By Peter Johnson, RealAg Agronomist, @WheatPete
It is COLD! There are lots of questions right now about how low nighttime temperatures in the forecast might impact Ontario’s awesome wheat crop. I am hoping this is just another one of those times that growers like to “kill” the wheat crop, but the reality is that there is some risk.
Freeze injury depends on many factors, but most importantly is the stage of growth. We are in the “stem elongation” phase (called jointing in the U.S.). That means the head is above ground and moving up in the stem. The further up the plant the head is, the more susceptible it is to injury from low temperatures.
When wheat is just beginning stem elongation, it is tolerant of about -8 degrees C (at a 2 hour exposure). That is important, as it is generally colder to the north, but those crops are less advanced, and thus are more cold tolerant. By the time the wheat head is emerging from the boot, it is only tolerant of -1.5 degrees C. The stem isn’t protecting the head anymore, and the delicate parts of the flower are forming. In between those stages is a sliding scale. The further advanced and higher up the stem the wheat is, the less tolerant it is, and the more exposed it is to those cold temperatures.
Very little wheat is much past second node, which means the head is still well down in the canopy. Again, that matters: a thick canopy insulates the lower part of the plant from the cold, making it less susceptible. Thin canopies are at higher risk. Fortunately, the most advanced wheat has the thickest canopies, which helps.
Wind matters, too, as it drives the temperatures into the canopy, but typically wind means more air mixing, and less chance of extreme cold. Soil moisture plays a role, as wet soil holds more heat to release into the air inside the canopy and moderates the temperature. Unfortunately, from a potential freeze scenario, we are relatively dry. But that helps everything else, so…
Will your wheat freeze? It is highly unlikely. In my rather long career, I have never YET seen extensive freeze damage in Ontario. Isolated low spots and frost prone areas, but not widespread. Hopefully you won’t need to worry about this, but if you start seeing lows of -5 degrees C or colder, then we will need to talk about how to determine if you do have frost injury.