This time of year, when the temperatures soar, we worry a lot about our lawns getting enough water. But not enough attention is paid to the rest of our landscape. Our trees and shrubs need water too – not as often as the lawn, but the water should go deeper into the soil to encourage and maintain deeper roots.
Rooting depth is a big factor in how long to let your irrigation system run. Lawns in our area are usually watered for about 20 to 40 minutes three days a week with a sprinkler system to get water to penetrate about 6 inches into the ground. That’s because turfgrass roots only grow between 6 and 8 inches deep.
Tree and shrub roots can grow from 18 inches to over 24 inches into the ground, so they need deeper water penetration. This means you need to allow your irrigation system to run for a longer time for trees and shrubs than you do for your lawn. We recommend using drip irrigation and placing at least four or five emitters around the circumference of each plant, but not right up against the trunk. A medium-size tree with five 2-gallon-per-hour emitters would need up to 10 hours of drip irrigation once per week, or 5 hours twice per week, depending on soil type.
How do you efficiently water landscape plants with different root depths? You do it by hydozoning. A hydrozone is a group of plants in your landscape with similar root depths and, therefore, similar water needs. Examples of hydrozones include an island planted with trees and shrubs, a perennial flower bed or a lawn area. Each hydrozone should be irrigated on its own irrigation timer, or on a single zone within a larger irrigation controller. This allows you to set the duration of irrigation and the frequency of irrigation that meets the unique needs of each planted hydrozone.
I’m often asked how best to water trees planted directly in the lawn. Although this is not an ideal situation, you can make it work. Trees and shrubs will benefit from lawn watering, but the water won’t go deep enough unless you overwater your lawn – and that’s not efficient. Trees that get all of their water from lawn irrigation tend to be shallow rooted. You may notice these trees develop leaf scorch, leaf drop and branch dieback in midsummer. Once every week or two, you should water trees in lawns deeply to encourage deep roots. These deep roots are needed to get water to the top parts of the tree and to avoid water-stress damage.
Another strategy is to smother the grass around your trees with a 4- to 5-inch layer of organic mulch placed a few inches from the trunk and all the way out to the tree’s dripline. This will kill the grass in that area. Then you can set up your trees on a drip system and schedule irrigation to be deep and less frequent. The mulch will help protect your trees by minimizing water evaporation from the root zone. It will also condition the soil around the tree roots, loosening it up for better root development and providing a slow release of essential nutrients.
Protect your landscape trees and shrubs from drought stress by watering them separately from the lawn. Both your trees and your lawn will be happier.
Heidi Kratsch is the Northern Area Horticulture Specialist for University of Nevada Cooperative Extension. Have landscape irrigation questions? Contact a master gardener at 775-336-0265 or firstname.lastname@example.org.