Water-wise Gardening - Where are we?
As a landscaper, I feel we pay lip-service to water conservation in the landscape industry. Despite the liberal use of the word ‘water-wise’ on plant labels at nurseries, and during design presentations, the fact is that many gardens and landscapes are watered at least twice a week, and often three times.
While some indigenous and water-wise species are indeed adapted to drought, the way that we water (too frequently, not deeply) prevents them from developing strong, deep root systems. Instead of ‘making rain’ with our irrigation systems – giving a long deep watering - we wet the leaves, mist the atmosphere and moisten the top few centimeters of soil – only to have most of it evaporate soon after. Crazy, isn’t it? When one considers that most gardeners are using treated potable water on their gardens, it becomes even more incomprehensible. Imagine emptying 500 bottles of mineral water in one area of your garden – this is the typical water-use of ONE station of shrub sprayers.
Clearly, it is time for a major rethink.
What is a water-wise garden?
Is a water-wise garden one that is watered once a week, or once a month during the dry season? Or not at all? Or since rainfall varies widely across the country, should it be a relative measure?
There is currently no benchmark in the industry for a ‘water-wise garden.' It is a vague relative term that implies ‘less than usual.' Personally, I think this is not good enough! Fourie Petersen suggests water-wise landscaping should be based on a philosophy of sound ‘water management practices’. And ‘management’ requires measurement. Imagine if all landscapers always installed a water meter and rain gauge at each site, and organized weekly data to be collected and submitted to a regional/ national database? If we measure the amount of rain and water applied to landscape, we will have an accurate record and understanding of water-use, and be able to develop a benchmark over time.
Our relationship with water
Is it fair to say that water restrictions help to increase general awareness and appreciation for water? I think so. Switching off automated watering systems has brought us closer to our gardens: we are now tuning into plants and their needs and having to make tough decisions which ones are going to receive the grey water. My pots have never looked better since I’ve put a plastic basin in the kitchen sink for rinse water – it provides a few liters several times a day. In the days when I still had a bath with my young son, and it took me ten full buckets to empty it, I realized that even a shallow bath used a hundred liters. Needless to say, we have been showering since!
Saving and storing water
As landscapers in the Cape, we are often faced with the question of whether to install water storage tanks. This has led to healthy debate when one considers the following:
The Western Cape experiences a long dry season – often 4-5 months with no rainfall
One irrigation cycle can require 5000 -10 000 liters for a medium to large garden, so one can empty a full tank in one watering cycle
Tanks require space and have a visual impact
Water is pressurized with a booster pump, so electricity is used
Tanks are made of plastic, which has a significant environmental footprint
Looking at the above, it would suggest the value of storage tanks is questionable in the Cape. However, it is predicted that as a consequence of climate change, rainfall will be more erratic and less sharply seasonal. This summer in Cape Town seems to be a case in point: we received a big rainfall event in December and another downpour a month later – very unusual for January. If gardens were properly zoned into water-use zones, a full tank could last much longer, when used judiciously for selected high and medium water-use areas. Watering will make the difference between Scadoxus flowering, for example, or barely surviving during a long dry summer.
Storing rain water has another less tangible benefit: it brings us closer to the weather and rainfall and increases our appreciation of water. Recently I visited my friends who had put a 200-liter plastic drum under a gutter in their courtyard. It seemed so insignificant compared to the 2 x 5000-liter storage tanks they already had, but Pat pointed out it was enough to make 200 liters of foliar feed for his veggies. He requires unchlorinated water to make activated worm compost tea, so the rainwater is valuable. This experience has shifted the way I think: even small water storage tanks are worthwhile.
Understanding water in the soil
In his excellent book ‘The Water-wise garden’ (Penguin Australia, 2008) Jeffrey Hodges explains how plants use water and the way water is stored in the soil. I found his book fascinating, and the chapter on ‘How water works in your garden’ has transformed my watering practices. Understanding the physical aspects of water and soil has changed the way I look at water. He describes that ‘when water falls onto the ground, either by irrigation or natural rainfall, soil scientists divide it into 3 categories’ (notice only a small portion of it is actually available to plants):
‘Run-off water is water that washes off the surface without soaking into the ground. It may cause erosion and is unavailable to plants.
Gravitational water is the water that soaks in and drains away into the subsoil – not available to plants.
