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Rain
and Gardens Water – all gardens need some,
but how much is too much? How much is too little? Helen
McKerral looks at how to maximize your water assets. |
I’m writing this just after 90mm of blessed autumn rain fell
onto my dry South Australian garden – hallelujah! As the east
coast was flooding earlier this year, South Australian,
Melbourne and Perth gardeners had been looking in vain to
stubbornly sunny skies. Even when skies turned grey, clouds in
Adelaide refused to cooperate until the end of April when
gardens and gardeners were at last able to breathe a sigh of
relief.
In typical fashion, after months with just a few millimeters of
rain, April in Adelaide brought four or five consecutive days in
which 90mm fell. To the dismay of many gardeners, much of that
water went first to our stormwater drains, and then out to sea.
Cities, with their large proportion of hard surfaces, create a
huge amount of stormwater – stormwater that constitutes a
precious resource when retained, but which becomes polluting
runoff when allowed to pour into the sea.
At state and council government levels here, plenty of things
are happening to minimise stormwater. Wetlands are being
established to capture and filter stormwater, and inject it into
aquifers for later extraction. Housing estate developments must
now meet increasingly stringent requirements to minimise runoff.
Rebates have promoted the installation of new rainwater tanks
attached to pre-existing houses (see Global Garden Dec-Jan 07)
and a tank is mandatory for every new home.
Home gardeners can play their part, too. By retaining the
maximum amount of water on their properties, those properties
contribute less to the stormwater load. With recent drought
conditions – and possibly more permanent changes in climate -
too much water is a problem for few southern state gardeners,
and the more water we can retain, the better for us, our gardens
and the environment – one of those rare win-win situations!
Unfortunately, as we’ve recently seen, Australia is indeed a
land of drought and flooding rains! How nice it would be to have
neat, evenly distributed rainfall throughout the year, with
slightly more falling (at convenient, regular intervals) during
the dry season, and slightly less in the cooler months. It’s
never like this, of course, so any home rainwater management
system must aim to retain as much as possible, without at the
same time flooding gardens or neighbours. Rainwater tanks are
one obvious solution, but many more options are available.
The Soil as a Sponge
One of the simplest things gardeners can do to maximize water
retention is to improve the water holding capacity of their
soil. Field capacity is the maximum amount of water a particular
soil will hold. Organic matter incorporated into soil acts like
a sponge, holding water between the soil particles and
increasing the soil’s field capacity, while at the same time
providing bulk to maintain airspaces, thereby preventing
waterlogging. Organic matter is especially important in deep
sandy soils, where water can otherwise rapidly move through the
profile and into deeper water tables. Some organic matter, such
as peat, is famous for its water-holding capacity, but any
well-rotted compost will do the trick. Various commercial
preparations can help, too – I’ve had excellent results with
Hydrocell in my garden.
Improving Water Penetration
For rain to get into the soil, it must first penetrate the
surface, or it all just runs away downslope into the street or
neighbours’ yards. The steeper the slope, the faster water will
run off, and the less ends up in your soil. An open, friable
soil allows more water to penetrate than compacted soil or that
with a hard, dense crust. Sandy soil with large particles and
spaces between the particles generally allows water to penetrate
quickly, compared to clay soils that have smaller particles
packed more closely together. All of these soil and
topographical characteristics can be manipulated to keep as much
water as possible where you want it.
Loosening hardened surface soil with a metal rake or hoe, or
“roughing up” smooth surfaces, provides instant, albeit
relatively short-term results. Adding gypsum at recommended
rates to heavy clay soils will make them more friable: fork the
soil into cloddy chunks while it’s moist but not wet, then
sprinkle over gypsum. Leave for a month in the wet season before
further work.
Level topography by terracing, or by contouring the ground so
that water flows more slowly. Shape raised garden beds with a
shallow concavity rather than a convex mound to catch – rather
than shed – water. Cover bare, smooth soil with groundcovers or
mulch to slow the flow of water over the surface.
Commercial soil wetters act to break the surface tension of
water, allowing it to penetrate soil more effectively. Soil
wetters are enormously variable in efficacy – you may need to
try a few before finding the best. If you’re intending a
once-off application just before rain, low-phosphorous household
detergent works just as well, but not for as long.
On site retention basins and other earthworks
Creating deep or shallow unlined basins in the low-lying areas
of your garden will catch and hold a surprising amount of water.
In my garden, just before pipes enter the street, I’ve dug a
deep “pond” of perhaps 4,000L capacity, fed by a shallow
“creek”. The overflow from all rainwater tanks, surface drains
and roof surfaces is directed to the street via this retention
basin. Because the pond is unlined, a significant amount of rain
is required before it overflows, even when my tanks are full;
these last few winters have been so dry that the soil has not
been anywhere near field capacity and soaks up as much as I can
put in. The water from this pond supplies a number of large gum
trees and a melaleuca, all of which would be sucking more
thirstily elsewhere if not for their steady supply. I could buy
a cheap submersible pump and move water elsewhere onto garden
beds.
Tailor the size of your basin to hold the maximum amount of
water you expect from a heavy event (state water departments
provide tables to calculate volume for rainwater tanks for any
given roof catchment), and ensure the overflow from the basin
won’t flood other areas.
A shallow “creek” that flows only after heavy rains is an
alternative; sometimes it’s possible to simply replace a section
of pipe with a pebble-lined channel. Place a few rocks near the
pipe inlet to prevent erosion, and a grill over the outlet to
prevent blockage (but remember to clear it regularly!).
A more complex alternative is a rain garden, which is designed
to grow purely on captured stormwater.
Finally, when paving or hard surfacing driveways or entertaining
areas, consider options that direct runoff into garden beds
first, and then the street, or that allow water to penetrate.
For example, plant groundcovers between crazy paving, instead of
concreting. Lay two strips of concrete in the driveway for car
tyres to run along and plant the center with groundcovers, or
use gravel instead. New driveway surfacing constructions
designed to prevent compaction and allow maximum water
penetration are now on the market – some even include dedicated
storage systems directly underneath!
So the next time you see all that water rushing wastefully along
the gutters of your street, consider how much of that water is
yours – and what changes you can make to score a win for the
environment… and your garden!
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