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HOME:    GARDEN BASICS:   IRRIGATION

Introduction Gardening Equipment: Tools for getting started Soil Preparation Composting Fertilization Use of Pesticides in the home landscape Organic Gardening Irrigation

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 kinds of irrigation systems available. As a result, it is a task that 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 saturated with water in the late fall. 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 the 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 to make sure that the application is uniform. The objective should be to make sure the water fills the root zone area and then allow time for the soil to dry out a bit to ensure adequate oxygen for the roots. The root zone for lawns is about 10-12 inches, vegetable gardens 18 inches, shrub beds to 2 feet, and trees to three feet. In order to irrigate properly, three important pieces of information are needed: 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. Access this information at: info.ag.uidaho.edu/pdf/CIS/CIS1098.pdf. Soil type has a large impact on irrigation practices. Sandy soils may hold enough water for only a single day of plant growth. Heavier silt or clay soils may hold enough for 5 or 6 days. Consequently, it is important to know your soil type and adjust accordingly. 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 type does not change how much water plants need or use, but it does change the schedule 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, every third or fourth irrigation should be timed to apply enough water to fill the root zone to 2 to 3 feet. This does not mean the interval to the next irrigation will change, only that the next irrigation can be scheduled to fill a 12 inch root zone. 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. Also, mulching flower beds and around trees will conserve water by limiting evaporation from the soil surface. For additional help with calculating water needs for lawns and trees, pursue the following University of Idaho link: www.uidaho.edu/extension/lawn/. For a general discussion of home landscape water management, see the following University of Montana publication: www.montana.edu/wwwpb/pubs/mt8915.html. The University of Georgia supplies an outstanding discussion of irrigation systems, at: www.engr.uga.edu/service/extension/publications/b894-cd.html. For a listing of water conserving plants adapted to Idaho, follow this Washington State University link: www.wsu.edu/~lohr/wcl/. top