Growing Degree Days

Like many of you, I am watching the weather very closely this time of year, trying to predict which way the thermometer is going to move. Timing is everything in garden planning, especially in the spring. A calendar date means nothing; as we all know, the beginning of March can be wildly warm or crazily cold. One thing I do is match up the ten day forecast highs/lows against monthly average high/low trends. This gives me a fairly good idea where we are moving in the short-term. This information assists me in deciding not only what garden tasks to focus on in any given week, but relative timing for starting certain crops like all the vegetables and the few bedding annual starts we do in house.

Slightly more reliable methods may be found in the worlds of phenology and Growing Degree Days (GDD). Phenology, the study of cyclic changes in plant and animal behavior, is a much more reliable indicator of what is occurring (or about to occur) than weather modeling. Humans have relied upon it for millennia, and plant and animal DNA are finely attuned to seasonal transitions. We know fall is coming because birds fly south; you know to plant your corn when oaks leaf out, or to put down crabgrass preventer when forsythia bloom. Growing Degree Days is a concept developed for farmers to aid in knowing when to plant certain crops or when to expect certain pests and diseases.

Wellfield staff rely upon all methods mentioned above to plan. GDD Tracker 4.0 is an online reporting tool, developed through a partnership between Purdue and Michigan State University and others, which we use to bolster Wellfield’s IPM, or Integrated Pest Management, program. The first step is to identify what plant health problems to monitor. Then, we determine if there is any scientific literature providing a GDD number to various lifecycle stages of the pest or disease. As an aside, there is a lot more information available for agronomic and turf issues than there is for ornamentals. The GDD number we are looking for is associated with the most vulnerable point in the pest or disease lifecycle (i.e. the best time to apply control measures). Next, we frequently check GDD Tracker to see if we are getting close to the right numbers. We need to know what GDD baseline to follow for the pest. For example, if we are monitoring crabgrass, we will use baseline 32. For spring broadleaf weeds and most ornamental pests, baseline 50. We begin to monitor the garden for those issues as the GDD approach the right numbers. We are beginning to match up our garden spray logs to actual GDD numbers for even more accurate local forecasting. We also researched any phenology cues we could find tied to various pests and disease we frequently manage. For example, we know to take control measures for magnolia scale when redbuds are blooming.

However you choose to track the seasons, I promise you spring is coming. I foresee it in my GDD crystal ball.

Josh Steffen, Horticulture and Facilities Manager

6 thoughts on “Growing Degree Days

  1. The groundhog does nothing to predict accurately in the sense that he or she is nor more accurate than flipping a coin – about %50 percent accuracy. Plants tend to know more about the seasons than we give them credit for, even if they sometimes get caught of guard by blooming just before a frost. I sort of trust the thickness of the coats of dogs, horses and cats to predict winter, but they are not very helpful this time of year.

    1. hspaulding111

      great points, Tony. And the evolution of native plants in any particular area makes them a good choice for those who worry about hardiness in the case of a “late” winter. Anomalies may occur, but generally they’ll be successful.

      1. Natives are aware that anomalies are possible; like people. Those who complain about drought or the potential for flooding are not native. Those of us who are know that it is all quite normal, and how our climate works.

    1. Mary, here is the reply from Josh Steffen, Horticulture and Facilities Manager:
      “Usually, seeds are packaged with a “packaged” date, meaning the year they were packaged. I have yet to see an expiration date on one, though I am sure they are out there. A lot of seed has a shelf life of more than a year. I look up how long different species of seed keep and add that to the package date to know whether the seed is likely much good or not. The older the seed the more I sow per hole or trench as a means of increasing my odds of good % germination.”
      -Josh Steffen

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