Features and Occasionals

Water Your Trees!

By Katherine Pioli

When it comes to precipitation, Utah’s trees are always living on the edge. Fifteen inches of rain is the norm. A year or two of drought makes the situation more precarious. As more Salt Lakers quit watering their lawns or switch to xeriscape in an effort to cut down on water consumption, one unintended side effect could be the loss of our beautiful trees.

Salt Lake City’s 91,221 street and park trees belong to all of us. But it is each household’s responsibility to care for the tree in its own parking strip.

To find out what we can do to keep our trees healthy and strong, I talked with Ben Harris, current owner of Branch to Bud Tree Care and former arborist for the City of Logan and Utah State University as well as former president of the executive committee for the Utah Community Forest Council.

“It’s a common misconception that the roots of a tree are the mirror image of the branches,” Harris told me. Instead of the roots being like an inverted tree, says Harris, they look more like a piece of broccoli smashed on its head on a dinner plate. The roots of a tree normally extend down into the soil only about 36 inches and most of the roots are in the top eight inches. In addition to being very shallow, a tree’s roots spread far beyond the edge of the tree’s drip line, extending sometimes two to three times farther than the width of the canopy. That means there is a large area that needs to receive water to satiate a tree. You can’t (nor should you) just water at its base.

Harris’ tips for good tree watering methods

• It’s hard to overwater young trees. Young trees, especially those from containers, should be watered every day for first few weeks while their roots, which have been compacted into a dense ball, have time to spread out and push deep. Even if the soil looks wet, a dense root ball might still be dry. Taper watering back to every other day after the first few weeks, but remember, lots of water for the first year or two is critical to a tree’s lifelong success.
• Mature trees like deep and less frequent watering – so it’s still possible to let a lawn go dormant and keep a tree happy. In the summer, give big trees a deep soak every two weeks during particularly dry periods, every three to four weeks during periods of moderate moisture.

Look for signs of stress:

• If leaves are starting to wilt and look dry, it’s a good indication they need extra water.
• Your soil can also tell you when you need to water. Stick a screwdriver or shovel into the dirt. Does it go in easily, or is it dry and hard? If it’s difficult, you might want to water.

Twenty inches of rain a year is ideal for growing trees. Here, we get about 15 inches if we’re lucky. So there’s never a time when watering won’t do a lot of good.

• When laying drip irrigation, make sure that a line circles more than just the trunk. While this may not be feasible for parking strips, try to water as much of the entire root circumference as you can.
• To water trees with a garden hose, turn the water pressure to a slow stream, one that soaks in right away without making a puddle. Leave it on for an hour or two, moving periodically to various locations under the tree. Another good option is watering with a small sprinkler head on the end of the hose so that the spray reaches more of the root system at once.

Salt Lake City’s water conservation site adds these tips:

• Water away from the tree’s trunk (or root crown).
• Mulching around (not at) the tree’s base helps conserve moisture.

Other sources suggest:

• In clay and highly compacted soils: stop watering when runoff begins. This type of soil may absorb only a quarter-inch per hour.
• How long to water: drip irrigation should run about 90 minutes; and-of-hose sprinkler (conventional spray head): 45 minutes.

Salt Lake City has been putting a lot of time and money towards documenting the state of our urban trees. To find out the condition of the tree in your parking strip, visit http://bit.ly/1D6EtfZ and click on Urban Forestry. From there you can find an inventory of trees in the city, a map of diversity and a map showing tree vulnerability – each tree is rated “good health” (this is the majority), “fair health” and “poor health.”

You can also find the department’s list of suggested trees (natives, small and large species) for our climate and suggestions for suitable planting sites at bit.ly/1h3xFFO.

To find out the condition of the tree in your parking strip, visit http://bit.ly/1D6EtfZ and click on “urban forestry.”

Additional information is available at slcgardenwise.com, www.slcgov.com/waterconservation.

This article was originally published on August 1, 2015.