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Understanding Growing Degree Days and how it impacts the home gardener

Photos: Ron Patterson | UI Horticulture Educator
Most of the potatoes and grain have been planted to take advantage of early spring moisture, but growth is slow. It’s been a cold spring. Soil temperature is critical for good germination. Once seedlings have emerged, air temperature is also critical. Air temperature is used to measure what we call Growing Degree Days.
What does all this mean for the home gardener? Vegetables and flowers will grow slowly until temperatures get closer to normal.
Seed packets usually indicate the number of days until harvest. That’s a bit deceptive. Days to harvest are based on average GDD. The problem is that average GDD are different in Ashton and Driggs than they are in Blackfoot and Pocatello. In addition, the “GDD sweet spot” is not the same for different species and maybe even cultivars within a species. This is one of the reasons it is difficult to grow bell peppers in Driggs. The table below is a simplification of GDD, but it shows why even the cool season vegetables have not shown much progress without something to bring the temperatures up during the day.

Another factor that will affect growth rate is night temperatures. Cool season plants do fine with night temperatures in the forties, but warm season plants need night temperatures to be at least in the fifties, and some in the sixties. When it gets too cold, even if they don’t freeze, the plants just hunker down and wait until it warms up.
What about when it gets hot in the summer? These plants will also have a maximum temperature, beyond which they take a siesta until it cools off.
So, what it all boils down to is the longer you can have your plants in the GDD sweet spot, the more growth you will get each day, and maybe even beat the “days to harvest” listed on the seed packets.
How can you get the temperatures into the sweet spot? Gardeners are quite creative and there are many ways to meet these temperature needs—hot caps, cloches, cold frames, Wall o’ Water (commercial or homemade), row covers, high tunnels. These devices are not really an effort to beat nature, but to work with nature and provide ideal, or close to ideal, growing conditions for our plants. Just remember, too hot is as bad as too cold.
The real nice thing about the cold spring temperatures is that the daffodil and tulip blooms will stay vibrant longer. Enjoy it while it lasts.

The post Understanding Growing Degree Days and how it impacts the home gardener appeared first on East Idaho News.

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