Growing Degree Days

sundance, originally uploaded by nice+smooth ultramedia.

How fast plants develop is largely dependent on the weather. Cold weather means slower growth whereas warmer weather brings faster growth. Flowers don't bloom because it's April. Plants bloom because April (as well as March, February, and January) have brought warmer temperatures. While calendars are a great way to keep track of plant development, a better way might be to track just how much warmth a plant has received.

This can be done by logging growing degree days ("GDD"). A GDD is a simple calculation. In essence, it's a running tally of the number of degrees above a base point. Although there are many ways of calculating GDD, the easiest way is to take the average daily temperature (high + low / 2) and subtract a base temperate. 50 F is a good base point. Below that, most plants don't grow so well. So, for example, if today's high were 70 and the low was 40, we'd have 5 growing degree days.

Great. So what's that mean? From this events like bloom and pest emergence can be predicted. Plants develop at somewhat specific times. For example, corn is ready at about 1360 GDD. Red maples begin to bloom at 75 GDD. Forsythias get going at about 86 GDD.

From Weather Underground to your local extension agency, GDD data is plentiful. The NWS NOW Data has sensors all over faithfully logging GDD. In fact, there's one not more than a mile away from the grange, which should make it easy to log events based on GDD. While there are plenty of resources for commercial growers, there appear to be few geared toward gardeners. For example, a tomato might take 70 days to bear ripe fruit, but, how will colder or warmer weather affect fruit set and maturation? As such, I'll try to log events and key them to GDDs.

Sources
National Weather Service, National Corn Growing Degree Day Tally
Mont. St. Univ., Using Growing Degree Days to Predict Plant Stages
Ohio. St. Univ. Ext., Biological Calendars (fantastic power point)
Ohio. St. Univ. Ext., Summary of Phenological Events
National Phenological Network
Ohio. St. Univ. Ext. Growing Degree Days as a Method of Rating the Maturity of Corn Hybrids
Weather Underground, Degree Days
NYS GDD Tracker
UMass GDD Checklist (providing information about the timing of blooming and pest emergence based on GDD)