Regional Gardening Reports :: National Gardening Association

In the Garden:
Southwestern Deserts
August, 2006
Regional Report

Share |

This rootbound tree did not have a strong anchor and blew over in a windstorm. (Photo by Robyn Baker, City of Scottsdale Water Conservation.)

Tree Planting Mistakes

Lots of trees blew over during the Southwest's fierce summer monsoons last month. In some cases, a microburst was just too strong for whatever was in its path, and there's not much we gardeners can do about that. A microburst is caused by intense downward thrusting of air that diverges when it reaches the ground, creating 70+ mph winds. Microbursts are situated over a small geographic area and last just minutes, but the winds are tornado-like in their destructive capability. A microburst hit southern Scottsdale last month, taking out dozens of trees.

There's another problem that contributes to trees blowing over during a storm, and it is one that we can actually do something about during the transplant process. The majority of a desert-adapted tree's root system grows in the top 3 feet of soil. Roots grow horizontally, far past the canopy, spreading outwards to absorb scarce water near the soil's surface. This runs counter to most people's conception of a deep tap root anchoring a tree. However, it's important to grasp the concept. Your goal is to prepare a transplant hole that will help the roots' natural spread.

Preparing the Hole
Till an area that is three to five times as wide as the container, but only as deep as the container (or rootball). Don't dig deeper because as the soil settles, the tree sinks into the ground. For a 1-gallon container, you would need to loosen a circle of soil about 1 to 3 feet in diameter; for a 15-gallon container, the area would increase to 3 to 6 feet in diameter.

Loosening the soil over a wide area allows roots to grow outwards. This helps prevent the tree from becoming rootbound within the confines of the hole, just as roots would do in a pot that's too small. It's also a good idea to rough up the sides of the hole, by scratching the dirt with a rake or poking holes in it. It sounds silly, but anything you can do to facilitate roots growing outwards (think about how hard our soil is) will translate into a tree with a stronger anchor.

Don't amend the backfill with organic matter. This is another common practice that research now refutes. As the backfill decomposes and settles, the tree sinks. In addition, tree roots get comfy in that rich organic environment and don't transition well into the native soil. It's better to start them off in the same soil environment that they'll have to survive in.

Of course, the bigger the transplant size that you buy, the bigger the hole you have to dig. This can be exceedingly difficult in many desert soils, which can charitably be described as cement-like. If you're a do-it-yourselfer, it's okay to buy small plants. Research shows that they catch up fairly quickly in size because there is less transplant shock to the root system.

If you can't wait for a tree to grow, and prefer to purchase a mature 24-inch box tree, you'll likely hire someone to plant it. Before purchase, quiz them on their planting methods. I know from numerous horror stories that few companies want to dig a large hole. Even experts in the horticultural field have difficulty getting some companies to do so. Or a company may say they will dig a big hole, but the guys who actually arrive to perform the job are less thrilled with the assignment and will try to tell you that you are crazy.

Don't be intimidated. Those big trees are a big investment of time and money. Ask around for referrals for reputable firms. Hiring a company that will use digging equipment instead of laborers is an option. I saw a tree recently that had tipped over because the hole hadn't been well prepared and the roots had grown around and around instead of spreading outward. That's quite an expensive mistake.

Care to share your gardening thoughts, insights, triumphs, or disappointments with your fellow gardening enthusiasts? Join the lively discussions on our FaceBook page and receive free daily tips!


Today's site banner is by nmumpton and is called "Gymnocalycium andreae"