September 18, 2020

Climbing the walls

mark laurence

Landscaper and arborist Mark Laurence discusses how to help vertical gardens thrive in arid conditions.

In the October issue of Pro Landscaper Gulf, I discussed the benefits that green walls bring to an urban environment. In November, meanwhile, I talked about the over irrigation of landscape that tends to occur, particularly in arid landscapes.

Maintaining life in a thin layer of growing medium stretched up the side of a building may, after all, seem counter-intuitive to say the least. With that in mind, the question is under what conditions can we make arid climate vertical landscapes work?

Water management

Firstly, for living walls, I am a firm believer in the use of hydroponic growing methods over soil, compost or sand-based mixes. An inert, neutral growing medium does not become exhausted, unlike soil-based mediums. Nor do plants rely on the medium for their nutrition, which is provided via irrigation.

You might expect hydroponics to use excessive amounts of water, but this is not so. In temperate climes, we use about 1L/m²/day, while tests in the UAE used around 5L/m²/day – far less than is applied to horizontal landscapes. That said, many of the most suitable plants for use in an arid region might not like having a consistently wet root-zone, as any living wall medium is likely to provide. Such plants are used to putting out long, deep roots to seek out moisture, and many of these will be halophytes (salt-tolerant).

Ideal plant species

If we want to move living walls – and landscaping in general – towards a more naturalistic feel using appropriate plants, we have to accommodate their needs. In this respect, I suspect the optimal living wall system does not yet exist. I have previously worked on trials in the UAE, and successfully established a set of plants that appear to survive full exposure all year round.

However, to move to more naturalistic planting we need plants such as low forms of Atriplex, Acacia prostrata or Heliotropium (not necessarily natives but halophytes that are appropriate in arid landscapes).

These also give us the possibility of using more saline water for irrigation without adverse effects. (Hydroponic mediums don’t bind mineral salts, as they do not have a cation-exchange capacity.) This is important for the long-term health of plants grown in essentially artificial and isolated cells, as per any living wall or container.

New development

I have two patented hydroponic systems to my name, yet don’t feel that either of them – or any other system – perfectly fits the needs of arid-environment living walls and certainly not the use of halophytes. I am therefore developing a system which I believe will work well for these conditions and am looking to test this in the UAE and/or other areas in the Middle East. Any interest from readers is welcome.

My experience in various parts of the world suggests to me that we have largely been using the wrong kinds of plants in our living walls for the past decade. We need to move towards a low shrub-type plant selection for long-term effectiveness and durability. The Middle East has hardly started exploring the use of living walls, so now is the time to get it right.

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