Shallow Root Systems: Why Young Trees Need Regular Watering
By Steven Quick
Urban forests are invaluable to cities, providing shade, improving air quality, reducing urban heat islands, and supporting biodiversity. However, urban trees face unique challenges, especially concerning water use under heat stress.
Tree watering with water bags at SENCO Community Space. Photo: Julia Wolf.
Young trees are particularly vulnerable to rapid losses of surface moisture, while larger, established trees can take advantage of deeper water sources, but heatwaves exacerbate the water demands of both young and old trees. What is different is how they are impacted. Young trees, with their shallow root systems, may struggle to become established without consistent watering, making them particularly vulnerable during heat events. Older trees, while initially more resilient due to their deeper roots, can still suffer if deep-water reserves are depleted during prolonged heatwaves. Different species, such as conifers, have varying abilities to regulate water use and withstand heat stress. Understanding these differences is key to effective urban forest and water management.
Trees planted in the right-of-way throughout Tacoma tend to be younger than those found in natural areas. These young trees have developing root systems, limiting their ability to access deep soil moisture.
The shallow rooting of young trees makes them heavily reliant on surface water, which can evaporate rapidly during heatwaves.
Regular watering is essential to support their growth and preventing heat stress. Although these trees are typically broad-leafed and deciduous, their canopies are also still developing, limiting their ability to release water vapor to cool themselves and their surroundings. As a result, these small young trees can rapidly become heat-stressed in high temperatures, leading to stunted growth and even mortality if not adequately watered.
Larger, older, and more established trees with extensive root systems can access deeper water sources, making them more resilient to short-term drought conditions. Their larger canopies enable higher rates of transpiration, helping to moderate surrounding temperatures. However, during prolonged heatwaves, the increased water demand can strain even deep-water reserves. This can lead to structural weaknesses exacerbated by heat stress, branch drops, health declines, and increased vulnerability to pests.
Sun filtering through on a hot September day. Photo: Julia Wolf.
The sustainability of urban forests hinges on understanding and addressing the distinct needs of trees at different stages of their life cycle. Young trees require careful nurturing to overcome their initial vulnerability, while established trees need consistent support to manage their higher water demands during heatwaves. By implementing strategic watering practices and fostering awareness of these vulnerabilities, we can help ensure that urban forests continue to thrive and provide their myriad benefits for generations to come.
So, how do we help these trees thrive and continue delivering premium ecosystem services to the urban community? Water more trees!
TREE WATERING 101
By Eden Standley and Adela Ramos
The 3 pillars of tree-watering:
Age: For young trees, 15-20 gallons a week is the recommended amount. For established trees, 10 gallons of water for each inch of the tree’s diameter.
Pace: Regular and deep watering promotes deeper root growth and enhances drought resilience. Focus on watering slowly to allow the water to soak deep into the soil.
Time of day: Water either in the morning or evening when there is less evaporation.
Never water the tree’s trunk, as this can promote bacteria and mold growth which can make trees sick. Note that trees experiencing drought stress are already more susceptible to disease and pests.
Tools and Resources
Water bag or a 5 gallon bucket with holes or a timed soaker hose
Weeder or hoe for grass and weed removal prior to bag or bucket installation
Mulch, to conserve moisture and provide nutrients
Young Trees:
In the days following planting, the tree may need some extra water.
For the first three to five years, you should give the tree 15-20 gallons of water per week during the summer.
Create a system to help with regular and deep watering:
For smaller trees or trees with a lot of foliage towards the base of their trunk, a watering donut or ring will work best.
For larger trees that do not have lots of lower foliage, a gator bag may work best.
Other methods include watering buckets, soaker hoses, or timed soakers, and are good options for most trees, independent of size or foliage.
Mulching around tree bases can reduce surface evaporation and maintain soil moisture. When mulching, do not mulch up to the tree’s trunk, instead create a ring of mulch around the base of the tree, leaving a few inches of space around the trunk.
Established Trees
Watering older trees may only be necessary during heat events.
When watering, use slow-release watering systems such as soaker hoses or timed soakers which will reach their extensive roots and maintain soil moisture.
Water where your tree can reach, but not too close to the trunk. A tree’s roots typically extend out as far as the tree is tall, but a good rule of thumb is that a tree’s watering zone is about two-thirds of the tree’s height. For example, if your tree is 30 feet tall, you should water within 20 feet of the tree’s trunk.
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