Published on Wednesday, 30 November -0001 00:00
Gardening success and the weather go hand in hand. Usually we worry about rain because it is easy to see when we have too much or too little. Temperature, my friends, also can be a problem. I am just back from the Master Gardener's Conference at the University of Minnesota and almost everyone was talking about how the cool weather has impacted their gardens. It has been good for the flowers but hard on the vegetables.
If you've wondered why some years your garden grows better than others, it probably has a lot to do with growing-degree days. Growing-degree days is simply a way of measuring how much heat your garden gets during each day. Each plant has its own requirements as to how much heat it takes to germinate, grow and ripen. Some plants, such as corn and tomatoes, take much more heat to reach maturity than say lettuce or green beans.
Growing-degree days, usually abbreviated to GDDs, can be measured many different ways, but the simplest and most common is to add together the high and low temperature of the day, divide by two, and anything over a threshold of 50 is considered a growing-degree day. So if the high temperature of the day is 60, and the low is 46, when added together you get 106. Divided by two you get 53, which gives you three GDDs.
So, you ask, just exactly how cool has it been this summer? Well, Mr. Potato Head can tell you pretty exactly. If you planted your garden on May 20 this year you have had 1,215 growing-degree days so far this summer. The average for this same period for the past five years is 1,500. In 2004 we had it worse with only 1,152 and in 2006 we had a warm summer with 1,676 growing-degree days. It has been a great summer for low air conditioning bills, cool weather crops and flowers. It has been not so good for corn, tomatoes, peppers and melons. For example, most tomatoes will need between 1,500 and 2,200 GDDs to ripen. So, now you can stop wondering why your green tomatoes are staying green so long.
Growing degree day measurements tend to be very localized. You may find you get more or fewer heat units in your garden than your next door neighbor, depending on how your garden is sited and how much sun it gets. It can even vary in different locations within your garden. Planting a garden for maximum southern exposure and maximum sunlight ensures you'll get the maximum heat available.
Is there anything we could do to hurry the ripening process along? Unfortunately, there isn't much we can do but wait for the warmth. This week's weather will really help. For each day with a high of 85 and a low of 65 degrees we will gain 25 GDDs. Those green tomatoes will ripen. My watermelons, however, might just be a lost cause.
If you have gardening questions or suggestions for Mr. Potato Head please e-mail him at
Mr. Potato Head is Stearns County Master Gardener and Kimball resident Rick Ellis.