We can be good stewards of our precious water resources by using just the right amount of irrigation while managing our home lawns and gardens.  Irrigation amounts are usually expressed as the depth of a layer of water (in millimetres or inches), spread over the garden or lawn.

How much water do plants use?

The amount of water used by happy plants that are not under water stress depends mainly on sunshine.  Here are approximate daily summer water use values for a sunny lawn or garden:

So, by simply watching the sky conditions, you can estimate your plants' water use, note it on your calendar each day, and add it up for a week.  You'll find that happy plants in a sunny location, with a mix of sunny and cloudy days, will typically use about 25 mm (1 inch) of water during a summer week, or 100 mm (4 inches) per month.  But average summer rainfall in Guelph is only about 80 mm (3.2 inches) per month, which explains why we usually  need some irrigation.

If you are comfortable with allowing a turf area to go dormant (turn brown but not die), then only about 5 mm of water per week will keep the grass alive, and it will even survive three or four weeks with no rain or irrigation at all. 

Measuring rainfall

Once we see signs of browning on the lawn or wilting leaves in the garden, we know the plants have used up the reserve of water left in the soil from winter snow and spring rains.  Now it's important to start measuring the rainfall.  Simply purchase an inexpensive plastic rain guage from the hardware store and set it up in a location that's open to the sky, and away from your house and trees as much as possible.

Deciding how much irrigation is needed

You can see where we're headed now.  If you've had an average week when your sunny garden used about 25 mm of water, but only 10 mm of rain fell, you need to replenish 15 mm of water to keep the plants happy.  To determine how long it will take your sprinkler to deliver 15 mm of water, you can do the following:

Save 5 medium-sized tin cans (about 800 mL size is suitable).  Next time you water the lawn, space the cans out along a line from a pace beyond your sprinkler to a pace inside the sprinkler's range.  Put a small stone in each can to keep them level if necessary.  

Turn the sprinkler on for exactly 1 hour.  Then pour the water from four of the cans into the fifth one (remove the stone if you used one) and carefully use a ruler to measure the depth of water in the can.  Divide this depth by 5 (gives average depth per can) and now you know how many mm of water your sprinkler delivers per hour.

At a 5 mm per hour rate, you would require 3 hours of irrigation to replenish 15 mm of moisture.  If you later adjust your sprinkler to change the size of the area it waters, this will change your delivery rate.  For example, changing from a full circle to a quarter circle of watering will deliver four times more millimetres per hour. 

If you water your garden from a rain barrel, here's the recipe you need:

1 mm of water is provided when 1 litre of water is sprinkled over 1 square metre.  So you would need 15 litres per square meter of garden to replenish the desired 15 mm of water in the above example.

Whether you water by sprinkler or by hand, apply the water in two sessions -- half in the morning and half in the evening -- to allow time for it to soak into the soil and avoid any losses due to runoff.

It's easy to be water-wise when managing lawns and gardens!