GDD models can be a great tool for forecasting plant protectant applications critical to landscape management. If you do a search on the Internet for GDD Tracker you will find the site that many universities contribute information and models for. Some of the areas covered are Seed Head Timing, Broad Leaf Timing, Crabgrass Pre Applications, Grub Control Timing just to mention a few.
You can set the site up with your zip code and track timing of some of your most important applications of the season. Each model has a color-coded map that indicates application timing in your area. There is also a data section, which tells you the base used for the model, total accumulated GDD, and the date. Below this section is a graph area that contains the date, number of GDD, and a look at the timing for the model expressed as Early-On target-Late.
Unfortunately, the site only provides you with information of where you are today on GDD, not where you will be in the future. However, this is a fairly simple process for you to calculate. To do this calculation you need to know three things:
1. High temperature for the day
2. Low temperature for the day
3. Base of the model
Let’s say we have 145 GDD and we want to figure where we will be three days from now.
Forecast:            High Temp                        Low Temp
Day #1                        70                                    41
Day #2                        62                                    38
Day #3                        75                                    54
The base for the model is 32.
To calculate a GDD add the high temperature and the low temperature. Divide that number by 2 and then subtract the model base. This number is a GDD.
Day #1 70+41= 111 /2 =55.5 – 32 =23.5 GDD
Day #2 62+38= 100 /2 =50 – 32 =18.0 GDD
Day #3 75+54= 129 /2 =64.5 – 32 =32.5 GDD
Total for 3 Days 23.5 + 18.0 + 32.5 =74.0 GDD
By adding the current total of GDD and the forecasted total you have now estimated how many GDD you will have.
So, if you add the current GDD 145 + 74 new calculated GDD, your forecast in three days is 219 GDD.
Please use any GDD model as a tool and not absolute timing for making your applications. Using GDD models with your knowledge of non-target plant responses (e.g., forsytha bloom), soil temperatures, and overall growing conditions should help ensure you are making your applications at the most cost-effective time of the year.