Small grains are cool-season annuals and are most productive when they grow and develop during cool weather.
The crop’s yield potential is largely determined by the 6-leaf stage. Cool temperatures during this period are particularly important to develop high yield potential.
For example, the number of spikelets per spike is determined during the 4- to 5.5-leaf stage. Spikelet numbers negatively correlate with temperature; spikelet numbers are greater when temperatures during the 4- to 5.5-leaf stages are cool.
Benefits of early planting
Planting early is one way to improve the chances that these early growth stages occur during relatively cool temperatures. Plant as soon as it’s practical, but on or before the optimum date indicated in Table 1.
Table 1: Average optimum seeding dates and last recommended date for small grains
| Minnesota | North Dakota | Optimum seeding date | Last planting date |
|---|---|---|---|
| South of U.S. Highway 12 | - | First week of April | First week of May |
| South of Minnesota Highway 210 | South of Highway 13 and 21 | Second week of April | Second week of May |
| South of U.S. Highway 10 | South of Interstate 94 | Third week of April | Third week of May |
| South of U.S. Highway 2 | South of U.S. Highway 2 | Fourth week of April | Fourth week of May |
| South of Canadian border | South of Canadian border | First week of May | First week of June |
Research has shown that, on average, yields decreased 1 percent per day when planting is delayed past the optimum planting date. Planting after the last possible date is not recommended because the odds are greater for reduced grain yield and quality (test weight) due to heat stress.
Temperatures and germination
Spring wheat (and spring barley and oats) will start germinating in earnest when soil temperatures reach 40 degrees Fahrenheit. Once the imbibition phase starts, there’s no return to dormancy and the germination/emergence should be as quick as possible to establish a healthy, vigorous seedling.
Long-drawn-out emergence will predispose the seeding to attacks of soilborne fungi like Pythium damping off or common root rot, ultimately reducing stands. Daytime highs in the 60s and night temperatures around 40 are great, and will allow the crop to emerge in eight to 10 days and make for a robust stand.
During this whole germination and seedling emergence and up to the 5-leaf stage, the growing point will be located about an inch below the soil surface. At this depth, it’s protected from the ambient temperatures.
The crown can sustain temperatures down to 28 and probably even handle short periods of temperatures as low as 22. Even if above ground leaves freeze, the plant will survive and continue its development as long as the crown does not suffer any freezing injury.
Even if the weather in mid-March feels more like the middle of May, there’s a risk of cold weather returning.
However, the odds of really cold temperatures that could damage the crown appear to be relatively small. If any snow accompanies the cold weather, the snow will act as insulation and reduce the risk of the crowns freezing.
How to assess risk
As a starting point, see if the extended temperature outlook is favorable for germination and emergence.
To assess the risk of winter returning in April and the first half of May, we looked at weather records from the Northwest Research and Outreach Center in Crookston dating back to 1890. We found that winter can still return in April, according to the latest 30-year climate normal (1981 through 2010).
Likelihood of low temperatures
When winter does return, the number of days the minimum temperatures go below 22 between April 1 and May 15 is relatively small, at 9 percent (Table 2). The number of days the nighttime temperatures dips below 28 is much greater at 25 percent.
If, however, we look at the 30 warmest Aprils on record, these percentages are cut in half. Taking the warmest five April months on record cuts those percentages in half again.
Approach to calculation
This is a somewhat rough approach, as each individual day has its own probability function, meaning that it has its own mean and distribution around that mean.
To do these calculations in a statistically correct way, you’d have to calculate the probability that temperatures dropped below 22, 28 and 32 degrees for each individual day and then average them out over the same time period. Intuitively, you would understand the risk is greatest in early April and diminishes with each day the season progresses.
Table 2. Shows the percentage of days that temperatures dropped below 22, 28 or 32 between April 1 and May 15 in the last 30 years, the warmest 30 April months on record and the 5 warmest April months on record.
Table 2: Historical climate data for April and May
| Temperature | 30-year average (1981-2010) | 30 Warmest April months | 5 Warmest April months |
|---|---|---|---|
| Less than 22 F | 9% of days | 4% of days | 4% of days |
| Less than 28 F | 25% of days | 15% of days | 6% of days |
| Less than 32 F | 45% of days | 36% of days | 20% of days |
| Average April temperature | 43 F | 47 F | 50 F |
Reviewed in 2018