As more gardeners become concerned with conserving natural resources and saving money, alternatives to sprinkler irrigation are gaining popularity. For planting beds and most trees and shrubs, drip irrigation is a smart solution. Knowing the water needs of your shrubs can help you regulate your drip lines and get the most out of this type of irrigation.
How Shrubs Are Different
Trees and shrubs differ from other types of plants in that they have deeper root systems. Therefore, they need to be watered for longer periods of time, but less frequently, than flowers. Watering deeply further encourages the downward growth of shrub roots and creates a more drought-tolerant plant. Conversely, watering lightly and frequently causes shallow roots and makes the shrub more drought sensitive.
How Deep to Water Shrubs
Shrubs should ideally be watered to a depth of 18 to 20 inches. The depth of soil moisture can be assessed by simply probing the soil with a metal rod or long screwdriver. The probe will pass easily through moist soil, but will stop in dry soil. You might also dig a small hole to a depth of about 8 inches, and get a handful of soil for a feel test. Squeeze the soil to determine its moisture content. Soil with an ideal water content can be squeezed into a ball, leaving just a light layer of moisture on your hands.
The Right Drip Line for Your Plant
The size of your shrub will dictate the number of emitters on the drip line that should be delivering water to its base, and their flow rate. For shrubs less than 4 feet tall, one emitter delivering water at a rate of 1/2 to 1 gallon per hour will be sufficient. For taller shrubs up to 8 feet, two to four emitters dripping 1 gallon per hour is a good ratio. Take into account the type of soil that your plant has, as this has a major impact on water absorption. Sandy soils can absorb around 2 inches of water per hour, loamy soils can take in around 3/4 inch of water per hour, and clay soils have the hardest time, absorbing only about 1/2 inch per hour.
How Long To Run Your System
How long to run your system will be influenced by the previously mentioned factors, such as plant size and soil type. But run it long enough to thoroughly wet the soil to a depth of 6 to 8 inches at least twice per week. Newer shrubs benefit from more frequent irrigation. Older shrubs with more established root systems that draw moisture from a larger area can go longer between waterings.
Since plant size, soil water retention and rainfall can drastically affect water needs, the number of gallons of water needed by a single shrub can range from 5 to 50 gallons per week. To maintain efficiency and avoid over-watering, manually checking the moisture of the soil is essential.
Other Drip Line Thoughts
You can simplify the watering questions in your landscape by grouping shrubs with similar water needs together. This allows you to irrigate in a uniform way without a lot of extra work and worry. It's also beneficial to mulch around the base of your shrubs. This covers and protects the flexible rubber tubing of your drip system, reduces evaporative moisture loss from the topsoil, and reduces weeds that may compete for water and other nutrients.