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Back to Agricultural Climate of the Eastern
Prairies
Soil, Water & Climate
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Heat
| Spring Frosts, Autumn Frosts and Frost-Free Periods Frost
occurs at temperatures of 0°C or colder. The occurrence of frost is one of the most
important concerns for agricultural production on the eastern Prairies, since this region
is close to the northern limit for agriculture. Unusually late spring frosts or early fall
frosts can have serious economic consequences.
The average date of the last spring frost is particularly important because it
determines the date of planting. In addition, a late spring frost can do serious damage to
young plants The date of the first fall frost determines the end of the growing season for
most crops.
A particularly important frost characteristic is the frost-free period, defined as the
period between the last spring and first fall frost This is the time available for crop
production. |
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The average date (50 percent chance of occurrence) of the last spring frost shows that
two regions within Manitoba have the most favourable conditions for early planting (Figure 2). After May 20 there is less than a 50 percent chance of
frost in the Portage la Prairie, Langruth and Vogar regions, and in the Pilot Mound,
Morden and Altona areas. In most other areas of Manitoba, the last spring frost occurs, on
average, after May 25. On higher elevations above the Manitoba escarpment, the average
date of the last spring frost is somewhat later. The central Interlake and the
southeastern regions of Manitoba can expect frost during the first week of June in one out
of every two years. In southeastern Saskatchewan, the last spring frost usually occurs
between May 20 and May 25, while in Saskatchewan's northeastern agricultural zone, it can
be as late as June 9
The one-in-four year risk map (Figure 3) shows a similar
geographical pattern, but the last spring frosts occurs about eight days later than
average. This means there is a 25 percent risk that the last spring frost will occur
after May 25 in the Portage la Prairie and Vogar regions, and the Altona and Morden areas.
In most other regions of Manitoba, one in four years has a spring frost in the first week
of June. In the Riding Mountain and Hodgson regions, there is a 25 percent risk that a
spring frost will occur later than mid-June. Eastern Saskatchewan shows a similar south to
north pattern.
At the 10 percent risk level the one in 10 year occurrence the last spring frost occurs
later than June 4 to 14 in most regions (Figure 4) |
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Areas susceptible to late spring frost also risk an early fall frost (Figure 5). The earliest average first fall frosts are in
the central Interlake (August 26 to 30) and southeast regions of Manitoba and to the west
of the Manitoba escarpment. Likewise, the latest occurrence of first fall frosts - about
September 17 - are in those areas with the earliest occurrence of last spring frosts.
These regions are the Portage la Prairie, Vogar, Morden, Pilot Mound, Morris and Winnipeg
areas. In the most southern part of Saskatchewan, September 12 is the average date of the
first fall frost. This date is about five days earlier in the northeasterly part of the
agricultural zone. The "coldest spots" are around Kamsack and Porcupine Plain.
Maps showing the date of the first fall frost at 25 percent and 10 percent risk levels
show a geographic pattern similar to that revealed by the map of average dates. At 25
percent risk, or a one-in-four-year occurrence, the first fall frosts range from August 28
to September 15 in southern Manitoba, and from August 23 to September 7 in Saskatchewan,
with colder spots around Kamsack and Porcupine Plain (Figure 6).
For the 10 percent risk, or a one-in-10-year occurrence, the dates of the first fall
frosts range from early August to mid-September (Figure 7). |
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The frost-free period determines time available for crop production. As such, it also
determines what crops can be grown in a region. Table 1 shows the number of days required
for selected crops to reach maturity.
TABLE 1. Number of Days Required for
Selected Crops to Reach Maturity
| Wheat |
90 - 100 |
| Barley |
60 - 90 |
| Oats |
85 - 88 |
| Flax |
85 - 100 |
| Buckwheat |
80 - 90 |
| Yellow Mustard |
80 - 90 |
| Brown or Oriental Mustard |
85 - 95 |
| Corn (Grain) |
110 - 120 |
| Canola - late Argentine |
92 - 102 |
| Canola - early Polish |
73 - 83 |
| Field Peas |
90 - 100 |
| Lentils |
85- 100 |
| Canary Grass Seed |
95 - 105 |
| Navy Beans |
90 - 100 |
| Sunflowers |
120 - 130 |
| Sugar Beets |
120 - 140 |
| Coriander |
90 - 100 |
| Fababeans |
105 - 115 |
| Black Beans |
95 - 105 |
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The average frost-free period is the shortest - 75 days - near Kamsack, Porcupine Plain
and Riding Mountain (Figure 8). The longest average frost-free
period - 120 to 125 days - is in the Morden, Portage la Prairie, Vogar and Selkirk areas.
