Watering – Is it really that complicated?
It’s time to revisit some irrigation basics
A RECENT online newspaper article headlined “Stop using so much water! Reminder of water restrictions” indicated that our region, as a whole, was using more than our expected 200 litres per person per day requirement.
And once again it was gardeners who were targeted by being reminded that we should comply with the somewhat dodgy “Efficient irrigation for water conservation guideline” (EIFWCG) document, first published by the now defunct Queensland Water Commission quite a few years ago.
I’m not sure if anyone has read this document, but to do so will confuse and mislead most gardeners (as I mentioned in my February 2016 article).
Our region is currently in the grip of a prolonged dry spell and recent heatwave conditions have exacerbated the situation considerably.
Supplementary watering is needed to maintain our garden plants in an acceptable condition until good, steady rainfall can replenish the soil reservoir to a good depth.
Understand that, with the soil so dry, a heavy downpour will see a large percentage of runoff due to water-repellence, with very little soaking into the soil profile.
In Toowoomba, we’ll need around 100mm of steady, soaking rainfall to wet our clay loam to around 40-50cm depth, good for most plants.
So what makes for a good watering regime?
Firstly, know your soil type. Sands and sandy loams will accept high rates of irrigation, but will drain quickly and hold less moisture for plants to take up.
Heavy clays will need slow, light irrigation to allow it to enter the soil surface, will hold moisture for long periods of time (sometimes too long when saturated), but can withhold a fair amount of moisture from plant roots due to surface tension between its tiny pore spaces.
This leaves loam and clay loam, allowing a moderate rate of precipitation through its surface, and holding a good amount of moisture for a reasonable period of time.
Of course, the amount of moisture a soil can hold depends on its depth as well.
All these soil types can have their moisture retention abilities improved through the addition of organic matter.
Secondly, unless you’ve planned your garden perfectly, you won’t have “plants of similar water needs” all in the same area, so they will all have different moisture requirements and root systems of varying depths.
On average, it’s best to try to irrigate to around 50cm depth for most plants. Now there’s a fairly involved mathematical calculation we use (taking into account evaporation, plant and mulch cover, plant type, root depth, soil type, irrigation output etc.) to decide on how often to water and for how long, but let’s simplify it here.
Let’s assume that you’re following the EIFWCG document religiously, and have limited your sprinkler to around 9L/min output.
I’ve looked up the specifications of a “low-output” sprinkler, and it can irrigate up to 8m diameter (giving it a total area of about 50 sqm.) which means its precipitation rate is around 10mm/hr over that area.
Hold onto your hats folks! Using that “approved” sprinkler, it will take nearly seven hours to sufficiently wet a dryish clay loam (like that in most of Toowoomba) to 50cm depth! Some conclusions to be drawn here:
1. If we used an “approved system” (from EIFWCG) for the 1/2 hour the document suggests, then we’re only applying 3.5mm of irrigation, wetting a dryish soil to only a couple of cm depth (less if there’s a lawn or mulch in the way!)
2. The EIFWCG document does not promote efficient irrigation at all.
If you want to water efficiently, then follow these basic points:
Know your soil type (and how deep it is too).
Use a sprinkler that applies around 7-11 mm/hr output (so that most soils will absorb the moisture readily)
If plants are wilting and the soil is quite dry, you may need to water for many hours to get moisture deeply into the soil profile (around 50cm depth)
At this time of year, and if you have mulched your soil properly, you may not need to water again for up to 17-25 days, longer if we get reasonable rainfall (10mm or more) or temperatures remain cooler than normal.
Irrigating for short periods of time (e.g. ½ hr) is simply inefficient – you’ll promote shallow, weak root systems and much moisture will be lost to evaporation.
Healthy gardens are essential to maintain the health and well-being of residents in our region – without them we’d simply be existing in a tin, tile, timber, brick, concrete and asphalt wasteland.
Most gardeners I know respect the value of water for the growth of their plants, and understand the need to conserve this precious commodity.
Why is it then, that when dry seasons occur, and water use becomes a serious topic again (it should never lose importance), it’s gardeners who cop the lion’s share of criticism about excessive water use?
Ever thought about how much water is used to wash all those clean, shiny cars on our region’s roads every day?