Capillary water is the water that remains ‘stuck’ in the smaller pore spaces of the soil – this is the water which is available to plants.’ This water which surrounds the soil particles is what root hairs and mycorrhizal threads (mycelia) can absorb.
It is worth noting that the free capillary water keeps moving up, towards the surface, as water evaporates into the atmosphere. It is as if water is constantly sucked up through the soil. This is why a thick layer of mulch really helps because it cools the soil and slows down the upward movement of soil water and loss to the atmosphere. Hodges explains that ‘as soil dries out, the moisture films around soil particles become thinner and at a certain point the root hairs cannot ‘extract’ the water, because of strong attractive forces holding the water to the soil particle.’ At this point - called permanent wilting point - the plant goes into water deficit.
Above: Sketch from 'The Water-wise Garden' by Jeffrey Hodges Raindrops that fall on bare soil damages its structure. Covered soil remains undisturbed and the water soaks in where it falls.
There are many ways one can reduce the need for water in your garden, and create landscapes that can survive and thrive on natural rainfall. Some if it requires careful planning (grouping plants according to their water needs, planting in the right season) and an element of design (dividing your garden into high, medium and low water-use zones). Planning the high water-use zone close to the house will enable you to optimize the use of grey water and stored water.
Choosing plants at a nursery
Buy plants which are small and stocky, and which look in balance to the pot or bag. Plants which are large and full, more than 1.5x the height of the pot, have been generously fertilized and watered, and have too much foliage for the small root system to sustain. They are also often root bound. Instead choose small, multi-stemmed plants that are actively growing.
Buy plants that are grown in soil – i.e. where the bag or pot has some weight to it. Plants grown in light coir or bark mixes which are very light, require daily watering and are therefore not suitable for water-wise gardens.
Plant at the beginning of the rainy season – this gives plants 6 months to establish before the onset of dry weather.
In loam or clay soil, fill the plant hole with water and allow it to soak in before planting.
In sandy soil, line the bottom and sides of the hole with wet corrugated cardboard, add plenty of compost and plant.
There is a school of thought that says do not add any compost or fertilizer, as this promotes slow, sustainable growth and root development. I think it depends on the soil – why not experiment in your garden and see how adding nothing compares to a composted hole? Remember that the latter will grow more quickly and look best, but which of the two will handle the first dry season?
Create a watering basin of at least 60cm in diameter, and 20cm depth, so that it can hold 10-30 liters of water – that is 1 to 3 watering cans or large buckets!
Mulch with 4-8 layers of wet newspaper and 5 to 15cm of organic material (shredded garden waste, dry grass, chipped wood) on top of the soil surface. This will suppress weeds, keep the soil cool and radically reduce the need for water.
Above: Using cardboard: Line the hole with corrugated cardboard, place compost in the bottom of the hole, and add a 5 - 15 cm layer of mulch on top.
Do not use sprinkler irrigation as it favors the development of surface roots.
Water directly onto the plant’s root zone – for most plants this is the drip line, i.e. below the outer edge of foliage.
Understand that water does not move sideways from wet to dry soil, and plant roots will not grow in search of moisture. If you are not watering the actively growing roots, you are simply wasting water.
The type of watering given during the first summer affects the root development of the plant. Deep, infrequent watering makes plants more resistant to drought.
Create a watering basin around each plant. Water deeply and infrequently e.g. 20-30 liters once a fortnight in clay /loam soil.
If you have had a small rainfall event (say 10-20mm), consider watering directly after the rain so that you capitalize on the fact that the mulch and top layer of soil have been moistened. Adding another 20-30mm will make it a more effective deep watering.
Understand that it is okay for plants to look water-stressed at midday, so long
as they recover during the night.
Even gentle slopes result in water run-off and waste. Create swales and little berms to slow down and direct water run-off into the root zone of trees or shrubs.
Install a dedicated water meter to measure and record weekly water usage in your garden. If you have no water restrictions, measure the amount of water used by each station of your irrigation system. This is how I discovered my drip system had a dozen leaks – it was using way more water than the spray station!
Weed your garden. Weeds take up moisture from the soil and lose it from their leaves by transpiration – i.e. they suck away moisture, leaving less for your garden plants.
Keep mulching and adding organic matter to your soil – not only is it food for earthworms and beneficial microorganisms, but it also becomes humus which can absorb up to twice its weight in water, thereby improving the water-holding capacity of the soil.
Hardy succulents, large