At the 25 percent risk level, or one-in-four year occurrence (Figure
9), the most favourable localities are the southern most parts of Saskatchewan and
Manitoba, and northward along the Red River Valley where the frost-free period is 105 to
115 days. In these regions, many of the crops in Table 1 will have adequate time to
mature.
In areas with short frost-free periods such as Kamsack, Porcupine Plain and Riding
Mountain, temperatures may remain above 0 degrees C for periods shorter than 60 to 75 days
one year in every four. Many of the crops in Table 1 would not reach maturity in these
areas in one out of every four years. At a 10 percent risk level, or one year in 10, the
frost-free period is shorter in all areas (Figure 10).
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Plants and insects require heat energy to grow and develop. It is not enough that
temperatures remain above freezing. Each organism has a base temperature below which
growth or development will not occur. Heat accumulated above a given base temperature is
usually reported as "growing degree-days" (GDD). |
- Base Temperature Values for Selected Crops and Insects
- Spinach, Lettuce, Tomatoes, Pumpkins
- Peas, Asparagus
- Corn & Beans
- Grasshoppers & Corn Borers
- Cutworm, Cabbage maggot
- GDD above 5° C
- GDD above 10 ° C
- GDD above 15 ° C
- Corn Heat Units
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Calculation of GDD is relatively straight forward: heat accumulated each day is
determined by adding together the maximum and minimum temperatures and dividing the total
by two to obtain a daily average. The base temperature specific to a given crop or insect
is subtracted from this average (Table 2). This value represents the daily heat useful to
a crop or insect. Results greater than zero are added to determine the weekly, monthly or
yearly GDD accumulated.
GDD can be used to:
- assess the suitability of a region for production of a particular crop;
- estimate the growth-stages of crops or life stages of insects;
- predict maturity and cutting dates of forage crops;
- estimate the heat stress on crops such as canola;
- help estimate the yields of cereals and canola, as well as the protein content of
cereals and the oil content of canola; and
- as a planning tool for spacing planting dates to separate harvest dates in vegetable
production.
TABLE 2. Base Temperature Values for Selected Crops and Insects
| Crop or Insect |
Base Temperature (°C) |
| Spinach |
2.2 |
| Lettuce |
4.4 |
| General Plant Growth |
5.0 |
| Peas and Asparagus |
5.5 |
| Cabbage Maggot |
6.0 |
| Variegated Cutworm |
7.0 |
| Corn and Beans |
10.0 |
| Grasshoppers, Corn Borers |
10.0 |
| Pumpkins and Tomatoes |
13.0 |
| General Insect Development |
15.0 |
| House flies |
15.0 |
GDD above 5°C
Cereal grains such as spring wheat, barley, oats and buckwheat, as well as oilseeds
such as canola, require a minimum accumulation of about 1,200 GDD above 5°C to reach
maturity. The map of the average GDD above 5°C (Figure 11)
illustrates that all regions within the eastern Prairies are climatically suited to cereal
and oilseed production. The Riding Mountain, Kamsack and Porcupine Plain regions have the
fewest average accumulated GDD above 5°C - 1,250. Plum Coulee in south-central Manitoba
has the highest average annual total of 1,810. Maps of the 25 percent (Figure 12) and 10 percent (Figure 13)
risk levels for this parameter show that cereals and oilseeds will mature over 90 percent
of the time in most regions. However, one year in 10, most cereals and oilseeds would not
have enough heat to mature in the Riding Mountain, Kamsack and Porcupine Plain areas.
