Instructions on seed packets like "Plant in warm soil" really annoy me. What precisely is "warm"? Or descriptions like "Today is a beautiful day". There is something beautiful about every day, but does an emerging corn seedling agree it is a beautiful day?
So I like numbers and measurements. Temperatures. Growing degree days. Now we are talking specifics.
My trusty soil thermometers measure the soil temperature at 52 degrees. That soil is the coolest due to the cover crop. It retains moisture and shades the soil. At the other extreme are the raised beds; this soil is dry and has no shade. This soil temperature measures 62 degrees. Soil testing convention suggests sampling each morning at 9:00 AM. When the temperature meets or exceeds the minimum planting temperature for three continuous days then the soil is warm enough to plant.
Another fun measurement is growing degree days or GDD. It helps quantify the difference between a day in late April and a day in late July. As the growing season days come to pass, GDDs can be summed together to help predict plant maturity and harvest dates.
For example, sweet corn. Seeds are labeled with relative maturity expressed in days. If the seeds are not also labeled with GDDs, then the number of days can be multiplied by 20 to estimate the GDDs. Applying a little math to the daily high and low temperatures can predict whether the corn will mature ahead or behind schedule. Knowing the average GDDs over the season, you can pick target harvest date to calculate a planting date. You can also use GDDs to properly delay subsequent early season plantings if unusually cool weather delays emergence of prior plantings.
The sum of the average GDDs for the first 18 days of April are ~46. The sum of the actual GDDs for the first 18 days of this April? 197. That is a lot of beautiful days!
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