Know your degree days, know your plants

  • Posted: Friday, March 1, 2013 12:59 a.m.
Degree days determine when plants will bloom and insects become active.
Degree days determine when plants will bloom and insects become active.

SALISBURY — Have you ever noticed how plants seem to burst forth each spring in a definite order? For example, daffodils always appear before irises and irises always open before coneflowers. Amazingly, every year, the order is always the same. Why? The answer comes from the concept of degree days.

Each plant requires a certain number of “degree days” to know when to start growing again. A Growing Degree Day (GDD) is a measure of each day’s heat value. A Growing Degree Day is calculated by subtracting the plant’s temperature base, (the temperature below which the plant stops growing), from the average temperature for the day.


Let’s use wheat for example. Wheat has a temperature base of 32 degrees, (meaning it won’t grow below 32 degrees.) If the average temperature for a day was 50 degrees, then that day counts as 18 degree days (50 minus 32 equals 18). Wheat requires a total of 257 GDD’s to emerge and 2,800 GDD’s to reach maturity.

The GDD for daffodils is 20, dandelions are 50, eastern redbud is 197, and lilacs is 238. Therefore, you will never see lilacs flower before daffodils (unless you artificially manipulate the temperature, as in a greenhouse) This GDD never changes for each plant. So if you know the current number of degree days that have accumulated for a certain date, a gardener or farmer can then predict when a certain crop will flower or bear fruit

Furthermore, not only is GDD valuable for predicting when a plant may bloom or fruit, it is also valuable for predicting when certain pests will arrive. You see, invertebrates also operate on the GDD principle as well. For example, adult Japanese beetles require 966 GDD to hatch. Therefore, you will never see Japanese beetles devastate a field of daffodils. However, you might see them destroy roses which have a later GDD. The GDD for tent caterpillars is 92. Therefore, you will see tent caterpillars before Japanese beetles.

The farmer or gardener can access the degree days from standard weather information sources. If the farmer follows the accumulated degree days for his region, he can then use this information to gauge planting times, as well as timing for herbicides and pesticides.

Sweet corn producers will often use this gauge to stagger sweet corn for a constant supply.

Because each year brings different weather patterns, Growing Degree Days are more accurate that just following the calendar.

So just as musicians warm up before their symphony performance, so, too, plants need to warm up before putting on their spring symphony.

Kathy Richards is a volunteer with Cooperative Extension in Rowan County.


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