Seedheads

Its that time of year again when I am watching "Growing Degree Days" to determine when our first application of Primo and Proxy should be applied to the greens to prevent the Poa Annua from producing seedheads. These chemicals help to prevent seedheads from forming thus allowing the traditionally weak Poa Annua to spends its energy on growing roots instead of generating seeds. Most members like the fact that the greens are smoother in the spring without the presence of seedheads but I am really controlling the seeds for the health of the greens allowing for longer roots and larger carbohydrates in reserve giving the Poa Annua a better chance at surviving the summer months. The following explains how I calculate "Growing Degree Days."

Definition and history: Degree-days are units of total accumulated heat that are used to forecast everything from plant growth to pest infestations. The value of using temperature to predict plant and animal growth was recognized by the French scientist Renè A. F. de Rèaumur in 1735. By adding together the daily average air temperatures that occurred during the life cycle of several different plant species, he discovered that the sum of temperatures for each species was the same from year to year, even though the number of days required for development varied. In other words, a specific amount of heat (what de Rèaumur called a thermal constant) was required for a plant to reach maturity, and this amount of heat was characteristic for each plant species or variety.

At the turn of the 20th century, scientists began to propose that the thermal constant be expressed in units which came to be called degree days (DD). Today, over 100 DD models now exist that forecast everything from the harvest dates of grains, vegetables and fruits to optimal crop planting dates to the best crop varieties for different geographical areas.

Calculating degree days: Each stage in an organism's life has a characteristic DD requirement. For example, from the time an insect egg is laid until the time larvae hatch from the egg may require 100 degree-days, while the time period between larval hatch and the emergence of a butterfly may require 175 degree-days. Each organism also lives within a relatively narrow range of temperatures, above or below which their survival becomes difficult. The lower developmental threshold is the temperature at or below which growth stops. The upper developmental threshold is the temperature at or above which growth slows down or stops. The number of degree-days accumulated in one day is the total amount of heat between these lower and upper thresholds.

For example, the development of annual bluegrass (Poa annua) has a lower developmental threshold of 50F (10C). When you hear about "degree-day base 50" models, it is this lower threshold that is being referred to. To calculate the number of degree-days base 50 that occur in a single 24 hour period, the following simple calculation is made:

average temperature for a 24 hour period minus 50 = number of degree days

If the average temperature is 50F or less, then there are no DDs accumulated for poa for that day. But if the average temp is 51F, there is one DD for that day, if the temp is 60, there are 10 DDs and so on. Counting DDs usually begins on January 1, so that the DD accumulation at any given time is the sum of all the DDs from the start of the year.