Lightning Protection Systems for Trees

  • LIGHTNING PROTECTION for TREES - CONTENTS: Research & design recommendations for lightning protection for trees, flashover, causes of tree damage. Guide to Using the patented Arborbolt for connection lightning protection cabling on trees. Design specifications for lightning protection systems for trees.
  • POST a QUESTION or READ FAQs about lightning strike protection system for trees
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Protect trees from lightning:

This article explains research and recommendations on how to protect trees from lightning strikes.

This article series describes common lightning protection systems, certification, installation, and lightning protection system inspection. We provide information about lightning strikes, lightning hazards, related equipment, sources of lightning protection system installers, and lightning strike risk assessment.

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Lightning Protection and Trees

- Ben Fuest

Introduction - Lightning Protection Systems Fitted to Trees

[The research and recommendations for protecting trees from lightning strikes, and the concomitant protection afforded to nearby buildings discussed in this article focus on conditions in the U.K. However the principles of tree protection from lightning discussed here are generally applicable to other countries as well, though the statistics on the frequency of lightning strike and other local conditions vary. -- Editor]

This research on lightning protection systems for trees was established for the purpose of developing a better understanding of lightning protection systems specifically designed to be fitted to trees. Coming from a background in sylviculture my initial concern was to enable important and intrinsically valuable trees to be protected from damage resulting from lightning strikes.

However it quickly became apparent that the protection of nearby structures and buildings that might be liable to collateral damage in the event of strike was of equal significance.

Research showed that on those occasions where lightning protection had been installed in trees, the system employed had been based upon designs originally intended for use on buildings and other essentially non-dynamic man made structures.

The particular problems of installing the necessary hardware into living and growing trees did not appear to have been adequately addressed. Thus the direct effect on the tree of the installation of the required hardware is not factored in, with the result that trees are likely to be caused some degree of long-term harm in the very process of attempting to protect them.

The Approach to Lightning Protection for Trees, Passive Cloud-to-Ground Lightning Protection

This document refers to lightning protection for trees in the UK. The type of lightning considered is cloud to ground and not inter-cloud as this type is not at present considered to be a threat. In the UK, we can expect approximately 450,000 - 500,000 strokes per year.

Of this approximately 40% is cloud to ground. (EA Technologies cloud to ground database) This is quite low compared to some countries, but is still considered significant enough in some instances to warrant lightning protection. I will only be referring to passive lightning protection.

Active systems are not considered, as there is insufficient evidence to support their performance claims. (The University of Manchester Institute of Science and Technology Test report No: 43427): The need for lightning protection is evaluated either to conserve specimen trees or due to a concern about collateral damage to surrounding people and property.

We can ascertain the extent of lightning activity in the immediate area around the subject tree by performing a cloud-to-ground density analysis. Cloud-to-ground lightning produces extra long wave radiation (low electromagnetic frequency). Low frequency emissions have very good transmission characteristics in the atmosphere and potentially can travel for hundreds of kilometers before decaying to an undetectable level.

This frequency is at 1070Hz and transmission is further enhanced by the entrapment of the ionosphere; this is referred to as wave-guide effect, a useful tool in the evaluation process. It should be noted that this is a two-dimensional representation of a three-dimensional phenomenon. As such it will be necessary to look at the topography of the site taking into account other tall structures, dynamic and non-dynamic.

A lightning protection system is comprised of three main components: air terminal, down conductor and earth (ground) termination.

The earth (ground) termination is where we will be attempting to direct the huge current in a manner that minimizes risk to property and people. In the horticultural context we look at soil as a medium for growing. In the context of lightning protection we look at it as a medium for conducting electricity.

The ANSI A300 Part 4 recommends designing the earth (ground) termination based on a visual inspection of the soil and its moisture content. This is not possible as water is an insulator not a conductor; it is the dissolved salts in the water that give it its conductive properties.

These salts are not detectable with the human eye. We can ascertain the electrical value of the soil to a given depth using a system sometimes referred to as the four-point test or the Wenner method. This was first introduced in a paper published in October 1915, A method for testing soil resistivity, by F. Wenner.

This test enables us to prescribe the number of earth (ground) electrodes to employ for a given ohms resistance on completion of the earth (ground) termination. This is important if we are to achieve an effective earth (ground) without over specifying the number of electrodes causing unnecessary expense and disruption, or too few resulting in an inadequate earth (ground). It has been my experience that multiple electrodes are required in almost all installations.

