When to water trees

Overuse is a problem

First, it is important to remember that trees offer significant environmental, economic, and social benefits. These are delivered at the homeowner level as well as the entire community. It is not easy to replace well-established trees, which is why trees should be a high priority for preservation during drought. 

Second, it is important to realize that most municipal water use in the summer is for lawn irrigation. Estimates vary, but Nebraska Extension suggests many of us overwater landscapes in the range of 30 – 300 percent on average.² Lincoln for example, which utilizes more than 80 million gallons of water per day during dry periods ³, would by conservative estimates waste more than 24 million gallons. That is 600,000 bathtubs worth of water every day! ⁴

Treat the symptom

In a severe drought, municipalities may restrict lawn irrigation to help save water for drinking and other important purposes. Early in a drought, if we moved away from a lawn watering mindset to a tree watering mindset, significant amounts of municipal water could be saved and watering restrictions could possibly be avoided.

If it comes to the choice of saving your lawn or saving your trees, try to remember that it’s a lot easier to replace turf grass than it is to replace trees that have been growing for years or perhaps decades. On the subject of turf, those wanting to minimize their environmental footprint further may want to visit this Nebraska Extension resource for a more sustainable alternative (i.e., less watering, less mowing, less herbicide).

Prioritizing your landscape

The good news is that most trees have at least some drought tolerance and are able to survive abnormally dry periods of at least a few weeks. Some species such as juniper, hackberry, bur oak, and gambel oak have remarkable drought tolerance. However, during severe droughts, even long-lived and well-established trees can be significantly stressed. So whenever possible, watering around trees during a drought is generally a good idea. How should trees be prioritized for watering? Here are some suggestions:

• Young and newly planted trees (1-3 years old) should be given the highest priority. Such trees may need to be watered weekly during a drought.
• Established trees more prone to drought stress should have the next priority. In Nebraska, this would include species like red maple, sugar maple, baldcypress, ginkgo, tulip tree, aspen, crabapple, and spruce among others. In a yard with a big bur oak and an old tuliptree, for example, the tuliptree should be watered more.
• Since water runs downhill, trees on drier sites such as the top of slopes or in sandy soils should be prioritized over trees in swales or at the bottom of slopes, where soil moisture will persist longer. Also, south-facing slopes dry out faster than north-facing slopes.
• Trees well adapted to drought should be a lower priority. This would include things like bur oak, honeylocust, elms, hackberry, limber pine, and juniper/redcedar among others.
• Don’t just guess at the dryness of the soil but use a probe of some kind like a screwdriver to see how dry it is several inches below the surface. If the soil is still moist, hold off on watering.

Watering with a purpose

How should trees be watered? Ideally, irrigation should be concentrated under the tree's canopy (also referred to as the drip line) long enough so that it soaks into a depth of 8-12” or more. Compared to lawn watering, trees generally require less frequent but deeper watering. For established trees, watering about once a week or every other week should suffice during severe drought. The goal is not to keep the soil saturated all summer but rather just moist enough to keep the tree alive and healthy. One way to think about it is that roughly speaking, Nebraska receives about 1” of rainfall/week on average during the growing season. Mimicking that would be a good way to approach it.

• Watering from the end of a hose (for small and individual trees). Using a water breaker at the end of the hose helps slow the pressure and achieves better soaking and less runoff. Setting a hose on a slow trickle and leaving it near a young tree can also help achieve deeper penetration of the moisture.  
• Lawn sprinklers. The best lawn sprinklers are those that spray low to the ground so less is lost to wind and evaporation. Avoid sprinklers that throw the water high into the air, especially on hot days.
• Soaker hoses and drip tubing buried underneath mulch are good choices for trees and shrubs in landscape beds.
• Watering bags including gator bags are often used on new/young trees. The bag is filled with water that slowly seeps out over several hours.

Recommendations for better drought tolerance

1. Try to incorporate and maintain a 2-3” layer of wood mulch around individual trees and across planting beds. Mulch helps water penetrate the soil more thoroughly, maintains soil moisture, and reduces soil temperature. It also keeps mowers and weed trimmers away from tree trunks. Don’t pile the mulch too deeply and don’t mound it against tree trunks. Avoid inorganic mulches such as river rock, pea gravel, shredded rubber, etc., as they can increase soil temperatures
2. Pick the right trees and other plants for the landscape–and try to emphasize regionally native plants! There are many, many drought-tolerant trees, shrubs, perennials, and grasses that can be selected. For a list of drought-tolerant trees and shrubs, see the resources at the bottom of this page.
3. Don’t scatter trees across a landscape. Instead, plant trees and shrubs together in complementary groups and use them as anchors for additional landscape plantings – including perennials and ornamental grasses. Mulch them in mass and you’ll have a grouping of plants that are much easier to manage with similar irrigation requirements.
4. Limit the use of high-input turf grass to where it is truly needed – and begin converting parts of the lawn to more drought-tolerant species. If possible, allow a cool-season lawn to go dormant or have some brownness in the heat of the summer. 
5. Think healthy soil! The more organic matter in the soil, the more drought-tolerant it is. Organic matter comes from the decomposition of mulch and plant debris left on the soil. Such soils better absorb and retain moisture and are much better for tree health. Remember that the most resilient landscapes begin with healthy soils. Design and manage irrigation systems wisely! Irrigation systems can be a good tool for landscape care, but they also often waste a LOT of water. Irrigation systems should be shut off when not needed, and yet many systems run even when it’s raining. Most modern systems utilize sensors that detect rainfall. Keep in mind that it’s very possible to plant and manage beautiful and functional landscapes in Nebraska that need very little supplemental irrigation.
6. Design and manage irrigation systems wisely! Irrigation systems can be a good tool for landscape care, but they also often waste a LOT of water. Irrigation systems should be shut off when not needed, and yet many systems run even when it’s raining. Most modern systems utilize sensors that detect rainfall. Keep in mind that it’s very possible to plant and manage beautiful and functional landscapes in Nebraska that need very little supplemental irrigation. 

Drought-tolerant trees for Nebraska

Tree list for eastern Nebraska

Click to download

Tree list for western Nebraska

Click to download

Sources

1. Climate Change Implications for Nebraska (2014). Retrieved from http://snr.unl.edu/research/projects/climateimpacts/index.aspx?utm_source=Google&utm_medium=email&utm_term=&utm_content=&utm_campaign=Sector%20based%20Roundtables
2. Avoid Overwatering Lawns and Landscapes (2012). Retrieved from https://unlcms.unl.edu/educational-media/droughtresources/lawns-turfgrass \
3. Why Conserve? Water Conservation (2000). Retrieved from https://www.lincoln.ne.gov/files/sharedassets/public/ltu/utilities/water-system/conservation/why-conserve.pdf
4. Water Science School, United States Geologic Survey (2021). Retrieved from https://www.usgs.gov/special-topic/water-science-school/science/a-million-gallons-water-how-much-it?qt-science_center_objects=0#qt-science_center_objects