April 20, 2021

Irrigation isn’t always the answer

irrigation

Landscaper and arborist Mark Laurence says that in light of shifting climates, the goal has to be minimal water use.

Irrigation is considered a necessity when landscaping in arid climates. It is a view I wouldn’t like to completely contradict, yet I have seen a fair bit of evidence that tells me that many plantings, trees especially, are over-watered.

Of equal importance is the fact that many irrigation methods waste water and sometimes damage the trees themselves. We have to discern the different requirements of trees and understand that what is necessary for one species is overkill for another, particularly when mixing native with exotic species. You will spot Ghaf (Prosopis cineraria) and Sidr (Ziziphus spina-christi) growing wild without irrigation, but imported exotics need a regular supply of water. I have seen Ghaf trees blown over in roadside plantings caused, I suspect, by shallow root development encouraged by excessive surface irrigation.

Even exotics such as the flame tree (Delonix regia) tend to be over-watered, causing a reduction in flowering and a susceptibility to bacterial wetwood. How the water is applied is just as important. Pop-up sprinklers in lawns can damage the trunks of trees, causing aerial rooting in species such as palm or fig, discolouring bark or causing stress-induced rots to occur. Drip irrigation is better but still not ideal, as it applies water at the surface and still encourages shallow rooting.

In the UK, we are used to putting a subterranean irrigation ring around trees, which gets water to the roots at a deeper level. For watering established trees, perforated tubes can be used, inserted vertically throughout the root zone, with the trees either manually watered or connected to standard irrigation systems. Supplying water at a slightly deeper level means less water is used. A word of warning, though. Most feeding roots occur in the top 300 to 500mm of soil, so watering too deeply can also be wasteful.

In coastal cities, problems can arise from a naturally high-level, saline water table. Halophytes (salt tolerant plants) have evolved to cope with this but for some imported species, salinity can be a problem. You also have to be aware of the quality of the irrigation water itself which, if drawn from the ground, may have a high saline content. Get your water supply tested if you are unsure.

Ultimately, I believe that planting styles and expectations of “landscape” must change. A more natural style, with more xeriscaping (landscaping that reduces or eliminates the need for irrigation) and use of natives or other arid-loving plants from different parts of the world (but from similar conditions), will emerge. Building plant communities that function and thrive in place without much human care or maintenance is more important, in my view, than using strictly native species.

As climate zones shift rapidly around the world, nature cannot keep up and it will be down to us in the industry to create landscapes that sit well in their altered environments, whether native or not. I believe we can do this with considerably less use of irrigation. The water we do use should be grey water (from taps and sinks), which is a much better way to conserve processed water use.

The goal has to be minimal water use, natural, ecologically benign planting, and urban environments which feed our need to connect to nature.

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