Irrigation systems


Watering plants is the single most time-consuming part of the service technician’s job. It is also boring and wastes the time of our skilled staff. For this reason, several companies have developed what they consider to be a panacea – the ultimate irrigation system.

This means that there are now dozens of products, all making competing claims about their efficacy.

Different types of irrigation system

There are several different categories of irrigation systems, such as self-contained subterranean irrigation, semi-hydro subterranean irrigation, electronically controlled built-in systems, polymers, chemicals and other soil additives, and hydroculture. What we have done for this article is to compare different subterranean irrigation systems and give you our thoughts on their benefits.

Self-contained subterranean irrigation
These systems are very popular. There are also many different products available. One look at any issue of ‘Interiorscape’ magazine or in an Interior landscape supplies catalogue will show just how many of these systems there are. Most subterranean irrigation systems operate in the same way. A tank of water is placed in the soil, under the plant, and water is drawn into the soil through wicks or soil plugs by means of capillarity. Good examples include the well-known ‘Mona’ system, the ‘Rentokil Irrigation System’ and the ‘Tanker’ system.

Rentokil Initial R&D trials conducted over several years have shown that subterranean irrigation has some real benefits. These trials have consistently demonstrated that sub-irrigated plants require less water, grow more vigorously and retain their condition for longer. A dramatic illustration of this is shown below (from an experiment on Spathiphyllum wallisii).

Comparison of plants watered with a subterranean irrigation system (left) and top watered (right). Note, the plants that were top watered in this experiment had an irrigation tank fitted so that the total volume of soil in both sets of plants was the same.

One subterranean irrigation system that doesn’t rely entirely on capillarity is the CWI irrigation system. This product stores its water in the side walls of the insert and uses a vacuum sensor to control the flow of water into the soil. The system is self-regulating. The vacuum sensor only allows water into the soil when the soil has dried out. There are several benefits to this. First, service staff do not have to worry about whether there is sufficient water in the tank to last until the next service visit – just top it up and let the system control itself. Secondly, the capacity of the system is such that watering intervals can be extended to four or more weeks in the right circumstances. Finally, because the water is held in the walls of the unit, not in a deep tank at the bottom, planting depth is increased, allowing greater flexibility in container choice.

More information on the CWI irrigation system can be found on the manufacturer’s web site.

Two subterranean irrigation systems. CWI system (left) and Rentokil Irrigation System (right)

Semi-hydro subterranean irrigation
Semi-hydro subterranean irrigation is very popular in many European countries. It is a very simple, cheap and effective method of watering plants, although setting up the system can be a little tricky. The bottom few centimetres of a container are filled with LECA (Light Expanded Clay Aggregate) granules. On this, a layer of capillary matting is laid. Capillary wicks are sometimes fixed to the bottom of the mat to trail into the LECA layer. Ordinary potting compost is then added to fill the remainder of the container. A filler tube is included that runs from the soil surface to the LECA layer. The LECA layer acts as a sump and reservoir for irrigation water.

The plant is then planted into the compost as normal and installed on site. To begin with, the plant is top-watered as well as having water added to the LECA layer.  Once the roots of the plant have grown sufficiently, watering via the filler tube into the LECA layer is all that is necessary.

There are two main disadvantages to this system.

First, if this system is used with ceramic containers, the container must be lined or made completely waterproof. This can increase preparation time and add to complexity.

The second drawback is that there is a body of free water in the container, not held in a separate tank. This means that even the slightest crack in the container could result in a leak.

The benefits of subterranean irrigation

• Most systems allow an increased interval between watering, sometimes to as much as four to six weeks.

• Subterranean irrigation reduces the risk of sciarid fly infestation. This is because sciarid flies (fungus gnats) can only breed in the top few centimetres of the soil, which has to be damp. Subterranean irrigation keeps the soil surface dry at all times.

• A dry soil surface also reduces the risk of soil fungi.

• Plant longevity is increased.Experiments have shown that plants live longer, grow bigger and look better when subterranean irrigation is used.

• Subterranean irrigation allows the use of a wider range of decorative top-dressings that would otherwise be washed into the soil by top watering.

Click here for more information on watering large feature beds.

Author: Kenneth Freeman