone fills the spaces the soil is too dry to support plant life.
The ideal state for the vast, majority of plants is for a film of moisture to surround each particle and air to fill the remainder of the spaces.
In well-drained ground this condition normally prevails for some distance from the surface and gardeners endeavor to maintain it.
But if you dig a deep enough hole, you will finally reach a level where all spaces between the soil particles are filled with water, where the soil is saturated.
The upper surface of this zone of standing water is called the water table. The depth at which it occurs varies greatly in different places and may fluctuate considerably in the same place.
The chief source of the water that plants take from the soil is rain and melting snow and ice. Not all that comes into contact with the ground penetrates.
A portion is lost by run-off and evaporation, the amount varying with the type of soil, grade, state of cultivation, rate of application and other circumstances.
In gardening practice, rainfall is also often supplemented by watering and irrigation.
Let us consider what happens when rain or water from a hose or other irrigating device descends on dry garden soil.
But first realize that even the dryest soil as it occurs outdoors contains some moisture.
This hygroscopic moisture, as it is called, comprises from one to five per cent of the weight of air-dry earth but it is completely unavailable to plants and so is of small importance to gardeners.
Rain or other water that comes in contact with dry soil surrounds each particle with a film of moisture and the surplus moves downward by capillarity or gravity until, if it is sufficient in quantity, all particles from the surface to the water table are surrounded by water films; any excess water serves to raise the level of the water table.
It is easy to understand how water moves downward, not so simple to comprehend that it moves upward in the soil too. This upward movement is by capillarity, the same phenomenon that causes liquid to rise above its surface level up a paper towel dipped into it.
Because of this capillary rise, water taken from upper layers by roots and lost from the soil surface by evaporation is gradually replenished from below as long as the soil below contains more moisture than that above.
But such capillary rise is not believed to make available to plants any considerable amount of water from the zone of saturation if the water table is more than fifteen feet beneath the surface.
Capillary rise is slow, especially in clay soils, and modern investigators believe that its importance in providing water for plant needs has, in the past, been over-emphasized except in so far as it applies to river bottom lands and areas close to lakes and ponds.
In urban and suburban developments, grade changes and drainage systems often materially raise or lower the natural water table and change completely the amount of moisture plants can obtain from that source.
These changes frequently harm established trees.
The amount of water retained as film surrounding soil particles after the soil has been thoroughly wet and all surplus has drained down to the water table varies tremendously with different soils.
A high clay or humus content makes for the retention of substantial amounts; soils containing sand or clay and those deficient in organic matter hold much less.
The water content of the soil when the equilibrium described above has been reached is known as the field capacity of the soil, and this normally varies from 5 to 35 per cent of the soil weight.
When the moisture content of the soil is at or near field capacity, garden plants in growing condition normally function best.
If the soil contains substantially more water, the air spaces between the particles are reduced and the roots are damaged or killed because they cannot obtain sufficient oxygen they are, in effect, suffocated.
When the soil contains substantially less water than its field capacity, plants are unlikely to be able to obtain sufficient water for optimum growth.
Cultural Practices Used to Control Water are:
- Drainage, usually by the installation of agricultural drains, to lower the water table where it is permanently too close to the surface
- The addition to the soil of bulky organic matter to act as a sponge and so retain additional moisture
- The incorporation of clay or clayey soil in light sandy soils to improve their moisture-holding abilities
- Mulching the soil surface to check excessive loss of moisture by evaporation
- The maintenance of a loose top inch or so of soil by frequent surface cultivation to check evaporation
- The addition of water by sprinkling and other irrigation methods
Soil Water Plant Needs
When the soil contains insufficient water for plant needs, better results are obtained from applying water before wilting takes place.
Experiments have shown that if a plant is allowed to wilt, even though it recovers after watering, many hours or even days elapse before photosynthesis and other life processes that require an adequate water supply in order to function are able to do so normally.
Gardeners must know that wilting due to lack of water can occur even though the soil contains adequate amounts. Anything that interferes with the plants’ ability to rake up water will cause this.
Obvious causes are severe damage to roots due to transplanting, insect damage, over-fertilization and even an over-wet condition of the soil (which causes the roots to rot).
On windy, sunny days water may be lost by transpiration more rapidly than it can be replenished by the roots even though the root system is undamaged and the soil contains plenty of water.
Gardeners should aim to supply optimum amounts of water that plants can use advantageously.
At times, when the plants cannot take up sufficient water to prevent wilting, even though it be present in the soil, everything possible should be done to check loss of moisture by transpiration.
Methods of accomplishing this are shading, protecting from wind and other currents of air, humidifying the air by sprinkling the plants and ground with water and spraying with transpiration retarding liquids (used at transplanting time).
Newly-set plants that have foliage should be soaked thoroughly so that the roots (which of necessity are damaged somewhat at planting time) have the maximum amount of moisture available to them.
This watering also serves to settle the soil and bring the moisture film that surrounds each particle into intimate contact with the roots.
When watering established plants, give sufficient water to thoroughly saturate the soil to a depth of 8 inches or more; then do not water again until the plants need it.
The rate of application should be adjusted so that it is not greater than the soil can absorb. Very dry soils often absorb water slowly at first.
While watering in the very early morning or late evening is a little more economical (not as much moisture is lost by evaporation), there need be no hesitancy about watering in sunshine.
This does not harm plants. (July and August showers occur without harm on sunny days, don’t they?) Late in the day watering may induce fungus diseases if the foliage remains wet for more than an hour or two.