Chapter 28
Growing Degree Days
This chapter is included to help clarify the relationship between air temperature and tomato plant growth and development. Growing Degree Days (GDD) is a tool used extensively by corn farmers in the management of their crops. It is also used to predict the life cycle of various insects and the proper timing of insecticide use to control them.
Growing Degree Days, sometimes referred to as Accumulated Heat Units, is a technique based on air temperature used to predict the time required to go from one stage of plant growth to another. GDD relies on the concept that it takes a certain amount of heat for a plant to go through its growing cycle from seed to mature fruit production.
Plant growth and development depend on numerous factors. These include weather (air temperature, soil temperature, rainfall, and sunlight), soil fertility, cultural practices, and genetics to name but a few. Of these, air temperature is a very important factor. By measuring air temperature and observing plant growth, the number of degree-days required to complete the growing cycle has been determined for many different plants, including tomatoes. It appears that giant tomatoes need 1700 to 1850 degree-days of heat to go from seed to mature fruit. By knowing this, predictions can be made about a tomato's growth cycle, and when it will mature based on the amount of heat it has received.
Most home gardeners continue to use calendar days, based on past experience, to predict plant growth and development. An example of this is the days to maturity (DTM) listing on most tomato seed packets. This number represents how long it will take to go from transplanting in the garden to harvesting a mature fruit. However, calendar days can be quite misleading, especially for seasons that are warmer or cooler than usual. For example, a cool June can greatly delay the growth of tomato plants that were transplanted to the garden in late May or early June. On the other hand, two weeks of warmer than usual weather in July can markedly increase the growth of tomato plants resulting in earlier than usual maturation. These variations from expected temperatures during the growing season make the DTM labels on tomato seed packets unreliable. Research has shown that measuring the actual heat accumulated over time (degree-days) provides a more accurate estimate of plant growth and maturity than does the use of calendar days.
GDD can be utilized to estimate and predict the growth and development of tomato plants during the growing season based on outside air temperatures. It does this by assigning a heat value to each day called degree days. By adding up these daily heat values, one can predict where a plant's growth will be at a certain time in the growing season. The basic concept is that plant growth will occur only if the temperature exceeds some minimum threshold or base temperature. The base temperature for tomatoes has been established at 50˚. Below 50˚, tomato growth slows significantly and then stops.
To calculate GDD, you must first find the mean temperature for the day. The mean temperature is found by adding together the high and low temperature for the day and dividing by two. If the mean temperature is at or below the tomato base temperature of 50, then the GDD value for that day is zero degree-days. If the mean temperature is above 50, then the GDD value equals the mean temperature minus the base of 50. For example, if the high temperature for the day were 80, and the low temperature 60, the mean temperature for that day would be 70 (80 plus 60 divided by 2 equals 70). The GDD for that day would be 20 degree-days (mean temperature of 70 minus the base of 50).
Tomato growth is dependent on temperature, and it is the consensus that at temperatures below 50˚, tomato growth nearly ceases. 50˚ is considered to be the low developmental threshold temperature for tomatoes. This number is used as the base temperature. Above the base temperature, tomatoes grow at increasing rates until the temperature rises to a high developmental threshold where growth slows and then stops. The high temperature threshold for tomatoes is 86˚. To account for these threshold temperatures relative to GDD, most tomato growers now use Modified Growing Degree Days as their standard.
Modified Growing Degree Days are similar to GDD with several adjustments based on the low and high developmental threshold values. For purposes of modifying the GDD, keep in mind that tomatoes slow or stop their growth if the temperature exceeds 86˚, and may stop growth if below 50˚. If the high temperature for the day is above 86˚, it is reset, for calculation purposes, down to 86˚. If the low temperature for the day is below 50˚, it is reset up to 50˚. Once the higher and lower temperatures have been modified, if they have exceeded the set limits, greater than 86˚ or less than 50˚, the average temperature for the day is computed and compared with the base temperature of 50˚. Examples of these calculations are as follows: If the high temperature for the day is 70˚ and the low temperature 45˚, the 45˚ is adjusted up to 50˚ and these numbers are then added together and divided by two and then the base of 50˚ is subtracted for a GDD value of 10 degree-days (70 plus 50 equals 120, divided by two equals 60, minus 50 equals 10). If the high temperature is 95˚, the top value would be adjusted down to 86˚, and the same calculations are made producing a GDD value of 23 degree-days (86 plus 60 equals 146, divided by two equals 73, minus the base of 50 equals 23.)
GDD is the final accumulative value of all daily degree days added together from the day of transplanting to the garden to the day of harvest. It would appear that giant tomato varieties require between 1700 to1850 degree-days to produce a ripe tomato though this number will vary from one variety of tomato to another. It would be worthwhile to determine for yourself the number of degree-days for your own garden and the varieties you grow, and use those in the future for your predictions. GDD is the most accurate way to compare maturity dates for different varieties of tomatoes and is consistent across all climates and temperature zones.
Growing Degree Days can be used to predict tomato maturation. It helps answer the question as to whether tomatoes might ripen in late July or early August, or not until September. The use of GDD data gives you a method that allows you to make predictions, and can take some of the calendar day variations out of predicting events affected by temperature.
There are several ways growers can manipulate garden temperatures and thereby the accumulated heat units. Some gardens may not receive enough heat to ripen certain varieties of tomatoes. Some gardeners may want their tomatoes to ripen earlier. The use of greenhouses, plastic row covers, or planting against a south facing wall are tricks used to build and retain heat. They are used by determined gardeners to get ripe tomatoes earlier, or to grow long day (DTM) varieties.
One weakness of GDD is that plants may be set back if temperatures are extreme. Perhaps in this instance, some heat units should be subtracted. If a plant is exposed to 40 degree or 100 degree temperatures, should some sort of a negative degree day be applied? This is not taken into account presently and perhaps further research is indicated.
If you understand the concept of GDD then you may realize why it is so difficult to be certain that you will have a tomato ready for a given weigh-off day since the weather is one variable you cannot control.
Home Grown Tomato from Keepers – Sugar Hill Records – 1997
When I die don't bury me
In a box in a cold dark cemetery
Out in the garden would be much better
Where I could be pushin' up home grown tomatoes
Home grown tomatoes, home grown tomatoes
What'd life be without home grown tomatoes
There's only two things that money can't buy
That's true love and home grown tomatoes.
