University of Vermont Extension 
Department of Plant and Soil Science

Summer News Article 


By Dr. Leonard Perry, Extension Professor
University of Vermont

All plants need water. When natural 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's 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 time absorbing it.

Watering will need to be done more often on sandy soil than on clay. You also must 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 means 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).

If it rained in the previous week, consider 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. The excess merely runs off.

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.

So, 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. (This will only provide a rough idea as most sprinklers do not apply water evenly, so more water will accumulate in some containers than in others.)

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.

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