On the surface, watering home landscape or garden plants appears to be simple and straightforward. However, it is complicated by a number of factors including climate, available water source, weather, topography, soil type and the irrigation system available. As a result, irrigation is poorly accomplished by most homeowners. In fact, many homeowners fall prey to over-watering, wasting money and natural resources. Irrigation has a greater impact on plant health in Idaho landscapes than any other input, so doing it properly is critical.
Many landscape trees are lost each year because the root zone was not adequately watered in the late fall, and young trees are often damaged or killed by over-watering. Water-stressed plants are subject to increased insect and disease problems and decreased winter hardiness. Excess watering leaches out nutrients and pesticides that can pollute ground water. As you can see, it is very important to understand how to apply the proper amount of water. Important decisions associated with proper irrigation include choice of equipment, determination of irrigation frequency, and knowing how much to apply.
There are several types of irrigation systems, each with advantages and drawbacks. Most homeowners use some type of sprinkler application, some manually controlled and others automated, while some flood irrigate. Drip irrigation is becoming popular as it can conserve large amounts of water, as well as aid in weed control. Whatever the method, it is important to know the application rate of the system and the application is uniform. To properly irrigate, the root zone should be completely filled with water, and then allowed to partially dry between irrigations so to ensure adequate oxygen for the roots. The root zone for lawns is about 10 to 12 inches, vegetable gardens 18 inches, shrub beds up to 2 feet, and trees approximately 3 feet.
Three important pieces of information are needed to properly irrigate: 1) the application rate of the system, 2) the amount of water plants are using, and 3) the amount of water required to fill the root zone. The University of Idaho has published a detailed and valuable document on using this information to determine best irrigation practices, Watering Home Lawns and Landscapes.
Soil type (texture) has a large impact on irrigation practices. Sandy soils may hold enough water for only a single day of plant growth. Fine-textured such as silt or clay soils may hold enough for 5 or 6 days. Consequently, it is important to adjust watering practices based on soil texture. In principle, sandy soils will need a very light application of water on a very frequent basis. Heavier soils will need water less often, but will need a larger amount at each irrigation event. Soil texture does not change the amount of water plants need or use, but it does change the schedule (frequency) for supplying the water.
One of the problems of irrigating landscapes and gardens is that multiple plant species, each with their own water requirements and rooting depths, are grouped together. For most plants in a mixed garden or landscape, it is adequate to water to a depth of about 1 foot. However, if trees or shrubs are part of the landscape, for every third or fourth irrigation, the sprinkler system should operate long enough to fill the root zone with water to a depth of 2 to 3 feet. After filling the root zone to a 2- to 3-foot depth, the interval to the next irrigation should not change. On the next irrigation, run the irrigation system just long enough to fill the root zone to 12 inches.
In areas with limited water supplies, there are ways to conserve. One is to select plants that need limited amounts of water. Many of our Idaho native plants are adapted to dry summer conditions. Another way to conserve moisture is to use mulch in flower beds and around trees to limit evaporation from the soil surface.
Find additional help with calculating water needs for lawns and trees at University of Idaho Extension. For an in depth discussion on calculating amount of water to apply to a lawn, see University of Idaho bulletin Watering Home Lawns: How Much and How Often.
For a general discussion of home landscape water management, see the Montana State University publication Yard and Garden Water Management.
The University of Georgia supplies an outstanding discussion of irrigation systems in the bulletin, Irrigation for Lawns and Gardens. For a listing of water conserving plants adapted to Idaho, see Washington State University bulletin Hardy Plants for Waterwise Landscapes.