Sprinklers: What’s Good for the Grass is Not Always Good for the Trees

Sprinklers are a familiar sight on lawns and in gardens, but did you know that they have the potential to negatively impact trees by causing stress? Discover how watering with sprinklers can impact your tree and learn the key to properly watering in our newest blog.


When we think about sprinklers, we are often reminded of the ones we used to jump through as kids. But did you know that lawn sprinklers can come in many different forms? They can be either standalone and movable or fixed in place as part of an integrated in-ground irrigation system, and their sprinkler heads can be oscillating or rotary. Despite the variety of sprinkler types out there, they are designed with one focus: watering grass (aka turf). Lawn sprinklers need to be able to deliver frequent, evenly distributed, shallow watering to meet the high-water demands of the shallow root systems of grass. For most home landscapes, however, turf often shares the space with other plants that may have different watering needs.

Water is a vital resource for all plants but too much or too little of it can create stress, which reduces resiliency. Lack of resiliency opens the door to pests and diseases and can also lead to plant death. Although trees and grass both require water to live, they differ in how much and how frequently they need it. So, what happens when a tree is watered with a lawn sprinkler? Here are three ways sprinkler watering can impact trees.

 

 

Some of the most common types of sprinklers include fixed (top left), oscillating (top right), rotary (bottom left) and impact (bottom right). Although there are many types of sprinklers available on the market, they are typically designed and installed to address the needs of turf. © 2016 Aqua Mechanical & © 2015 Ester Merbt & © 2021 David Russell & © 2023 Jonathan Cooper

 

 

1. Underwatering/Overwatering

To target the needs of turf, lawn sprinklers typically deliver water to the top layer of soil where grass roots are growing. As such, less water infiltrates deeper into the ground where tree roots are found. On hot summer days, this shallow watering is also prone to evaporation leading to even less water infiltration. Overall, sprinkler watering can lead to underwatered trees because water is not able to effectively target tree roots.

Because turf needs to be watered more regularly than trees, certain sprinkler irrigation systems can be set up to prevent soil from drying out. This overcompensation means trees can end up being overwatered by sprinklers. Tree roots need oxygen to grow and survive, but when soil is always saturated with water, it reduces oxygen levels available for roots to thrive. This can prevent proper root growth and function, as trees prefer soil to dry out between waterings.

 

 

Both standalone sprinklers and sprinkler irrigation systems can overwater and underwater trees depending on how often and long they are used. Image credit: “Summertime” by Eric Sonstroem is licensed under a CC BY 2.0 license.

 

 

2. Fungal Diseases

Trees are also more susceptible to fungal disease when they are watered with overhead sprinklers. Overhead sprinklers deliver water from above and are ideal for covering large areas like a lawn. Picture a classic fan-like, oscillating sprinkler that attaches to a garden hose and sits in the middle of the lawn. Because these sprinklers are meant to mimic rain, the foliage (leaves) of taller plants like trees become wet. When used frequently, leaves stay wet for extended periods of time, generating humidity. Wet leaves and humidity combine to create the perfect environment for fungal disease development.

 

 

Overhead watering can lead to the development of fungal diseases such as powdery mildew, as seen on the leaves of this horsechestnut tree. Image Credit: “Chestnut powdery mildew” by Drazhnikova is licensed under a CC BY-SA 4.0 license

 

 

3. Trunk Rot

Both fixed-in-place and movable sprinklers can inadvertently catch tree trunks and branches in their crossfire as they spray water. This is because sprinklers are often installed or placed in lawns without accounting for trees. When tree trunks become wet but don’t have enough time to dry between waterings, the wet bark can become a breeding ground for fungi. The fungi in turn rots wood, which weakens the tree and opens it up to other stressors, such as pests or diseases.

 

 

This tree trunk is unable to dry out because it is repeatedly hit by water from a nearby sprinkler. This creates a wet environment on the bark, which is attractive to fungi, thereby predisposing the tree to rot. Image Credit: "Olive Tree and Sprinkle” by Zeevveez is licensed by a CC BY 2.0 license.

 

 

Proper Tree Watering

While sprinklers are attractive for their convenience, they can predispose a tree to stress because they are designed to meet the needs of grass. How should trees be watered then?

Trees prefer slow, deep watering targeted at the root zone where soil has a chance to dry out before the next round of watering. On average, young trees (approximately two years or less after planting), should be watered twice per week using approximately six gallons of water each time. That is the equivalent of using three full watering cans twice per week or using a hose with no nozzle on a very slow trickle for approximately 15 minutes twice per week. For older trees (three years or more after planting), you can water as needed throughout the growing season, particularly during the dry summer months.

 

 

Proper watering technique using a host on a slow slow trickle, for approximately 15 minutes twice per week

 

 

Because every soil type is different and because weather changes throughout the season, monitoring a tree is important to avoid overwatering or underwatering. A quick and simple soil test does the trick: just stick your finger into the soil under the mulch - if it is wet, the tree can be left for another day and if it is dry, the tree needs a drink!

 

Want more advice on how to care for your tree? Learn more Tree Care tips or consider booking a consultation with LEAF today to get help planting your next tree!

Jess Wilkin is the Residential Planting Programs Operations Supervisor and an ISA certified arborist at LEAF.

LEAF offers a subsidized Backyard Tree Planting Program for private property. The program is supported by the City of Toronto, the Regional Municipality of York, the City of Markham, the Town of Newmarket, the Regional Municipality of Durham, the Town of Ajax, the Township of Brock, the Municipality of Clarington, the City of Oshawa, the City of Pickering, the Township of Scugog, the Town of Whitby and Ontario Power Generation.