Growing Degree Hours and GDH30

Growing degree hours (GDH) are a weighted measurement of the temperatures to which a tree is exposed, based on the idea that trees have an optimum temperature for growth and that temperatures far above or below that optimum temperature have a greater impact than temperatures just above or just below it. UC Davis’s Harvest Prediction Model calculates GDH values based on the equation of Anderson et al., 1986 and temperature readings collected by the more than 100 CIMIS weather stations throughout the state.

Analysis of the accumulated growing degree hours for the first thirty days after full bloom (GDH30) have shown the amount of accumulation to be related to the number of days between full bloom date, reference date, and harvest date (Marra et al., 2002). Higher GDH30 values correlate with less time between full bloom and reference date in peaches (Lopez G and DeJong TM., 2007) and less time between full bloom and harvest date in peaches, nectarines, plums and prunes (Ben Mimoun M and DeJong TM, 1999; DeBuse et al, In press). Higher GDH30 accumulations with no change in management practices also result in smaller fruit at harvest (Lopez G and DeJong TM., 2007). The Harvest Prediction Model enables growers to better integrate knowledge of this relationship into their management practices.

Why Are GDH30 & Harvest Timing Related?

Peach fruit have been shown to have two phases of development. During phase I, the relative growth rate (rate of unit dry weight increase per unit dry weight per unit time, RGR) of the fruit is decreasing at a logarithmic rate. There is then a shift to a stable, linear RGR which constitutes phase II. During phase I, cells are dividing and differentiating. During phase II, cells are expanding and maturing. The timing of this phase change is genetically regulated but also affected by how warm or cold a spring is, and shortening or lengthening of phase I affect the timing of maturity, and thus the harvest date (DeJong TM and Goudriaan J, 1989)

Why are GDH30 & Fruit Size Related?

Trees are made up of a number of individual organs – fruits, shoots, leaves, etc. Each organ’s potential for growth and pattern of development is based on the genetics of that organ type. Once the potential for growth of that organ is activated by signals within the tree and/or from the environment, the actual "realized" growth is a result of environmental conditions (temperature, light, etc), resource availability (carbohydrates, nutrients) and competition with other organs for those resources. How competitive an organ is, its "sink strength," is dictated by that organ’s relative distance from the source of carbohydrates and the organ’s efficiency in unloading those resources from the supply stream (DeJong TM and Goudriaan J, 1989).

During the first stage of growth in peaches, fruit growth is limited by competition from other organs (DeJong TM and Grossman YL, 1995). The respiration rate of both vegetative organs and fruit also increases with increased temperature, approximately doubling when temperatures increase by 10° C (Grossman YL and DeJong TM, 1994; Pavel EW and DeJong TM, 1993). The demands of growth and respiration under higher temperatures quickly add up - between 20 and 30 days after full bloom, the daily potential rate of fruit respiration and growth, and subsequent demand for carbohydrates, can be 5 to 10 times higher during a warmer spring than during a cooler spring (Lopez G and DeJong TM., 2008). These increased respiratory and growth demands come at a time of low photosynthetic fixation because leaves are still small and days are still short (Grossman YL and DeJong TM, 1994).Thus not only do fruit have less time for growth because of an earlier growth phase change and an earlier harvest, they also have to deal with increased competition for assimilates from increased maintenance respiration.

Source demands are highest on trees with moderate and high loads of fruit (DeJong TM and Grossman YL, 1995). Decreasing competition for resources by thinning is thus an obvious solution, but the timing of that thinning is critical to its success. Limiting resources during phase I, cell division, irreparably decreases fruit size because fewer cell divisions leads to fewer cells, decreasing the maximum relative growth rate a fruit can achieve (Grossman YL and DeJong TM, 1995b). While thinning fruit at a variety of points throughout the growing season can boost fruit relative growth rate to near its maximum potential given normal temperatures, the absolute growth rate (rate of unit dry weight increase per unit time) cannot be recovered, resulting in smaller fruit (Grossman YL and DeJong TM, 1995b). This situation can be likened to a savings account: even given the same interest rate, an account with a larger initial investment will yield more than that with a smaller investment. And though fruit growth rate does increase with increased GDH accumulation, it does not increase enough to compensate for the shortened growing season, resulting in smaller fruit after warmer springs (Lopez G and DeJong TM., 2007). Thus, though thinning is traditionally done 30 days after bloom for early-maturing cultivars and 60 days for late-maturing varieties, these activities should be undertaken sooner with higher GDH30 accumulation.

