What are GDD's?

This question came up in another thread.

GDD=Growing Degree Days

It's the amount of time a plant must spend above a certain temperature to reach maturation.It also applies to the life cycle duration of pests.

It's used by commercial farmers to determine when to apply pesticides so they are most efficient. Also, to determine when to prepare for harvest and transportation to market.

The 'amount of time from planting to harvest' that you read on seed packages is just a guestimate based on GDDs. If you have a cool summer, it takes more days to accumulate the hours. If you have a warmer than usual summer, it takes less.

Corn and tomatoes need 1400-1800 GD to reach maturity depending on strain. Dandelions take about 75. It's hard to find values for different crops.

The calculations are basic, based on high and low temps for the day. I found an auto web site here that charts an accumulative graph of GDDs throughout the year, based on the data from an automated weather station that's 10 miles from my house. It also contrasts the current data to an historical average.

One advantage to home gardeners if this information was easier to find, is when to start looking for squash bugs. By tracking GDDs, the experts know at what stage of development a bug is in, and when it is going to start causing damage. They can pinpoint their emergence to within a matter of days.

There's some google savvy members on the forum, maybe they can find more info geared toward homeowners.

as soon as I figure out how to interpret this, the information on the referenced web-site will be really useful!