“Dear Bill: An oak tree fell on our garage last night. The tree trimmer said it was crown rot.  I am very concerned about a couple more trees behind the house ...”

That is an excerpt from an e-mail I received after the rain and wind storm in October. The first sentence gave me a sinking feeling, fed by memories of failed trees that crashed through roofs, right down to the floor.

Fortunately, in this instance, the tree was not as large as I imagined. The damage was not catastrophic. And the tree service was right about the cause. The base of the tree had been rotting away and finally gave out in the storm. It had presented no obvious signs of ill health and the decayed area was hidden in dense, irrigated vegetation around the trunk.

Trees with root disease and decay often show above-ground symptoms like reduced growth, thinning pale foliage, early dormancy and a heavy crop of seeds, cones or fruit, but severe decay may exist in the roots and trunk of a healthy-looking tree.

How can that be? 

A big tree can  absorb enough water and minerals from a relatively small number of roots to give it the appearance of good health. Some trees produce ‘adventitious roots’ while the main root system is rotting away. As it says in “Arboriculture” (Harris, Clark, Matheny): “This usually happens  where the base of the trunk is buried in fill soil, kept moist by rain, irrigation, deep mulch or dense ground cover plantings.”

Adventitious roots form on a large tree in a way similar to those on a rooted cutting from a houseplant. Moist conditions at the wound cause the cambial tissue at that point to differentiate into roots rather than wood or green stems. 

So giant oaks, in some cases, may be like giant rooted cuttings, living on a few  little roots, while the main structure of woody roots is slowly rotting away.

Conditions like this are often visible if you know what to look for and if you take the time to look.

This is why  methodical visual tree assessment (VTA) is a critical first step for arborists examining trees. It is like a visit to the doctor for any type of medical complaint.  They always take your blood pressure and weight, no matter why you are there.  It is good professional protocol.

A typical VTA starts with an overall look at the tree as you approach it, considering its species and how it should look at the time of year it is being inspected. Although not all arborists use it, many give the tree a “vigor” rating, which may range from “excellent” to “dead,” based on that first impression.

Another important data point is the trunk diameter. It is good “for the record” as a reminder of the overall size of the tree. An accurate measurement can help in locating a given tree in a group on a later visit and it is a critical measurement for establishing the value of a tree using the trunk formula method. 

Next, and most important with respect to this subject of “crown rot”, is careful observation of the root collar — the base of the tree where the trunk flares out into the buttress roots. It is a critical juncture, or crossroads, in the tree where everything coming up from the roots must pass through on its way to the branches and, vice-versa, where food coming from the canopy must pass through to feed the roots. If the root collar dies, the tree dies, unless it can grow adventitious roots as a backup survival move.

Most tree species, not all, show a visible flare — outward taper —  at the root collar. It is the classic look of gnarly old trees you see in paintings and the ever-popular company logos depicting trees.

If there is no visible flare, it is a sign that the grade might have been raised or the tree was planted too deeply. If so, the tree can be in for big trouble because burying the base of the trunk can suffocate tissues that need to breathe. It leads to death and decay.

In some cases dense vegetation might hide problems like an open cavity or mushrooms growing at the base of the trunk. Additionally, dense vegetation and irrigation to maintain it can suppress and kill sensitive root collar tissues.

A thorough VTA goes on to include observations of the condition of the trunk, limbs, branches twigs and foliage, as much as is practical from the ground.

That bout of nasty weather in October was a wake-up call. Even though the current outlook, as of this writing, shows nothing but blue skies on the horizon, it is a good time to take a close look at your trees.

Bill Pramuk is a registered consulting arborist. Visit his Web site, www.billpramuk.com, e-mail questions to info@billpramuk.com  or call him at 226-2884.