GDD above 10°C
The average GDD above 10°C is about 1,000 in the southern-most part of the eastern
Prairies and decreases northward (Figure 14). As in previous
maps, the coldest locations are near Riding Mountain, Kamsack and Porcupine Plain. At the
25 percent risk level or one-in-four-year occurrence (Figure 15),
the geographic pattern is similar; however the values are approximately 50 GDD lower than
average. The 10 percent risk map (Figure 16) generally shows a
further reduction of 50 GDD in all areas.
GDD above 15°C
For the average accumulation of GDD above 15°C (Figure 17),
the number of accumulated heat units decreases northward. Overall, the Morden to Emerson
area experiences the highest average accumulation at 550 GDD above 15°C, while regions
around Riding Mountain, Kamsack and Porcupine Plain, and the northern reaches of Grand
Rapids and The Pas, accumulate between 300 to 350 GDD above 15°C. At the one-in-four year
risk (Figure 18), the highest accumulated values are 500 GDD
above base 15°C and the lowest values are between 250 to 300. The 10 percent risk map (Figure 19) shows a further reduction of 50 GDD in all regions.
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Corn Heat Units
Corn is an economically important crop in Manitoba. As a result, the heat requirements
for this crop have been studied in some detail. In central Canada, where the majority of
corn is grown, studies have led to the development of the "corn heat unit."
The basic concept of the corn heat unit calculation is similar to the degree-day
system; that is, the rate of growth is assumed to increase with increasing temperatures.
However, day and night temperatures are treated separately. It is assumed that no growth
occurs with night temperatures below 4.4°C or day temperatures below 10°C. In addition,
maximum growth occurs at 30°C and decreases with higher temperatures, accounting for the
detrimental effects of very high temperatures.
Corn heat units (CHU) for each day are calculated by the formula:
Where Tmax = Daily maximum temperature (°C) and Tmin = Daily minimum temperature (°C)
In southern Manitoba and Saskatchewan, most corn hybrids have been specifically rated
for the region. A listing of these hybrids can be found in publications such as the
current issue of Seed Manitoba. On the
eastern Prairies, corn hybrids grown for silage require 2,000 to 2,100 CHU annually, while
the earliest grain corn hybrids require 2,200 to 2,400 CHU to reach physiological
maturity.
CHU were accumulated over the growing season (May 15 to the date of occurrence of the
first fall frost of -2.2°C).The map of the average CHU accumulation (Figure 20) shows that the best area for corn production is south
of Lake Manitoba and Lake Winnipeg, between the Manitoba escarpment and west of a line
from Steinbach to Pinawa. Within this area, Pilot Mound, Morden, Altona, Emerson, Portage
la Prairie and Great Falls all receive 2,600 to 2,700 CHU on average. In an average year,
the Interlake regions, as far north as Eriksdale to Vogar, and southwestern Manitoba, are
viable locations for successful grain corn production with a hybrid rating of 2,400 CHU.
Average CHU is lower in the Manitoba upland areas above the escarpment and in
Saskatchewan, the best areas accumulate only 2,300 CHU, while the Riding Mountain and
Kamsack to Porcupine Plain areas receive less than 2,000. In an average year, these areas
are not suited for corn production, not even for silage.
At a 25 percent risk level, or one-in-four year occurrence (Figure
21), a hybrid with a 2,400 CHU rating will mature in only five areas: Portage la
Prairie, Morden, Altona, Emerson, Winnipeg, Great Falls and Vogar. In other areas, a 2,400
CHU hybrid will not mature in one year out of four. For Saskatchewan, only the earliest
hybrids, that is, those with 2,200 CHU ratings, can be expected to mature three years out
of four, and only in southern areas.
Assessment of minimum CHU accumulation at a 10 percent risk level (Figure 22) shows that most of the area east of the Manitoba
escarpment, south of Lake Manitoba and Lake Winnipeg, and west of Steinbach to Pinawa,
will mature a 2,200 CHU rating hybrid to the grain stage nine out of 10 years. There are
no areas in Saskatchewan where maturity of even the earliest grain hybrids can be assured
nine years out of 10. As for silage, production is restricted to southern Manitoba. There
are no areas in Saskatchewan where high quality silage corn can be reliably produced nine
years out of 10.
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