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Growing Days
Tue Oct 05, 2004
Listen in RealAudio 
Hi, I’m Bryan Yeaton for The Weather Notebook. When you buy seeds for your
vegetable garden, the packets usually give an approximate number of days from
germination to maturity, a value that may vary with your Plant Hardiness Zone. In an
average weather year, such estimates are good for most growers. But why did one plot
of 68-days-to-maturity peas ripen at the same time as those planted two weeks later?
The answer likely was a cold spell.
Assuming no water stress or pest damage, the average air temperature most strongly
affects plant growth and can be used to track the path to maturity. Days to maturity is a
fine estimate in an average year, but a more accurate measure, used by agriculturists,
is the growing degree day.
This index is easily calculated. For each day, a base temperature for the given plant
variety — say 42 Fahrenheit for peas — is subtracted from the day’s average
temperature, estimated by adding the maximum and minimum temperatures and
dividing by two. If the resulting value is negative, a value of zero is assumed. For
example, if the mean temperature was 65 F, using a baseline temperature of 42, the
day’s accumulation would be 23 growing degree units.
This daily calculation begins at germination, adding up as the days pass. The plants
reach various growth stages at specific growing degree unit accumulations. Peas
generally take 680 growing degree units to go from emergence to bloom, and 1,200 to
harvest.
Thus, the likely cause of the aforementioned spaced plantings was fewer degree days
in those two weeks. The cooler weather provided few growing degrees for the first
planting and allowed the second to catch up.
Thanks to our contributing writer, meteorologist Keith Heidorn. The Weather Notebook
is produced by the Mount Washington Observatory, funded by Subaru of America.
Today's Links
Calculating Degree Days
Growing by Degrees
Growing Degree and Applications
http://www.agron.iastate.edu/courses/agron508/classes/503/lesson1/1.3.html
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