Predicting Plant Development Using Growing Degree Units
Planting dates and plant progression vary year to year due to variable temperatures. It’s not unusual for growers to use calendars to predict plant development, but the fact of the matter is that management decisions based off of planting calendars are not always accurate. Growing degree units, on the other hand, are based off of current temperature data and are a more accurate method of predicting plant progression.
What are Growing Degree Units?
Growing Degree Units (GDUs) or Growing Degree Days are a measure of heat accumulation leveraged to predict plant development rates, more specifically the date a crop reaches maturity.
GDU calculation allows growers to estimate the progression of their crops and compare their progress with that of regional or national averages.
Different varieties and types of crops have different growing degree unit requirements.
How do you calculate GDUs?
Figure 1. Midwestern Regional Climate Center
Growing Degrees (GD) are daily temperature values found by averaging the day’s maximum and minimum temperatures and subtracting the base temperature from that value. In mathematical terms, GD = ((daily maximum temperature + daily minimum temperature) / 2) - base temperature. The base temperature varies by crop; it is the temperature below which there is no plant growth (see Figure 1). For example, if the low for the day was 36 degrees and the high was 64 degrees, then the day had an average temperature of 50 degrees (64+36 / 2). If the plant in this area had a base temperature of 32 degrees, then the growing degree for that particular day would be 18 degrees (Fahrenheit scale), as ((64+36)/2) - 32 = 18. The date one begins recording and adding GD is called a biofix. This is the point at which one begins to observe a seedling or when an organism’s buds start to open for plants and for insects is when the first adult specimen is caught in a trap. GDUs are calculated by cumulatively adding together GD values as the season progresses.
Aside from predicting plant maturity, what else can GDUs do?
Figure 2. Midwestern Regional Climate Center
Pesticide Application
Degree units can also be counted to determine the growth stage of insects. Knowing this, farmers can apply pesticides accordingly. Insects’ different stages of life require different methods of treatment (see Figure 2).
When to apply pesticides can also be dependent on the plant’s stage of growth. Apply it too early and it will not affect the pest because their eggs are immune to most chemicals. Apply it too late and the damage is most likely already done (see Figure 3).
Irrigation Management
How many growth degrees a plant has over a growing season also helps farmers determine if the region their fields are in is suitable for particular crops. The region must be warm enough to have enough growing degree units to fully develop but not too hot to halt the growth all together.
Heat stress is one of the factors that determines the amount of water the plant needs. These needs vary depending on the stage of growth the plant is in at a particular point. Knowing the recommended amount of water a plant needs is crucial to its growth. If more heat stress is present, the plant will need more water. Growing degree units can be used to make sure crops are not over/under-watered.
Timing Hay Harvest
Cutting forage crops is different from harvesting, but GDUs can still take the guess work out. If the farmers are able to determine the plant’s maturity, they will be able to cut them at the optimal time to balance maturity and nutrition.
“The importance of harvesting a hay crop on time can make a big difference in the fiber and energy levels for alfalfa. There is usually a short window of opportunity to harvest alfalfa at a particular goal since neutral detergent fiber (NDF) levels can change rapidly with increasing temperatures.”
Things to keep in mind
Location is important. If the farmer knows the amount of growing degree units needed by their crops and is aware of the climate of the region, they can coordinate their crops accordingly. If they plant two different crops with different GDU requirements at the same time, they will harvest them at different times. They could also plant them at different times so they are ready for harvest at the same time, depending on their preference.
Need help determining your base temperature and GDUs?
Use the resources on this page!