Plants’ needs and tolerances of soil moisture depend on the climate to which they are adapted. Plants adapted to areas with long dry spells have evolved methods for avoiding water stress, and can tolerate periods of dry soil. However, arid soils usually have few soil fungi, and these plants have few defenses against root-rot organisms. Some plants are adapted to wet areas that occasionally dry up, and these plants can tolerate wet or dry soil. At the opposite climate extreme, plants adapted to rain forests "expect" it to rain every day, and have little tolerance for dry soil.

Weather and Other Factors

When you irrigate, you are replacing the water that has evaporated from the soil surface and the water that the plant has transpired (water extracted from the soil moves up the plant and is lost as vapor from the leaves). Together these two processes are known as evapotranspiration. The evapotranspiration rate is influenced dramatically by such factors as temperature, humidity, wind, light, day length, and whether or not the soil is mulched. The faster the rate of water loss, the sooner you will need to add water.

During hot, dry, or windy weather, plants transpire at a faster rate and more water evaporates from the soil surface than during cool or humid weather. Plants in full sun transpire more rapidly than plants in shady locations; in addition, more water evaporates from soil in a sunny site than in a shady one.

Day length is an important factor because plants transpire only during daylight. June is usually the month with the greatest water demand because it has the longest days, even though it may not be the hottest month of the summer.

Mulching also affects the evapotranspiration rate. A thick layer of mulch keeps the soil cool and cuts down on the amount of water that evaporates from the surface. Thus, mulching reduces the amount of water that must be added to your garden.

The Touch Test

There are many methods, some of them very sophisticated, for figuring out when to water a plant. A very simple but reliable method is to feel the soil in the root zone of the plant. Don't just feel the surface since it dries out first and is not a true indicator of soil moisture. Use a probe or dig down to the root zone with a trowel. Depending on the plant, you should irrigate when the soil in the root zone reaches a certain level of dryness.

Plants can be divided into three broad groups based on moisture requirements:

1. The first group of plants can't tolerate drought. They thrive in soil that is moist but not wet. Water these plants when the soil is damp. When you touch damp soil, it feels cool and wets your finger but does not muddy it. If you squeeze a ball of damp soil, water will not run out.
2. The second group of plants is made up of those that require an average amount of water. The soil in the root zone should be just barely moist before watering. Barely moist soil feels cool and moist but doesn't dampen your finger. The soil is crumbly but it is not dry and dusty.
3. The third group of plants can't tolerate wet soil. Although these plants can usually withstand long periods of drought, they grow better when watered periodically. Water these plants as soon as the soil in the root zone feels completely dry and no longer cool to the touch. Keep in mind that these plants are drought tolerant only after they are well rooted. When they are put into the ground, they should be treated as needing average water until they are established. This usually takes one to two full seasons.

Watch Your Plants

In addition, it is always a good idea to observe plants for signs of water need.

• Curling leaves are usually the first indication of stress. The surface area of the plant is being reduced to cut down on transpiration (loss of water from the leaves).
• Normally shiny leaves grow dull. Bright green leaves take on a blue or gray-green appearance.
• New growth wilts or droops and older leaves turn brown, dry up, and fall off.
• Flowers fade quickly and drop prematurely.

In most cases, these symptoms signal a lack of water, and the plant will recover if watered soon enough.

Several instruments can be purchased to measure the water content of soil. Some of these are left in place permanently, some are carried around to take instant readings, and others regulate automated systems, turning them on when the soil needs water. The most commonly-available instruments are called tensiometers.

Applying Water

Basically, there are four ways to distribute water to a garden - by hand watering, sprinkling, flooding, or through drip irrigation. Each of these watering methods has benefits and drawbacks.

1. Hand Watering
   Hand watering usually means holding the end of a hose to deliver water to plants. The hose might have a sprinkler nozzle, a watering wand, or just a thumb to break up the stream of water. Hand watering might also include hauling water to plants in buckets, but this is seldom necessary in modern gardens. Hand watering relies on the gardener's skill to know when each part of the garden has received enough water. If the gardener is impatient or inexperienced, it's easy to water too shallowly.
2. Sprinkling
   Sprinkling, with a hose-end sprinkler or an underground sprinkler system, works best where plants are spaced close together, as in a lawn or ground cover. It's also effective when plant roots cover a large area, such as in an orchard or grove where the trees are spaced far apart, but the roots need water over the entire surface of the area.
3. Flooding
   This method of irrigation, which does exactly what it says, is particularly valuable for directing large amounts of water into basins around deeply rooted trees and large shrubs. It's also useful for watering long rows of vegetables.
4. Drip irrigation
   This system of flexible tubing that distributes water directly to a plant's roots is valuable in areas where water is in short supply. It has the added advantage of keeping moisture off leaves of plants that might be affected by mold or other diseases, but it's not well suited for watering expanses of lawn.

To avoid run-off, apply water at a slow enough rate that the soil can absorb it. If your irrigation system applies it too fast, let the system run until puddles form or water runs off the garden, then shut it off for an hour or so to allow the soil to absorb what you've applied. This is called "cycling" irrigation. Many sprinkler timers have settings to allow two or three cycles at each watering.