Paulownia on The Green
New Castle, Delaware

(Paulownia tomentosa)

** Excerpts from a report submitted to the New Castle Tree Commission, March 3, 1998.

The history of The Green in Historic New Castle, Delaware, dates to over 300 years. While many of the trees now standing on The Green are majestic old specimens, none are quite that old. There are several that may date to over 150 years, or perhaps even 200 years. Each mature tree is considered to have significance, though.

Behind the Old Courthouse and Jail there were three large paulownia trees. Several years ago, an assessment of these trees was made and two of the trees were deemed very hazardous and subsequently removed. The third tree was decayed, but was left standing for the time. The New Castle Tree Commission recently decided to reassess the condition of this tree, and a decision was subsequently made to remove the tree, based on the following analysis of its condition.

Observations of Tree and Site

The Green is in the center of the New Castle Historic District, and is regularly used as a recreational park. The subject tree is located near the northwest side of The Green, behind the old Courthouse building.


Figure 1: The Paulownia on The Green, near Third Street

The paulownia tree has a trunk diameter of 40.3 inches, measured at 6 feet above the ground level (above a burl on the trunk). The height of the uppermost branches is 63 feet. The crown of the tree is asymmetrical, primarily due to past pruning and limb loss. There are a few large dead branches in the canopy, and many smaller branches and twigs are dead or declining. Twig growth overall is below average for the species.

Paulownia base
Figure 2: South side of tree

The most extensive pocket of decay at the base is on the northeast side of the tree. Other cavities on the east and south sides coalesce with the main area of decay in the trunk. About 30 to 40 percent of the circumference at the base is affected by the decay to some degree.

The base of the tree has several areas of decay and open cavities. The decay progresses from below ground level in the roots up into the main trunk of the tree.

On the south side of the tree there is a cavity near the ground line (Figure 2). Directly above it at 4 feet high is another open cavity associated with an old burl. The decay from these two openings coalesces inside the trunk, and form part of a long column of decay. Drill-testing of the trunk indicates there is an average of about 7 inches of solid wood around the tree, with the remainder of the trunk decayed.

cross section
Figure 3: Cross section through trunk

The main trunk has other cavities at higher levels, and there are many decay pockets in the main limbs of the tree. One of the holes, at 29 feet on the south side, is the main entrance for a honeybee nest. Other holes in the limbs are scattered throughout the crown.

Discussion

The tree is structurally weak, as indicated by the condition of the main trunk and buttress roots. Paulownia trees have a low wood density, resulting in weaker wood than many other deciduous trees. This means that more sound wood is needed to support the weight of the canopy.

The base of the tree trunk is severely decayed, as shown by the drill testing and core samples removed. Less than 7 inches of solid wood remains, forming a hollow cylinder. Analysis of this data by two different formulas, indicates the tree is unstable, and at risk of failure. Using a strength-loss formula, about 40 percent of total strength has been lost in the lower trunk. A threshold of 25 percent loss, or less, is recommended for trees in medium or high-use areas.

The second formula compares the thickness of the solid wood remaining with the radius of the stem. In this case, a thickness less than 30 percent of the trunk radius is considered to be at risk. This paulownia tree has a wood-to-trunk-radius ratio of 20 percent, again indicating high risk of failure.

The decay pockets in the canopy also are in danger of causing failure. Over the years, branch loss and pruning have lead to a contorted shape to many of the limbs. The sharp bends and angles, combined with internal decay increase the likelihood of a branch breaking. The torsion and bending against uneven patterns of growth in the wood make it more likely that a limb will fail. When decay is present, the risk increases, as there is less wood to support the bending and twisting of the limb.

When considering the fate of a tree such as this one, the potential targets, or objects that may be struck by falling parts, must be considered. If the tree is isolated in an area that is seldom used, the potential for damage is less, and so is the hazard associated with the tree. For this paulownia, the risk is high. The nearby building is close enough that it could be struck if the tree broke near the base, with some potential for damage. The Green is also an area of moderately heavy use at times by people. Of course, the value of human life cannot be measured in dollars.

Conclusions

Removal of this tree is the only alternative that will eliminate the risk of damage to objects or injury to people in the vicinity. The decay and other structural defects in this paulownia have weakened it significantly. The normal safety margin inherent in trees has been eroded. There is little strength left in the roots, trunk and limbs to withstand the forces of wind and gravity. The tree is at risk of failure, and it could happen at any time. While breakage is most likely to occur during high winds, it is possible that a limb could break even when there is no breeze.

Attempts to preserve the tree should not be considered. Pruning may reduce the risk of failure of the upper limbs, but will not significantly improve the risk of trunk or root failure. Pruning will also reduce the energy reserves of the tree, negatively affecting growth and vitality. Bracing the trunk cavities, or filling the trunk with cement or other filler, would be expensive and impractical, and may not significantly strengthen the tree or prolong its life. Those efforts will also increase the cost of removal, when the tree ultimately dies. The weaknesses in the limbs would still be present. Any of these treatments, singly or in combination, could be seen as efforts to prolong the life of the tree, but would still not eliminate all risk. Some of the treatments would also affect the health of the tree, perhaps speeding the decline spiral and resulting in death of the tree within a few years.

A few years ago a committee of arborists suggested that a protected area be established around this tree, to limit traffic and use close to the tree. A suitable barrier would have to be built that would assure a safe zone of about 70 feet in all directions from the tree. This would be a poor option on The Green, as it would disrupt the use of this park as open space.

This old paulownia has stood on The Green for many decades, or perhaps as much as a century. But as with all living things, its life is finite. It is now near the end of its useful life, and if left standing without intervention, it will eventually collapse and fall. It could possibly damage the historic Courthouse building, and perhaps even cause injury or death to anyone present at the time. Although the tree has received care over much of its life, it is now time to make room for other trees to grow and flourish.

The only responsible course of action is to remove this tree. Despite the historic nature of the site, and the appearance the tree makes in the landscape, it is the only reasonable and prudent alternative.


© Tree Tech Consulting 1998
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