According to NCSU Extension Specialists Bilderback and Powell, "water should be applied efficiently and effectively to make every drop count. Wasted water costs money and may lead to surface water or groundwater contamination."
Before we can begin to determine an "efficient and effective" level of irrigation, we need to assess the individual needs of the plants in our garden and landscape. We can broadly begin by separating our plantings into categories of lawn, trees and shrubs, and finally annuals and perennials.
As we drive through town, it is quite common to see folks watering turf. A more durable lawn results with less frequent, but more thorough irrigation. This watering technique helps to encourage a more deeply rooted plant that is less affected by fluctuations in temperature and moisture changes.
Research suggests that using the technique of "footprinting" (footprints on the turf will not disappear within one hour) helps to accurately pin-point the time when turf grass will benefit from irrigation. Tree and shrubs have a much deeper and more extensive root system and can therefore be placed into a "low-moderate use" category requiring supplemental watering only during establishment (first 8-10 weeks after transplanting) and during periods of limited rainfall.
With the root system extending 2-3 times the spread of the canopy, woody plants are able to extract moisture even when the soil appears bone dry.
Annuals and perennials often vary widely in their tolerance to drought, so locating plants in zones dictated by water requirements can simplify the burden of watering.
In general, herbaceous ornamentals have a shallower root system that exhibit water stress more quickly during dry periods. These plants benefit from watering once or twice a week.
When considering how to irrigate, we can break our options into three categories: hand-watering, overhead (sprinkler) irrigation and drip (micro) irrigation. Most of us are familiar with hand-watering; it is an efficient, although time consuming method of irrigation. Using a wand helps to extend the gardener's reach so the water can be directed to the base of the plant thereby protecting the foliage.
The research suggests applying 5 gallons of water per 10 square foot, which is approximately the amount of water delivered by a standard garden hose at medium pressure for one minute. Then, increase the watering duration by 15 seconds for each foot of plant height over 4 feet.
Another option is the sprinkler which may be a simple or more complex design, but simply applies water overhead in a uniform pattern over a given area. While many sprinklers are adjustable, it is a challenge to adjust for irregular areas and avoid unnecessary overspray and wasted water. One common area of confusion relates to the proper time to operate sprinklers. Research suggests that sprinkler irrigation is best applied after 9 p.m. and before 9 a.m. because there is generally less wind, lower temperatures, and less sunlight, resulting in less water loss to evaporation.
Finally, drip irrigation is probably the most effective and efficient option for many applications. Also known as trickle or micro-irrigation, water moves through small flexible tubing and control devices called emitters and is delivered slowly and directly to the roots of plants.
In addition to tubing and emitters, the drip system requires a valve (manual or automatic) to turn the system on and off, an in-line screen filter to prevent clogging and a pressure regulator that reduces the household water pressure and ensures the system operates efficiently.
During very dry weather, a mirco-irrigation system needs to run about three times per week for 4 hours each time to meet the optimum water needs of the plants.
Bob Filburn is an Edgecombe County Extension Service agent specializing in horticulture.