Lightning Protection System Electrodes

Electrodes [used for lightning strike protection] come in different sizes. The ones referred to here are 1.2m (4 feet) in length and 12.5 - 13 mm diameter, and are made from mild steel with a two-micron copper bond coat. The ends are threaded so that with the aid of a coupler they may be joined together. The minimum requirement for length is 2.4m (8 feet).

If the working space is limited then it is sometimes best to add one on top of another, 2.4 - 3.6 - 4.8. If there is room to work in, it is possible to install individual electrodes in parallel.

[Click to enlarge any image]

Tip depth is an important factor to be considered when deciding configuration. Tip depth determines the distance between electrodes. If we install them and they are too close they will interact and we lose the performance benefits. If they are too far apart we are wasting time, materials and causing unnecessary disruption (BS 6651 1999).

The photograph shown here is an example of how not to protect a tree and a nearby building from lightning strikes.

The owner of this home in New York reported recurrent lightning strikes on tall trees surrounding the building, but even a casual inspection showed that the lightning protection system had been badly compromised.

It is possible that these conditions actually increased the risk of damage to this tree and building from lightning.

Soil Conductivity & Lightning Strike Protection

When working with soils of poor conductivity it is possible to introduce soil-conditioning agents. Bentonite is de-composing volcanic ash; it has hygroscopic properties that enable us to enlarge the diameter of the earth (ground) electrode without the cost implication. Instead of driving the electrode into the soil we can drill a hole approximately 25cm (10 inches) wide, and as deep as the intended electrode, and then back fill with the Bentonite mix.

The lightning protection system electrode is then inserted into this. It should be noted that Bentonite is an excellent conductor while wet but works just as well as an insulator when dry. If we are employing this technique, it is useful to remove turf around the hole and put it back on completion resulting in a more professional finish.

There are alternative ways to overcome poor conductivity. In the past it has been recommended to install our earth (ground) electrodes just outside the drip line.

Observations show that if the soil around the tree has undergone de-compaction with the addition of organic mulch, it is possible to obtain a much-improved earth (ground) working just inside the drip line. With regard to the area affected by the dissipation of the current around the electrode, it has been observed that this will be contained within a radius of approximately 35-50cm (15-20 inches).

There will be occasions when driving the electrode into the ground it abuts an impenetrable layer or obstacle. The temptation to cut it will be strong, but I would not be in agreement with this practice. I would suggest removing the electrode and trying again only this time at an angle of anything up to 30 degrees from the horizontal; this way we retain the integrity of the electrode length without the requirement for depth.

We can test the value (ohmic resistance) of the earth (ground) during the installation and on completion; I have found the 62% method to be the most reliable and industry standard around the world. In the UK the target figure is 10 ohms or less (BS6651 1999).

Recommendations for Installation of Lightning Protection Systems on Trees

Having established the need for lightning protection and designed the earth (ground) termination we can look at the aerial aspect of the system. The ANSI A300 Part 4 recommends a maximum distance between air terminals of 35 feet. I am in agreement with this but, as with all things organic, there will be exceptions and we should look closely at the tree and see how it lends itself to the formula and what we think will work best for the circumstances that prevail in the canopy.

Lightning Protection System Conductors on Trees

The conductor should be firmly secured to the fabric of the tree. In the UK we fix at 1 meter (3 feet) intervals; in the US they are specified at a maximum of 1.8 am (6 feet). This difference is not of any real concern, as I believe we are all of the opinion that the distance will vary with the form of the tree.

It is however of greater importance to consider the type of fastener we employ. The ANSI A300 Part 4 recommends a type of fastener that is attached to the tree in a manner not dissimilar to a nail. The conductor is secured at the outer end in a pinch portion. As the tree grows it will envelop the fastener.

When the incremental growth reaches the pinch portion of the fastener ANSI recommends removing the conductor from it and installing a new fastener 30 cm (1 foot) above or below the old one, leaving the old one in the tree. This is a contradiction within the standard and ISA Best Management Practices (BMP) for Tree Lightning Protection Systems, creating other metallic conductive objects in the vicinity of the conductor that are not bonded.

The reasons for bonding metallic conductive objects to a component of a lightning protection system are well established (ANSI A300 Part 4, 46.1.7). The BMP is of the opinion that these old fasteners will not be subject to potential difference.

Lightning Arcing & Flashover Considerations

It is suggested that these components will be subject to potential difference and an arcing spark is definitely possible. There exists a common misconception that lightning always takes the path of least resistance.

It would be more accurate to say that although lightning has a preferred path, it takes all available paths simultaneously to a lesser or greater extent. Where we have two parallel paths for the current to flow, the total current will divide in inverse proportion to the resistances (impedance).

