F E AT U R E
Guiding Principle: Water plants efficiently Efficient watering is selective (only on plants that need it), and dispersed into the root zone with minimal loss through evaporation.
sportsfields. Spray irrigation has moved into the ornamental landscape industry with few modifications.
The recent 3 year drought and water restrictions in the Cape have led to a number of observations which can inform the way we look at irrigation: • Many established trees and plants survived on rainfall only - they do not require any irrigation. If watering is only required during the establishment phase, is hand watering an option, or a temporary irrigation system which is removed after two years?
Water is sprayed onto the soil and leaves of plants, resulting in an irrigation efficiency of 75%. One has to ask: does it makes sense to spray water, knowing that 25% is lost to the atmosphere during watering? In a typical Cape summer another 5mm may be lost by direct evaporation from the soil within 24 hours, so how effective is a 10mm watering, if only 2.5mm (25%) remains in the soil after this period?
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Some plants require water only during the hottest period of the dry season – occasional, selective watering is enough to keep them going. Can these plants be grouped together - spatially, or on the same irrigation station, with targeted watering?
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Plants from high rainfall and temperate regions did not survive without regular watering – clearly these are not suited to local conditions.
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In some landscapes hardy water-wise species died which was surprising. Upon examination these plants had shallow root systems because they had been incorrectly watered during the establishment period. Current watering practises - too frequently and not deep enough - make plants water dependent.
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While watering with buckets or watering cans is inconvenient, it is targeted and effective when plants have watering basins. Selective watering is efficient.
The irrigation efficiency of drip is published to be 81%, and the addition of a thick layer of mulch would help to further reduce water loss by direct evaporation from the soil. However subsurface irrigation (e.g. Novap) is more efficient, as drippers are covered with 25cm soil which cuts out any direct evaporation. Another advantage is that wetting the soil to a depth of 30-60cm encourages deep rooting.
Guiding Principle: Plant appropriately It is important to select plants that match the available water budget for the site. When there is very limited or no water available for irrigating, it is advisable to select indigenous or exotic water-wise plants that can survive on local rainfall only. Rainer and West (Planting in a Post Wild World, 2015) make the point that stress is an asset, something to embrace and use as a major determinant in plant selection. There is a general tendency in landscaping to ameliorate the conditions on site to be able to grow a wide range of plants. We improve the soil by digging it over, adding compost and fertilizer, and irrigation is installed to provide constant moisture. However by removing the constraints that limit plant growth, one is removing the very qualities that will create a strong sense of place. It is also the start of an intensive maintenance programme of ongoing feeding, watering and pruning. Careful plant selection and embracing slower more sustainable growth are key factors when creating low maintenance water-wise landscapes. Group plants with similar water requirements into hydrozones. This low water-use zone is planted with Cotyledon orbiculata, Hesperaloe, Anizoganthos, Pennisetum and Rhagoda histata and only requires once a month rainfall / watering.
The current method of spray irrigation originates from the northern hemisphere, from the turf industry, where sprinklers are used to water
Soil probes are an excellent way to see how far the moisture has soaked into the soil after a rainfall or watering event. How many mms will it take to give a deep watering, where the whole soil profile is moist? A thick layer of mulch will minimise direct evaporation from the soil surface.
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Pro Landscaper Africa | March 2019
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