How Can Growers Use GDH30 Figures?

Understanding the affects of increased GDH30 empowers growers to adjust their management practices to at least partially counteract the effects of warmer springs. Anticipating earlier harvests can allow growers to organize harvest logistics. Anticipating smaller fruit under normal management, growers can thin fruit earlier to provide more resources on a per-fruit basis over the growing season. As a rule of thumb, 6,000 GDH30 is a critical turning point for fruit size reduction and earlier harvest dates (Anderson et al., 1986 ; Lopez G and DeJong TM., 2008). When GDH30 goes above this threshold, growers should thin earlier in the season. More exact prediction models for harvest date have been developed for a number of stone fruit cultivars based on the relationship between GDH30 and harvest date (Ben Mimoun M and DeJong TM, 1999; Day et al., 2008).

For guidelines on predicting harvest date, proceed to the next page, Predicting Harvest Date. In subsequent pages,  the Harvest Prediction Model will help you find your GDH30, which is necessary for calculations.

There are three approaches to using the GDH30 accumulation figures for your location to predict an approximate harvest date: using a rough threshold estimate, using established models developed from historical data by previous research, or using your own site-specific trend calculations to generate your own model.

Threshold Estimating

It’s been found that 6,000 GDH is a threshold for changing maturation behavior. Accumulation of much more than 6,000 GDH in the first thirty days after bloom leads to earlier harvest dates and smaller fruit size under normal thinning practices. If the GDH30 accumulation figure shown for your location and bloom date this year is above 6,000, it’s recommended you thin fruit earlier than usual and anticipate an earlier harvest than normal. (Lopez G and DeJong TM., 2008; Lopez G and DeJong TM., 2007).

Established Models

The relationship between GDH30 and the number of days between full bloom (FBD) and harvest (HD) has been quantified for a number of different cultivars of peaches, plums, prunes and nectarines based on historical data from a number of years and locations throughout the state (Ben Mimoun M and DeJong TM, 1999; DeBuse et al., In Press). Though the exact numbers for every cultivar have not been calculated, the fact that GDH30 and FBD to HD are related is true for all cultivars. Preliminary data suggests that cultivars with similar chill requirements and similar maturation timing have similar slopes, with low chill cultivars having steeper slopes than medium chill cultivars. Given the GDH30 figure generated for your location and bloom date, you can use the models shown below to estimate the harvest date.

To get a ball-park figure of the days between full bloom and harvest for this year, find your cultivar of interest, or one with a similar chill requirement, approximate the GDH30 value’s location on the x-axis, and based on the modeled relationship approximate the associate number on the y-axis (see example below). Add the number of days to your FBD for this year to get an approximation of this year’s HD.

The equations in the figures below show the slope and intercept that was calculated for each cultivar following the formula Days from FBD to HD = (slope x GDH 30) + intercept. If an equation for your cultivar of interest is shown in the graph, you can get a more exact estimate by plugging in your GDH30 to calculate HD (see example below). If your cultivar is not shown here, you can plot your historic data of days between full bloom and harvest and GDH30 for the last five years onto the graph, see which cultivar your records best line up with and use that cultivar as an approximation.

Figure 1. Example of finding Days of Growth using GDH30 = 6,500 and existing O’Henry model. (Ben Mimoun M and DeJong TM, 1999)

It has recently been found in peach and nectarine models that cultivars of approximately the same chill requirement have the same slope (Day et al., 2008). If you are calculating for peaches and nectarines with a medium chill requirement, you may wish to use this more simplified approach. The table Peach and Nectarine Common Slope Models provides the intercept, cultivar parameter and slope. Research to similarly refine the models for other stone fruits is ongoing.

Generating Your Own Model

You can also use the GDH30 data for your area and bloom dates and your own harvest date records to approximate your own trend line. Gather data from at least five or six years to generate an accurate model and enter them into a calculator or computer spreadsheet. Set your GDH30 figures as the independent variable (x axis) and Days between Full Bloom and Harvest as your dependent variable (y axis), then use the linear regression function to generate a model line with an equation in the format of Days from FBD to HD = (slope x GDH 30) + intercept and plug in your GDH30 figure for this year [see example below using Excel and the XY (Scatter) graph function].