Until now, I was not really aware of crop heat units or growing degree days. I have noticed that the information on the seed packet regarding the days to maturity was not always accurate. It does seem that water, soil quality, sunlight and heat all play roles in the time it takes to produce a mature crop. If you do live in a climate that has four distinct seasons, such as Pennsylvania, knowing your average heat units can help you decide what varieties to plant and if they need started earlier indoors. So what are heat units anyway?
Heat units for the growing season are derived from a calculation using the mean of the average daily temperature using the daily maximum and minimum temperatures. A temperature of 50 degrees is used as a base temperature.
Author Chris Smith at SeattlePI.com explains it here in his article about heat units:
“Using a base of 50 degrees, let's say the high on July 30 is 82 and the low 58. Mean temperature for that day would be the average of those two temperatures, or 70. When you subtract the base temperature of 50, you have 20 heat units for the day. By making a similar calculation for all the days of the growing season, you can arrive at the seasonal heat units for your site.”
The article at Seattle PI is defiantly worth looking at. Another really interesting site with a lot of calculation tools is http://www.agclimate.org (would be very helpful if you live in Florida, Alabama or Georgia). For Pennsylvania you can access up to date climate data at the Pennsylvania State Climatologist Home Page. There is a data page with the current number of growing degree days for your region here. I was surprised that southeast PA has had 1,113 growing degree days for the year to date and northwest PA has had only 658 days! That is a bigger difference than I thought. Often a whole state is lumped into a growing season but it can vary regionally. The site also offers some agricultural maps showing the growing season for the state, first frosts and some individual county data.
SOme other good links
Here is a growing degree day online calculator
This website list growing degree day reccomendations for grapes - note that 1390 and above is most suitable
Wikipedia Article on growing degree days
Growing degree days can also indicate when garden pests will emerge! Many pest control measures are enacted for crops based on growing degree days.
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