Minimum temperatures are recorded at a height of five feet above the ground as well as at the ground surface.
The weather station at Ormstown recorded three nights with negative temperatures at the ground surface: Friday morning September 18 at minus 3.0; Saturday morning September 19 at minus 3.5; and Sunday September 20 at minus 3.0 degrees. All plants are more or less susceptible to cold temperatures. For sake of comparison, it is easier to choose one plant for consistency to determine killing frost. This has been grain corn over the last 45 years. The damage done in the Ormstown area has been restricted to slight damage on some upper leaves and is not considered a killing frost. The plants are still alive and continuing to benefit from additional heat units. Soybeans on the other hand are showing signs of shutting down which will encourage leaf drop and some competing weeds to dry up which is a good thing. Sensitive plants such as tomatoes and beans in vegetable gardens may or may not have survived depending on their exposure. Other pockets have likely recorded slightly lower temperatures as considerable damage has been observed in grain corn fields in the Howick and Huntingdon areas.
As of Monday morning September 21, the minimum temperature 5 feet off the ground was minus 2.0 degrees and minus 7 degrees on the grass surface. There was ice on the surface of a pail of water which is usually the best indicator of a killing frost for corn plants. For the record, the growing season for 2020 will end September 21 compared to the ten year normal of October 16. Total heat units beginning May 4 are 2999 compared to the ten year normal of 3335. The heat unit total last year beginning on May 21 was only 2910 and the grain was poor quality with a lot grading number 4 or 5 which all farmers wanted to forget about. This year, in spite of having only 90 more heat units, the quality seems to be normal for the most part. This goes to show there are many factors affecting crop maturity other than a mathematical heat unit formula.
The earliest killing frost date for Ormstown during the last 45 years was September 19 in 1979 followed now by September 21 in 2020; September 24 in 1992 and September 26 in 1978 and 1984.
Peter Finlayson