The lightning conductor cable would be the preferred path to ground (primary) but the higher the resistance of the earth (ground) termination and the higher the impedance in the conductor, the greater the current in the alternative path (flashover) (Dr V. A. Rakov pers. comm. 2007). Having seen the conductors in the ANSI system and the recommendations for earth (ground) termination, it is believed that the systems will be of high impedance.

If we look at the scenario of a system having just undergone an upgrade with new fasteners, the distance between old fasteners and down conductor is about 10-15 cm (4-6 inches) and is comprised of air space; so flashover would be in the form of spark/electrical energy. When the old fastener becomes completely engulfed by new wood the distance remains the same but the gap is no longer just air space but has a percentage of wood tissue. The difference in terms of resistance is considerable. In this context resistance is the property of a material to resist the flow of electrical current. It is specific for a given tissue, depending on its moisture content, temperature and other physical properties.

The photograph above was taken at Arlington National Cemetery (Arlington, VA, USA). The photo shows a system having undergone an upgrade with new fasteners. The old cable fastener is just visible to the left, having been nearly completely overgrown by the tree. Lightning flashover damage is visible as lost tree bark to the left of and below the lightning protection cable pinch-bolt. The flashover could be from conductor or from the fastener beneath the surface or both. As these fasteners are at six foot intervals there is plenty of opportunity for disaster.

Effects of Lightning Conductor Connectors on Transfer of Lightning Energy to Tree Interior

The higher the resistance of the tissue to the flow of current then the greater is the potential for transformation of electrical energy to thermal energy. So it is not that the flow of current is seeking to earth (ground) through the tissue of the tree but to convert from electrical energy to thermal energy as the easier of the two options.

This creates the possibility for the energy to convert again only this time from thermal energy to kinetic energy. (BMP While a trees inner bark and cambium are its most conductive areas, the heartwood is also conductive and, when lightning is conducted to the heartwood, the tree is often shattered). (Isolated metal bolts, nails and lightning fixtures can change lightning paths from exterior to interior: Dr Kim Coder. Spark of death. Arborist News June 2004)

Use of the Arborbolt Lightning Protection Connector on Trees

In the UK, extendable fastener referred to as the Arborbolt is used.

Unlike its American counterpart, the Arborbolt lightning protection connector will accommodate the incremental growth for an indefinite period relieving us of the dangers of potential difference and the arcing spark.

[The author has patented this lightning protection system connector in the U.K. and is pursuing similar protection and sales in the U.S.--DJF]

Final Tree Protection Lightning System Details

Once the system has been installed, all the components should be mapped; this would include any bonding to aerial braces and cables as well as underground utilities. A lightning protection system fitted to a tree should have a minimum functional life expectancy of at least 30 years, and a regular inspection and earth (ground) re-test is required. This is best done at 11or 13 month intervals and the results recorded.

The reason for eleven or thirteen month period is so that after twelve years we have tested throughout the seasons (BS 6651). In my experience the ohms resistance in the earth (ground) termination can change by an ohm or two but not much more. If Bentonite was incorporated and there is a significant rise in resistance then irrigation should be considered. Also it is observed that at near zero temperatures the resistance can rise significantly and, although lightning activity is unusual at these temperatures, it is possible.

Conclusions on Lightning Protection for Trees

To conclude, it is suggested that there are very few definitive solutions to lightning issues. All that can be done is to work with the technology available and importantly record all we do. This will then help gather information from practical; experience in order to develop a better understanding of the relationship between trees and lightning activity.

It could be said that the approach to lightning protection and trees described here is over engineered but, unlike a non-dynamic structure, we cannot re-point or replace a lightning-damaged tree. Lightning is a very powerful natural phenomenon and as such is capable of overwhelming our defence mechanisms. The better our systems are engineered, the better they may control this powerful force of nature.

Reader question: did a wireless driveway alert system cause a tree to be struck by lightning?

Lightning damage - did my driveway alert control box cause the strike? said:

I recently installed a wireless driveway alert system so I know when visitors arrive on my property. The system consists of a probe, presumably a magnetic coil buried under or next to a driveway, a control box/transmitter hard wired to the probe with a cable 50 to 200 feet long and a receiver in the home that sounds an alert when a vehicle passes over or past the probe. The manufacturer instructions say to mount the control box/transmitter on a post or TREE 4 to 5 feet off the ground.

