Today is Arbor Day, the traditional day for planting trees. Most of us treasure trees. Planting one is an act of faith, something we do for our children, and perhaps our grandchildren. Sadly, thousands of our nation’s trees now reaching maturity are destined to an early death. They were doomed the day they were planted.
Gardening practices have changed over the years. The days of digging a huge hole for a new tree are over. Research has shown that planting a tree too deeply is a sure recipe for trouble.
The problem is with the roots. Roots need water and oxygen to live, and those are mostly found near the soil surface. We used to believe the root system of a tree looked just like the top growth, only upside down. Then some horticulturists started digging. Now we know that the roots extend sideways for five or more times the diameter of the crown, but only grow a foot or so down into the soil. Picture the common scenario of a tree planted in a healthy lawn. I was stunned to learn that the grass has deeper roots than the tree does!
When a tree is planted too deeply, the roots at the bottom of the hole suffocate and die. In a desperate effort to survive, the tree produces new roots higher on the trunk. Because these “baby” roots are thin and flexible, they easily follow the path of least resistance—around and around the planting hole. As the trunk enlarges with age, these circling roots form a corset, constricting the flow of food in the cambium layer just under the bark. Eventually, the tree starves and dies, killed by these girdling roots.
Note the lack of root flare on the tree in this photograph. The root flare should be above grade when the tree is planted.
If the problem is caught before the circling roots become embedded in the trunk, they can be cut (often with a loud snap!), freeing the tree. Sometimes that saves the day. But usually it isn’t until the dead tree is removed that the cause of death becomes apparent.
So, how should we plant a tree?
In my previous post, I described how to plant shrubs, perennials, etc. The same principles apply—I urge you to review those directions. Trees have a few special considerations, however.
When digging the hole, the proper depth is essential. While wider is better, the hole should only be deep enough for the root flare (where the trunk widens as it turns into roots) to be supported a couple of inches above the soil surface. Another way to look at it is to place the first (highest) main root one inch below the soil surface. This prevents these crucial roots from being interred instead of just planted. Disturbing the soil deeper than necessary destroys the firm foundation that will hold that tree in place for years to come.
If the tree is sold “balled and burlap” (the roots have some soil around them, and are wrapped in burlap secured with a wire cage), it is important to remove the wires and fabric. Colorado soils aren’t warm and moist enough for the burlap to decompose on its own, and the wires can eventually constrict root growth. Remove the wrapping after the tree has been placed into the hole. It’s all right if some wire remains under the roots. They won’t grow in that direction any way.
There is some disagreement over whether it’s worthwhile to amend the soil in the hole. The roots will soon extend well past that area, so it would only help during the first year or two anyway.
You’ll notice that this tree’s trunk is wrapped with paper. No, that’s not the packaging. Painting the bark a reflective white, or wrapping it with paper or plastic designed for this purpose will help prevent sunscald and injury. Especially in winter, when the ground is frozen, our hot Colorado sun can draw moisture out of the bark faster than the roots can replace it. Wrapping helps prevent this.
If you are planning to water the tree with a drip system, install it now. A couple of concentric rings of laser tubing or soaker hose is a good way to go. Remember that the roots will be growing outward from the trunk, so you should plan to add additional rings in the years to come.
Once the tree is planted, a supporting stake or two may need to be added. Make sure the tree has plenty of wiggle room—it’s the bending and swaying that strengthens the trunk. To avoid injury, any wires should be well padded where they touch the bark. Finally, remove the support after a year or two, before the wires (or stakes) become part of the tree.
Finally, mulch above the roots. Create a volcano, not a cone—you don’t want mulch right up against the trunk where moisture can encourage rot.
In addition to all the other benefits of mulch, it keeps grass from competing with the tree roots (remember which roots are deeper). Plus, grass next to the trunk exposes the tree to the dangerous condition known as “string trimmer injury.” Those spinning strings that mow down weeds so nicely will also cut right through the bark, girding the tree and killing it. In general, it’s good to stay off the soil where roots are trying to grow, especially if the soil is wet. Roots will not grow in oxygen-less, compacted soil.
It’s too soon to hang up the hammock; planting a tree is something we do for future generations. Let’s just be sure we do it right!
Girdling root photo courtesy Linda Haugen, USDA Forest Service, bugwood.org