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From Wikipedia:

Growing degree days (GDD), also called growing degree units (GDUs), are a heuristic tool in phenology. GDD are a measure of heat accumulation used by horticulturists, gardeners, and farmers to predict plant and pest development rates such as the date that a flower will bloom or a crop reach maturity.

GDD is calculated as $\frac{T_{min}-T_{max}}{2}- T_{base}$ - so a quick check of the units would indicate that they are in the same units as T, i.e., degrees C. However, they are usually presented without units.

Why is this?

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Actually, the units are °C days  or °F days. The units are mostly given in the typical name, Growing Degree Days where which temperature unit is meant is understood from the context. Much more important is knowing the base temperature ("T_base") since this value can be different in different usages, even for the same type of organism.

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