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Early Season Irrigation

Yes it's dry this spring. Most of southern Ontario has experienced low precipitation in March and early April (particularly in the Lake Huron shadow and Eastern Ontario). Add to that, Environment Canada's outlook for April - June is for above normal temperatures for the entire province and below normal precipitation for all of southern Ontario. This is a year to really be prepared for irrigation - odds are you are going to need to irrigate earlier this year than the last few.

How to decide when to irrigate?

Don't guess - go out into the field, dig a hole and feel the soil. Is there enough moisture? How far down has the rain gotten? Do not wait till the seedlings are wilting - that is too late. Consider purchasing a soil moisture instrument to help you make your decision (see your local irrigation equipment supplier). You may never have irrigated in April or May before… but it might be necessary this spring - snow cover disappeared early leaving soils lots of time to dry out before planting.

Springtime irrigations require different techniques than our usual dumping of irrigation water in the hot summer months. Seedling root systems are tiny and fragile, so smaller, more frequent applications should be used for early spring irrigations.

How much to apply for the first application?

On a completely dry Sandy Loam a seedling with 6" rooting zone requires a maximum irrigation depth of 0.7". Calculate your own spring irrigation depth by multiplying your seedling root zone by your soil's available water capacity (see table 1 below). This soaking of the root zone is required where soils are completely dry and there has not been a significant rainfall. Reduce the application for soils that still contain some moisture.

Divide the desired irrigation depth (0.7") by the efficiency of your equipment (see table 2 below) to get the actual depth that you must apply. Example: using a travelling gun the grower would apply 0.7" / 0.65 = 1"

Second irrigation application

After the root zone has been wetted it should not be allowed to dry out completely. Normally irrigation should be applied again when the soil has dried down to 50% of it's holding capacity. This means the second (and subsequent) spring application(s) would be 50% of the first irrigation - in our example 0.7" x 0.5 = 0.35" inches.
Now let's divide that by the efficiency of our traveling gun system 0.35"/0.65 = 0.5"

How many days between each spring irrigation event?

Keep in mind that although the root zone is small, the plants are also small therefore the water use for that field is lower than what you would expect when the plants are mature (canopy fully closed).

If we don't get rainfall, the ½ inch irrigation should be repeated every 4 to 11 days depending on the evapotranspiration (ET). ET is a measure of how hot, sunny, humid and windy it is. Maps of Ontario evapotranspiration can be found starting May 1st on the Weather Innovations Network website http://www.weatherinnovations.com/irrHome.cfm.

More information on irrigation scheduling can be found in "Irrigation Management, Best Management Practices" book available at your local OMAFRA office or by calling 1-877-424-1300.

A Caution

If the growing season continues warm and dry your water supply may not be as resilient as in other years. This winter we have experienced low precipitation; reservoirs and ground water may not have recharged to the full extent that they normally would. Watch for future articles about "contingency plans for irrigation water supply", "water conservation plans" and "long term investments in water supply infrastructure".