Down to Earth - Episode 9: Watering basics (1990)

YouTube ref: https://youtu.be/INaxUXQW0ZM

Date: 2 April 2015

[Music plays and logo and text appears: Gardening Australia – 9 Watering Basics]

[Image changes to show a rainforest setting in a Conservatory]

[Image changes to show the narrator walking along a path inside the Conservatory]

Kevin Handreck:  No wonder these plants in the Adelaide Conservatory look so lush, there’s an automatic watering system that provides them with as much water as they need.  But more importantly there’s a fogging system that maintains very high humidity around their leaves all the time.

[Image changes to show fog being released and settling around the plants in the Conservatory]

And that means that they’d lose very little water.  The plants in our gardens are nowhere near as fortunate as that.

[Image changes back to the narrator]

Australia’s sunny skies and often very low humidity means that our plants lose large amounts of water, and what rain doesn’t replace we must provide through our sprinklers.

[Image changes to show various images of the plants in the Conservatory]

The question is how much water do we need to provide, or put the other way around, in a country where water is very expensive and there’s not that much of it, how little do we need to provide to just maintain our plants in the condition that we would like them to be in.

[Image changes to show various images of soil]

Think of the soil under your garden as a large reservoir of water.  Some of the reservoir space is taken up with solid particles of soil, but between them are holes called pores that are filled either with air or water.

[Image changes to show a garden bed being watered]

When rain falls, or when we water our garden, the water moves down through these pores.

[Image changes to show an animation of rainfall on a garden bed]

A rain shower giving five millimetres of water will on average wet the soil to a depth of amount 20 millimetres.

[Image changes back to the narrator]

Most of the roots of typical garden plants are within the top 500 millimetres of the soil, so that’s where we need to put our water.  Now it would take about 125 millimetres of water to percolate down to about that depth.

[Image changes to show the narrator indicating the depth of the garden bed with his hands]

[Image changes to show a sprinkler watering grass and a garden bed]

To get the most out of the water that you’re putting onto your garden you must know the rate at which your sprinklers are delivering that water.

[Image changes back to the narrator who is squatting on the ground, surrounded by cans]

Now for a lawn like this, the simplest way of getting that rate is to use a can test.  You can see here we’ve got a grid pattern of cans all over this lawn here; they can be any shape and size of can, as long as they’ve got straight sides.

[Image changes to show narrator demonstrating a can]

What we’re going to do is to turn the sprinklers on here for an hour and measure the depth of water that is put into each of these cans.

[Image changes to show sprinklers starting]

[Image changes back to the narrator]

Well the sprinklers have been on for an hour now.  What we need to do now is to measure the depth of water in each of the cans.

[Image changes to show the narrator picking up a can and placing a ruler inside to measure the depth of water]

Now I’ve done this before and I know that these sprinklers deliver about 20 millimetres of water each hour.

[Image changes as camera zooms in on the ruler inside a can]

[Image changes back to narrator]

Now if I want to moisten this soil to a depth of 500 millimetres I need about 125 millimetres of water, so these sprinklers need to go on for about six hours to fill up that soil to that depth.  That amount of water will keep this lawn going for three to four weeks in a typical Adelaide summer.

[Image changes to show a garden]

In most parts of Australia water is becoming more expensive, and we’re paying for every drop we use.  That’s a very good incentive for keeping use to a minimum.

[Image changes back to narrator]

The most powerful way of reducing water consumption in your garden is to delay watering until the plants are wilted, like this one here.

[Image changes to show the narrator indicating to a plant beside him]

What happens then is that the plant can’t get water from the soil, or it can’t get very much, and so it sort of shuts up shop for a while.  Of course it grows more slowly, but at least it uses a lot less water than it would otherwise if you kept the soil very moist.  Now this sort of strategy is not for you if you want a lush garden.

[Image changes to show the narrator walking through overgrowth]

The second most powerful way of reducing water consumption in the garden is to have a thick mulch on the surface of the soil.

[Image changes to show the narrator indicating to the mulch on the ground]

A mulch such as this here can reduce evaporation of water from the soil by about 70%.

[Image changes to show the narrator digging below the mulch with his hands]

If we dig down here we can see that the soil is still reasonably moist, even though this is the middle of the summer.  So the important points to remember are these – know how much water you’re putting onto the soil, fill up that reservoir to its total content, and then let the plants dry it down to as far as you dare go.  And of course in the garden beds, have a good mulch right over them.