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The Climate of Southwestern Ontario
Consistent,Warm Growing Seasons
Summers in the seed corn production region of southwestern Ontario are remarkably stable.
In Ontario, heat accumulation is measured in Corn Heat Units (CHUs), a system specifically developed to
track corn growth. On average, seed corn production areas receive 3200 to 3400 CHUs per year (Table 3).
In general, this equates in the U.S. system to 105 to 110-day maturities.
 Because of the moderating influence of the Great Lakes, a wide range of varieties are grown, covering most
maturities grown in Canada and those in the world's expanding cool-season production zones.
Risks of post-planting and pre-harvest frost are also reduced by proximity to the Great Lakes. The frost-free
days for this region average between 160 and 170 days for southwestern Ontario, with the last spring and first-fall
frosts occurring around May 1st and October 13th respectively. To put this in perspective, 130 to 150 frostfree
days are required for commercial vegetable production.
Abundant Rainfall
An average 12 inches (300 mm) of rain falls in the key months from May through August in southwestern
Ontario (Table 4). This is approximately one-third of total annual precipitation.
Drought risk is minimized by the fact that soils generally start the spring with excellent moisture reserves.
Southwestern Ontario soils also have high moisture retention capabilities.
Rainfall is more variable than heat unit accumulation. Even so, drought is uncommon with losses due to drought
only reported in two of the last 20 years of production.
More often, the potential risk is from too much rain. This is why southwestern Ontario growers have invested
in systematic subsurface tile drainage instead of irrigation for their high value crops, including tomatoes, other
vegetables, and seed corn.
Subsurface drainage, combined with the good internal drainage characteristics of southwestern Ontario soils,
permits growers to achieve critical objectives including early planting, consistent emergence and growth, and uniform maturity.
Microclimates
The relationship between southwestern Ontario soil zones, heat accumulation and rainfall patterns form
three distinct regional microclimates:
 South
- sandy, silt-loam soil, cool spring, warmer fall, lake effect moderates temperatures, later planting, last to harvest, 3200 heat units
North
- black-loam soil with excellent water-holding capacity, less heat, more moisture than West, earlier planting (similar to West), 3300 heat units
West
- sandy soil well drained and pliable, hottest of three zones, earlier planting, harvested first, 3400 heat units
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Existence of these three regions results in detasselling and harvesting schedules that are spread out over a
longer time frame, which is more accommodating for contracting seed companies.
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