Amy Lentz

Amy Lentz

Trees are an important part of Colorado landscapes, especially in our urban centers. However, it’s hard to be a healthy tree in Colorado! Many of the trees we plant are not native to our prairie and mountain ecosystems and must adapt to their new homes with poor soils, low precipitation and just wacky conditions compared to other areas of the United States.

There are a few native trees across lower elevations such as Cottonwoods, but these don’t make the best landscape trees due to their shallow roots and enormous mature size. And yes, there are Colorado Spruce and Ponderosa Pines that are native to the mountain ecosystems, but when planted at lower elevations along the Front Range, they are now outside of their native habitat and must adapt. And that starts with the roots!

Contrary to popular belief, tree roots don’t grow that deeply into the soil. In fact, most tree roots only go down to a maximum depth of 18 to 24 inches and very few trees have a long-lasting taproot (a main, vertical root – like a carrot). Trees like oaks, hickories and walnuts might have a taproot as a seedling, but after a few years in the ground, this taproot stops growing downward and is replaced by a wide-reaching, shallower network of fibrous roots. That means that after a few years, tree roots are no longer growing downward, but instead are moving out and away from the trunk. Once they are established, tree roots will be in the top two feet of soil extending to what is called the drip line and beyond which can be up to 2 to 3 times the width of the tree’s crown. So eventually, that tree in your front yard might have roots extending to the neighboring property or under the street and sidewalk. It’s important to understand how and where tree roots grow so that you can plan accordingly and provide what the tree needs, right where it needs it!

One big consideration when growing trees is providing water where the roots need it. Often when we plant new trees, we place drippers near the trunk because the root system is small. This is okay when the tree has been recently planted because the roots haven’t yet begun their journey outward. In time, those roots closer to the trunk will gain in thickness and become more structural and no longer uptake water. As that tree grows and starts to extend its roots, any drippers need to be relocated to the area containing the finer water-absorbing roots at the dripline and beyond. If your tree is planted in an irrigated lawn where the dripline is receiving water through the sprinkler system, your tree is likely to have the water it needs. But, if your tree is not in a lawn setting and instead watered solely with a drip system, be sure to move those drippers out and add more emitters so that the entire root system is getting access to water. You can also remove the dripper right next to the trunk as it is no longer needed.

Also consider how to provide adequate water to you trees year-round. Remember, your irrigation system is not running during the off-season. You will need to provide water to your trees, especially evergreens that are still actively growing during the colder months. For this, you will have to provide supplemental water from a bucket or hose. Water your trees at least once per month in fall and winter if there hasn’t been enough natural precipitation or recent snow cover. Providing the proper amount of water can have a major impact on your tree’s long-term health!

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By Amy Poston Lentz. Amy is the Home Horticulture Program Coordinator for Colorado State University Extension Boulder County in Longmont.