The record breaking temperatures of the past week make it feel more like the middle of May than the middle of March. Obviously, as the fields look ready, the question arises whether this early is too early. With the very late start of 2010 and the disappointing wheat and barley yields that followed still fresh in memory, everyone understands that early planting is paramount. What are the risks of planting too early? Is there such a thing as too early for seeding wheat and other cool season grasses?
Spring wheat (and spring barley and oats) will start germinating in earnest when soil temperatures reach 40⁰F. Once the imbibition phase starts there is no return to dormancy and the germination/emergence should be as quick as possible to establish a healthy, vigorous seedling. Protracted 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 sixties and night temperatures around 40 are great and will allow the crop to emerge in 8 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 at ~1 inch depth. At this depth it is protected from the ambient temperatures. The crown can sustain temperatures down to 28⁰F and probably even handle short periods of temperatures as low as 22⁰F. Even if above ground leaves freezes, the plant will survive and continue its development as long as the crown does not suffer any freezing injury.
Thus planting this early is a risk if winter returns and temperatures plummet. The immediate forecast, however, looks very favorable for germination and emergence as National Weather Service's extended outlook favors temperatures in the region to average 16-20 ⁰F warmer than normal through the end of March. The 10-day extended outlook looks for daytime highs in the 50 and 60⁰F and nighttime lows in the low 40⁰F or high 30⁰F.
To assess the risk of winter returning in April and the first half of May, I took the weather records from the Northwest Research & Outreach Center that date back all the way to 1890. If we take the latest 30-year climate normal (1981 through 2010), winter can still return in April and when it does, the number of days the minimum temperatures go below 22⁰F between April 1 and May 15 is relatively small at 9% (Table 1). The number of days the nighttime temperatures dips below 28⁰F is much greater at 25%. If however, the warmer weather continues and we look at the 30 warmest Aprils on record, these percentages are cut in half. Taking the warmest 5 April months on record, cuts those percentages again in half. The National Weather Service's outlook for April favors temperatures to average warmer than normal.
Obviously this is somewhat of a roughshod 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 statistically correct you would 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 that the risk is greatest in early April and diminishes with each day the season progresses.
Bottom line: there is a risk that cold weather returning. Frost is likely to return to the region but the odds of really cold temperatures that could damage the crown appear to be relatively small. Of course, if any snow accompanies the cold weather, the snow will act as insulation and reduce the risk of the crowns freezing.