Putting a Trench Under a Hedge

One of the most frustrating events for any gardener proud of their hedge is when they learn that a service such as water, drainage, sewage, electricity or cables must be laid under that hedge that has been developed and perfected over many years. Carefully watered, trimmed, fertilized and perhaps treated for threatening pests. There are many reasons why these services need to be installed. It could be a new service, or the renovation of an old one, or the replacement of one system with another – for example new fiber optics. There is no way to avoid it – they must pass underground directly beneath that lovely, established hedge. What a shame! You are now faced, you think, with years of re-building, because the usual approach to running a trench under a hedge is to dig out two or three of the plants, to make enough room for trenching equipment to pass through. In this blog we will look at a newer technique that makes it possible to run conduit, drains and other services underneath a hedge without disturbing the plants. Let’s take a look.

The Standard Approach

Once the service is then installed, and the soil replaced, there are two options. Some people try to put back the original plants, but in an established hedge those plants will have a very specific shape, the result of years of trimming, and even lining them up correctly again can be difficult. Much more important, it is very hard to remove an established tree in a hedge with a good root-ball. The roots are just too entwined with the trees on either side, and the tree has been in place too long to achieve this without a couple of years preparation. Consequently, an inadequate root-ball is prepared, the tree is often treated poorly while it waits for re-planting, and often allowed to dry out accidently. In most cases these trees that are confidently put back simply decline over a period of months, or even a year or two, before finally dying. They almost never restore the hedge or eliminate that nasty gap.

The second alternative is to put in new plants. This is usually a far better option. If the hedge is not too tall, and large replacement plants are used, in a few years they can grow in well and hide the space. But it does take a few years. If this is a 10-foot tall hedge, or more, you can easily be looking at upwards of five years for even a large plant to fill in well.

Getting a Good Match

There is another issue too. Are you sure you know exactly the plant species and variety that was used for this hedge? If you don’t, then the new plants will not perform in the same way. The color will be different. They will start and end growth at the beginning and end of the season at separate times. They will have a different growth habit, and a different visual texture. Rather than the smooth, continuous texture and color we look for in a hedge, it will look like a patch-job, always different. If you cannot get a reasonably close identification of the plant used, then you may not even get close to matching at all, and as the trees mature you will have a totally different look in that area.

An Alternative Approach

All in all, none of these options look too promising, and they all involve a lot of handling of large plants, expense, and lots of time – with no certain outcome. What to do? Luckily there is a relatively new tool on the market, often already in the possession of landscapers and arborists who do transplanting. It’s called an Air Spade.

An air spade is a simple piece of equipment. It’s a hollow lance perhaps 6 feet long, with a spray nozzle on the end. The other end has a trigger control and is connected to an air compressor through a hose. To be effective pressures of 90 pounds per square inch, and air-flow of 200 to 300 cubic feet per minute are used, but the secret is in the nozzle, which is designed to deliver an almost laser-like air blast, the cuts into the soil.

This equipment was first developed in the 1980s, but it has only been in the last decade or so that landscapers have begun to use it. The biggest use is for transplanting larger trees and shrubs, which can be done much more effectively with an air spade than by traditional digging. The air spade is used to peel away the soil from the roots, so that almost all of the root system can be exposed, and with a minimal amount of cutting it can be lifted from the ground. This is much lighter than regular dug root balls, so transport is much easier. The roots can then be spread out again in a large hole, the tree staked, and transplanting is successful and easier much more often than by traditional methods.

Trenching with an Air Spade

No, the idea here is not to use an air spade to remove the plants from the hedge, but to use it to dig the trench. That’s right. Air spades are widely used today by utility companies to access services for repair more quickly, and thus more cheaply. They are increasingly being used to dig trenches under street trees too, which brings us back to our original hedge problem.

If the trench for the drains, new sewage system, installing a water supply, etc. is dug with an air-spade, then it is possible to dig right under the hedge, between two trunks, without disturbing the root system very much at all. The picture at the lead of this blog shows you how much root is left after trenching close to trees. The same technique can be used through a hedge. The trees will not be disturbed, and the conduit or drain-pipe is threaded through the trench under the roots, with minimal cutting required. Once the soil is replaced, life continues as normal, as if nothing has happened at all.

So Next Time You Need to Do This. . .

Air spades are widely available today with many contractors and arborists. So if you find yourself needing to run a service through a hedge, tell your contractor you want them to use an air spade, instead of digging a traditional trench. It should not be hard to find a local contractor who has the equipment and staff to do it. No more dead trees and years of waiting for your beautiful hedge to be repaired, usually with limited success. This equipment can also be used for trenching near trees, and it has saved many specimens from serious damage. Let’s protect our hedges and trees, by encouraging contractors to use air spades instead of traditional, destructive trenching equipment.