Irrigation is the process of supplying water to dry land. It is one of the oldest and most important agricultural techniques, allowing farmers to grow crops on land that would normally be unproductive. In a residential context irrigation is only necessary for aesthetic purposes, although those wanting to grow their own vegetables would obviously struggle if the soil was too dry. Depending on the soil type and drainage, some sort of irrigation system may be required for an attractive and manageable garden to be maintained.
Things have advanced massively since the days of deliberate flooding. Simple canals, and computer regulated machinery, can now be used to carefully regulate soil saturation to the optimal level, with minimal water wastage. Such methods are usually confined to large agricultural developments, but scaled down versions are available to horticulturists. Installation of such systems is most commonly a professional task, below are detailed the different types of irrigation system available and conditions which dictate their use.
Infiltration, the rate at which water passes through the ground is mainly dictated by soil type. Plant cover and soil saturation also play a small part, but these are both directly affected by the irrigation system anyway and so need not be considered. The main categories of soil are as follows:
- Gravel- This soil type holds very little water owing to the large gaps between stones with water moving over 20 millimetres per hour.
- Sand and Silt – These soils have a medium to high infiltration rate, meaning they hold water for only a short time. Loam (a combination of sand, silt and clay) unsurprisingly in the middle.
- Clay – This is the most compact soil type found in the UK and has a very low infiltration rate. In the summer months it can contract in the heat, causing large cracks to appear, even if the ground has a decent covering of turf. These large cracks increase the infiltration rate massively, but the water moves well beyond the root level of most small plants and grasses and so will not help the turf itself.
As a general rule, small amounts of water should be applied at regular intervals to most types of soil. This means that water is not wasted on sandy soils, as large volumes would pass through the soil straight away, and provides a chance for the water to soak through more clay based soils to prevent them from becoming waterlogged.
In addition to irrigation there are two other techniques for treating very impervious soils such as clay. A roller with spikes that remove small cores of soil from the ground which allow roots more room to grow and facilitate water movement down through the soil. Another method is to add a chemical surfactant to the water which lowers the surface tension of the water, which again allows it to move through the ground more easily. Both these techniques can be used in conjunction with other irrigation systems.
When it can be done
- Garden landscaping - The installation of an irrigation system can involve a fair amount of work, this can be minimised with careful planning and foresight. If the property is to be built on a new plot and the garden to be landscaped onto bare ground, almost any of the systems detailed can be used (depending on which is the most suitable). If the area that is to be irrigated has already been deliberately landscaped with fences and plants in place, the options are slightly more limited as these can obstruct pipes and turf may need to be taken up. If a pipe-based drip system is to be used it is most convenient for it to be installed before the turf is laid to minimise work. Ideally this should be done when there is no risk of a frost. If this can not be avoided, the area should be drained down after the installation and then irrigation begun fully in the spring.
- Garden design and plant layout - Before the system is installed, the garden should be split up into irrigation zones. These are sections of land which have different irrigation requirements, which will of course depend on drainage, soil type, aspect, plant type and cover. The requirements of each area can then be catered to with either a different level of water, administered through the same system; or covered by a separate system altogether. Computerised synchronisation of each of the irrigation components may seem somewhat overzealous, but this can be done relatively cheaply considering the likely price of the entire project, and is easily made up be using water more efficiently. If the zones were not dictated by plant type, then the garden should be planted with these zones in mind. Plants can easily be damaged by over watering, and so the water requirements and tolerance to drought and flooding should be similar for all the plants in each zone. Careful consideration of the water requirements will also help to save water and energy. In conclusion, the system should be installed with the zones in mind, but this of course requires the garden to be completed. If working from scratch, it is worth designing the garden and irrigation system alongside each other.