All plants need water. When rainfall does not provide water in the necessary amounts, the gardener must. When to apply water and how much are decisions anyone growing ornamental plants must make.

All soils have air spaces, and it is in the air spaces that soil water is stored. When water falls on the soil, it moves into the spaces in the soil. The rate at which the water moves into the soil is called the infiltration rate. After rain or watering, gravity pulls water out of the largest air spaces deeper into the soil. The more quickly this happens, the better the soil drainage. Sandy soils have many large air spaces, so they are well drained, but the small spaces in clay make it poorly drained.

Water is lost from the soil in two ways: either it evaporates from the soil surface, or it is absorbed by plant roots and moved to the leaves where it is transpired to cool the plant. The combination of evaporation and transpiration is called evapotranspiration. As water is lost from the soil, soil particles hold the remaining water more tenaciously and plants have a harder and harder time absorbing it.

The progressive drying of the soil can be seen by watching the plants. After rain or irrigation, the plants look fine. As soil moisture levels decline, plants may wilt briefly during the hottest part of the day but recover quickly as temperatures fall. As soil moisture declines further, the wilting becomes more pronounced, but the plants usually recover during the cool nights. Eventually the soil becomes so dry that the plants are wilted all the time. This level of soil dryness is called the permanent wilt point.

The landscape manager essentially manipulates the relative amounts of air and water in the landscape soil by irrigating. Because roots need both air and water yet occupy the same space in the soil, the gardener must try to maintain a balance between the two. This can be complicated by soil types, the weather, types of plants and the capability to provide the proper amounts of water.

When is it time to water?

Watering will need to be done more often on sandy soil than on clay. You must also take into account the amount of rainfall since the last watering. One way to approach the decision of when to water is to try to replace the water lost through evapotranspiration. This would involve applying about an inch of water in any week without rain. This would have to be adjusted depending on soil type (drainage) and weather (sunny or cloudy).

Any consideration of the amount of rainfall that occurred in the previous week must also include how the rain was distributed. Large amounts of water can fall in a short time during thunderstorms, but the water may fall on the soil faster than the soil will absorb it. Excess merely runs off. The amount of water captured by the soil can be far less than the amount that fell. An inch of rain falling in a very short period of time on clay soil with a low infiltration rate does not translate into an inch of usable water for plants.

How should the water be applied?

Apply water in any way that provides the maximum amount of usable water in the shortest period of time. Two factors must be considered: the rate of application and the manner in which the water is applied.

The rate of application should not greatly exceed the infiltration rate of the soil. If the application rate is too high, much of the water will run off the irrigated area into the street or other areas where it will do no good.

You need to determine how long it takes your watering system to apply an inch of water over the entire area being irrigated. A simple way to do this is to run the sprinkler and catch water in containers at various points under the sprinkler pattern. Keep track of the time it takes to collect about an inch of water in all of the containers. Most sprinklers do not apply water evenly, so more water will accumulate in some containers than in others. But this will give a very rough idea of how long it takes your watering system to apply an inch of water.

The next consideration is whether the soil will accept an inch of water in one sprinkling. It may be necessary to move the sprinkler or stagger waterings over more than one session. The water should be applied in the shortest possible time, however, not spread out over the entire week.

The best way to water with overhead sprinklers is to start in the morning and stop early in the day so that the plants have time to dry off before the cool night temperatures occur. Foliage that remains wet for long periods, especially in cool night air, is vulnerable to attack by disease organisms that require moisture to get established.

Conserving soil moisture

Conserving soil moisture is one way to reduce the effects of dry weather or to reduce the amount of watering necessary or the frequency of watering. The most widely used method of conserving soil moisture is mulching.

Watering problems

A number of problems on landscape plants can be influenced by soil moisture, either how much water is applied or how it is applied.

Overhead sprinkling is a common practice in most landscapes, but water that sits on the leaves during the night can lead to increased disease problems in susceptible plants. Applying water overhead shortly after applying pesticides may wash off much of the spray.

Shrubs in the foundation planting may suffer from serious water-related problems. Plants placed directly in front of a downspout can develop root problems during rainy weather due to the large amount of water put in their root zone from the downspout. The other extreme also occurs in foundation plantings. If plants are placed under a large overhang, the overhang prevents rainfall from reaching them and they suffer from too little water.

Homeowners with swimming pools will sometimes water their landscape plants with the water used to backwash the filter. The water is no problem, but the chemicals in the water can injure plants.

Keeping the lawn growing can involve frequent watering, especially if underground sprinklers attached to a time clock are used. It is impossible to water the lawn without also watering the other plants in the landscape. When the lawn is being watered frequently, check the root zones of trees and shrubs to make sure they are not getting too much water. Plants growing in areas that may be slightly lower than the rest of the lawn are most likely to be overwatered.