All plants need water. When to apply water and how much are decisions anyone growing ornamental plants or lawns must make. But how do we know when to water and how much?

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

Water is lost in the soil two ways: either it evaporates or it’s absorbed by the 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.

Ever notice how after it rains or irrigation takes place, the plants look fine? As moisture levels decline, plants may wilt briefly during the hottest parts of the day but recover quickly as temperatures fall. As 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.

Watering essentially manipulates the relative amounts of air and water in the landscape soil. Because roots need both air and water and occupy the same space in the soil, you must try to maintain a balance of the two. What complicates this is soil types, the weather, the type of plant 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 clay. Take into account the amount of rainfall since the last watering. To replace the water lost through transpiration, apply an inch of water in any week without rain. Remember that this would need to be adjusted based on soil type (drainage) and weather (sunny or cloudy). The amount of water captured by the soil can be far less than the amount of water that fell from rain. 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 or lawns. Some of that rainfall will end up down the sewer as runoff.

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 the application and the manner in which the water is applied. The amount of water should not exceed the amount of water that is evaporated. Overwatering results in saturated soils or runoff of excess water that wastes money and resources.

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 a container 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 give you a rough idea of how long it takes your watering system to apply an inch of water.

The next consideration is whether the soils will accept an inch of water in one sprinkling. It may be necessary to 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 may be 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 for trees and shrubs is mulching. You can find more information about mulching on our website at www.owentree.com/mulch.

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 may lead to increased disease problems in plants and grasses. Shrubs in a 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. As you can see, there is great importance to watering your lawn and shrubs properly. It can seem very complex and with that, always know we are here to help in any way we can. You can find more information on landscape maintenance at www.owentree.com or call 800-724-6680 to have one of our arborists answer your landscape maintenance questions.