Indicator Species in the Environment

Development of native plant species in the environment can also be correlated to GDD accumulation. For example, dandelions first flower when 260 GDD have accumulated, and lilacs first flower when 460 GDD have accumulated. By watching these plants and others like it, one can predict the development of crops planted and the percentage of weeds emerged.

The advantage of using wild and perennial plants as an indicator of GDD accumulation is that it is precise and it is accurate for every location. No instruments, no wireless or Internet access is required. It is available at all times at all locations with no cost. Significant wild and perennial plant development events include the first sign of green, first bud, first flower, first seed set.

Since crop plants also develop based on GDD accumulations, the observation of development events in wild or perennial plants can be used to predict crop development. Crop plants do not necessarily start growing at the beginning of the season like wild and perennial plants do, but only after they have been planted. Planting can take place over a large period of time, so for the wild and perennial plants events to have any use in measuring crop development, the planting or start of the crop (zero GDD for the crop) must be matched to a wild or perennial plant event. Once this is done, the development of both wild, perennial and crop plants will occur according to the same GDD scale.

A fixed GDD scale of wild and perennial plant development beside a moving GDD scale of crop development can be used to match future wild and perennial plant development events to future crop development stages. Such a tool has been developed. Placing the zero of the movable crop development scale next to the wild or perennial plant development event at the time of crop planting sets the tool up to be used to predict crop development for the remainder of the year.

A GDD slide rule was developed based on this principle. The slide rule allows for adjustments in seeding date and timing of weed control. This slide rule is in its early stages of development and will continue to evolve.

Go to interactive GDD Slide Rule.

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