The most likely cause of tree problems in the urban setting in our area is buried root collars. I seldom have a day in which I do not diagnose a root collar disorder. In my experience, it is by far the most common cause of tree decline and death and is responsible for most of the secondary pests and diseases we encounter. Into this root collar disorder category, I also group all the girdling problems such as girdling roots, girdling straps and twine, girdling wire baskets; and containers such as plastic pots, nylon bags, and felt bags that are not removed when a tree or shrub is planted.
The root collar is the area of a plant where the trunk or stem joins the roots. Trees have a noticeable flare at the root collar that we call the root flare and we want this flare to be visible and exposed to the air.
The trunk and root flare of a tree are not genetically programmed to resist constant soil moisture. Respiration occurs in this area of the tree (the exchange of oxygen and carbon dioxide) and constantly wet bark restricts or stops this gas exchange. Over a period of time, this restriction causes this area of the tree to function poorly, interferes with the downward movement of food to the roots, and eventually results in root dieback and reduced water uptake. These trees are more susceptible to disease infection, insect invasion and to stress caused by drought or compaction or irrigation excess.
Early symptoms of root collar disorders include yellowing of leaves, margin burn or necrosis of leaves, and dieback in the upper canopy. These are the same symptoms we see from water stress (both too much and not enough) and most of us react by watering the tree which usually makes the problem worse. Often, we see no early symptoms and we get the phone call that starts out with my tree died overnight. My tree was fine on Friday and was dead on Monday. That kind of rapid death is almost always caused by a root collar disorder. The tree exhausts its reserves, often looking OK, and dies overnight. The problem that led to that rapid death has been going on for years. And it is not uncommon for that overnight death to occur 1 to 3 years after the installation of an irrigation system.
Checking for a root flare is easy, you can see it. If it is visible, the problem is something else. If you can not see it, you have found the most likely cause of your tree’s problem. The soil or mulch needs to be removed down to the level of the root flare and out away from the trunk of the tree about 6 to 12 inches. The distance away from the trunk is not critical as long as the trunk and root flare are exposed to the air. You will find that the deeper you have to dig to reach the root flare, the further out from the trunk you will dig just so you have room to work. Most excavations are 6 to 12 inches deep but we have one tree that is doing well that was excavated about 7 feet deep!
Do not use a shovel or pick to excavate around your tree. Do not scrape or scar the roots. Use a hand trowel or rock hammer. You can also excavate with water pressure; it’s messy but very effective for shallow excavations. We use an air spade which removes soil with air pressure, does not damage the roots, and allows us to stand while we excavate. You will appreciate the standing after you have tried to dig a tree by hand. As you excavate, you should correct girdling problems caused by tree roots or things that were left on the tree when it was planted. Sometimes, you can not correct all of the girdling roots and will need to return in a year or two to correct additional girdling roots.
Severely declining trees need to be removed. Do not waste your time and effort trying to save a hazardous tree.
The well you’ve dug should remain open. There are many options for edging or lining the well to help keep soil from falling into the well and to make the well attractive. If the well is a safety hazard, it can be filled with large, course gravel, but that gravel will need to be removed and replaced every 3 to 5 years. It is best to leave the well open, the root flare exposed to the air.
The excavated tree should be fertilized with a good organic fertilizer or a good slow-release fertilizer or one of the good organic sick tree treatments that are available at better nurseries.
You may need to water the tree but you should be very careful. Our native or adapted Texas trees are very capable of dealing with drought. They are not well adapted at dealing with too much water. Be very careful with how you water because your trees are usually better off with dry conditions. We normally recommend that you water to keep your grass, annuals and shrubs healthy and do not irrigate established native trees.
In our experience, root collar disorders are by far the most common reason for the decline or death of our landscape trees. Proper planting and careful use of fill soils for leveling and for landscaping are important to preventing this disorder. If the disorder already exists, excavation to expose the foot flare to the air can often save your tree.
For more information, see Root Collar Disorders.