I buried the probe under a new driveway before the concrete was poured and ran the cable to the nearest pine tree, mounted the transmitter box on the tree per instructions. We have over 150 trees on the 3 acre property, 50 of them mature pines in a double row 10 to 20 apart, approximately 50 to 70 feet tall. The tree I mounted the transmitter to is one of the larger trees. Within 200 feet we have a heavily wooded area amounting to several thousand acres of trees just as tall.

We had our first serious lightening storm 2 weeks after installation and a lightening strike hit the tree, The path of the strike can be seen candy cane striping down around the trunk, knocking off bark in a 1 to 2 inch scar with a 1/8" wide and deep groove down the middle of the scar almost as if mad by a circular saw. While we cannot see the top of the strike on the tree at the bottom it ended at the plastic weatherproof transmitter box blowing it apart and probably following the 2 conductor probe wire to ground.

My question is: Did I cause this to happen, or make it very likely to happen by mounting essentially a ground wire to my tree 5 feet above the ground, turning my tree into a conifer lighting rod?

Also, since the strike ended 5 feet above the ground, it do not impact the tree from 5 feet up the trunk down through the root system. Will this make a difference in helping the tree survive?

I have read that the tree needs lots of water following a strike so it can take up nutrients. What nutrients can I supplement the soil to aid recovery?

Reply: electrical fields & tree damage from lightning


A certain question of whether or not your converted a tall pine tree into a lightning rod is beyond my expertise but my thoughts are these:

1. as a taller tree among its neighbors the tree was already at risk of a lightning strike

2. the addition of an electrical box to a tree 4-5 feet above ground level a common view would argue that more likely the effect was that you added an additional vulnerable component that was likely to be blown to bits if / when lightning struck the tree itself, reasoning that the electrical wire was only at the bottom of a much taller device. I've seen this effect when voltage moved from a source to a building roof, through building walls, across a yard following tree roots, up to a metal railing abutting a home, into metal siding, into metal water piping and electrical wiring. The latter components were the ultimate receivers but probably not the originators of the damage.

But if the electrical component itself created an electrical field that modified the electrical properties of the tree OR that created a positive or negative charge in the ground, at ground level, or at the base of the tree I would argue that the first argument (above) is incorrect, particularly if your system creates a positive charge in the earth around the tree and / or in the tree itself.

To understand this possibility let me quote from NOAA's "Severe Weather 101" article:


Lightning is a giant spark of electricity in the atmosphere between clouds, the air, or the ground. In the early stages of development, air acts as an insulator between the positive and negative charges in the cloud and between the cloud and the ground. When the opposite charges builds up enough, this insulating capacity of the air breaks down and there is a rapid discharge of electricity that we know as lightning. The flash of lightning temporarily equalizes the charged regions in the atmosphere until the opposite charges build up again.

Lightning can occur between opposite charges within the thunderstorm cloud (intra-cloud lightning) or between opposite charges in the cloud and on the ground (cloud-to-ground lightning).

... A typical cloud-to-ground flash lowers a path of negative electricity (that we cannot see) towards the ground in a series of spurts. Objects on the ground generally have a positive charge. Since opposites attract, an upward streamer is sent out from the object about to be struck. When these two paths meet, a return stroke zips back up to the sky. - NOAA retrieved 6/14/14, original source

You might want to speak with the company that designs and sells the driveway alert system you purchased to ask about its electrical field properties and to ask for their advice about lightning protection for the equipment.

Helping a tree to recover from a lightning strike

1. Because a lightning strike can cause sever but hidden damage to a tree's roots by boiling the sap in the roots, causing them to rupture or explode below ground, your tree might not recover from the lightning damage it received. Therefore before spending on any costly treatments you might want to wait six months or more, watching the tree to see if it dies completely - which would have made the cost of other expensive treatments a wasted effort.

In the case of a pine, if the tree is dying you'll see its needles turn brown throughout the tree in the coming months. (Some browning of pine needles is normal, seasonally.) And the "barber pole" lightning strike damage, if it circumvented the tree completely, may have also been lethal.

If the tree is dead it will most likely need to be cut down to avoid a safety hazard along your driveway.

2. If it appears that the tree was not killed outright (after the waiting period), then if the lightning strike left widow-makers, hanging, broken limbs, they should be pruned away. Meanwhile, for safety, keep people away from the damaged tree.

3. In addition to giving the tree extra watering (making up for damaged roots), you might want to ask a local arborist (that's someone who saves trees or cares for them, not just an enthusiast with a bucket truck and a chain saw) about repairing damage to the tree and its fertilization. On some trees with limited bark damage I've had success using a tree paint to seal the damaged area in an effort to reduce the risk of insect pest damage.

References & research on lightning damage to trees


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