Degree Days

While longer daylight hours may energize us, it is warmer temperatures that really get plants and insects going.

During colder winter months, most plants and insects don’t do much. It isn’t until a certain number of days are spent within a range of warmer temperatures that growth can resume. This combination of time and temperature is called physiological time. The physiological time needed by any particular organism stays relatively the same, much like the chilling hours required of certain fruit and nut trees to produce a good crop. Physiological time is expressed in degree-days (°D), also know as growing degree-days (GDD).

How are degree-days used?

The number of degree-days needed for any particular species to move from one developmental stage to another (phenology) is still being researched, but you can this information to help you predict germination, vegetative growth, bloom times, and harvest time. Degree-days are also very important when using pheromone traps and other pest controls for things like San Jose scale. Beekeepers are beginning to look into degree-days as a way to predict colony lifecycle.

Temperature thresholds

Generally speaking, degree-days needed by warm weather crops are those with temperatures between 50°F and 95°F, while cool weather crops have a low end temperature of 40°F. These thresholds can also vary by individual species. When temperatures drop below the lowest temperature, called the baseline, development stops. Above that range, development slows or cuts off altogether. Baselines of common garden plants:
   35°F - onions
   38°F - carrots
   39°F - strawberries
   40°F - asparagus, barley, beets, broccoli, collards, lettuce, oats, peas, potatoes, rye, wheat
   45°F - squash, sunflowers
   50°F - beans, corn, musk melons, peppers, sorghum, tomato
   55°F - cucumber, watermelon
   60F - eggplant, okra, sweet potatoes

Calculating degree-days

There are several different models used to calculate degree-days, but, here in the U.S., they all boil down to the same basic idea. Degree-days are calculated first by adding that day’s high and low temperatures and diving by 2 for a mean temperature for the day. A plant’s baseline temperature is then subtracted from that mean temperature, for a number of degree-days counted for that day. For example:
     High 84°F
     Low 54°F        (84 + 54) / 2 = 136 / 2 = 66 mean temperature
     Base 50°F        66 - 50 = 16 degree-days

Unless you really enjoy this sort of thing, you do not need to worry about calculating degree-days for yourself. Agricultural researchers have already done that work for you. You can look up your local degree-days using the UC Davis CA weather data (assuming you live in California). You can also try OSU’s Croptime calculator. Or, you can invest in your own weather station and generate a more accurate, customized model. Since each garden and neighborhood is its own microclimate, the degree-days reported are only estimates anyway, but these estimates can give you the advantage when controlling pests and caring for your plants.

Degree-days to maturity

Most seed packets offer a “days to maturity” number. This number is a statistical average spread out over the entire country. Factor in things like local climate, drought, pests, and disease, and you can see that these averages are only marginally useful. You can use weather station information to generate your own, more accurate days to maturity measurement. Here are the degree-days needed by a few common garden plants and pests to reach maturity:

• Beans           1200 GDD
• Beets            1450 GDD
• Corn             1360 GDD
• Cucumber  1370 GDD
• Grapes         1390 GDD
• Peas              1200 GDD
• Tomatoes    1300 GDD

• Borers            207 GDD
• Cutworms     130 GDD
• Leafminer      250 GDD​

What surprises me is that the number of degree days for common garden plants, pests, and diseases is not yet readily available.

I’ll keep you posted as the research is published.