ISSUE 12   July 29, 2009


For most plants, development towards maturity is strongly related to the accumulation of heat or temperature units above a threshold or base temperature. Little growth occurs below the threshold or base temperature. This lower threshold temperature varies with plant species. The lower base temperature for sunflower is 44 °F. In some plants there is also an upper threshold temperature above which little or no growth occurs, but no upper limit has been identified for sunflower. Although temperature is the most important factor controlling the rate of plant development, other factors such as water, solar radiation availability and daylight length may modify its effects. The most common temperature index used to estimate plant development is growing degree days (GDD) which are calculated from the daily maximum and minimum air temperature. Growing degree days have proven useful to predict plant development rate and growth stages.

Sunflower Growing Degree Day Calculation

Sunflower growing degree days are calculated by subtracting the plant's lower base or threshold temperature of 44 F from the average daily air temperature in F. Average daily air temperature is calculated by averaging the daily maximum and minimum air temperatures measured in a 24 hour period. All daily NDSU NDAWN data are based on a midnight to midnight period.

The Daily Average Temp (F) = (Daily Max Temp F + Daily Min Temp F) / 2

And: Daily Sunflower GDD (F) = Daily Average Temperature F - 44 F

Constraints on maximum and minimum temperatures are used to eliminate the effect of low or high temperatures that prevent or retard growth. For sunflower there is only a minimum temperature constraint: If the daily Max and/or Min Temp < 44 F it's set equal to 44 F.


1. Given Daily maximum temperature = 80 F and Daily minimum temperature = 60 F

Then Daily average temperature = (80 F + 60 F) / 2 = 70 F and Daily GDD = 70 F - 44 F = 26 F.

2. Given Daily maximum temperature = 60 F and Daily minimum temperature = 34 F

In this case: The daily minimum temperature of 34 F is replaced with 44 F, according to the above mentioned constraint. Then Daily average temperature = (60 F + 44 F) / 2 = 52 F and Daily GDD = 52 F - 44 F = 8 F

Reasonable growth stage predictions based on GDD require that temperatures measured at NDAWN weather stations are similar to those experienced by plants in the field. Differences in temperatures can be due to differences in field elevation or relief, soil color, type, water content, and surface residue.

Sunflower Growth Stage Estimation Using Growing Degree Days

The time from planting to emergence varies somewhat because of variations in planting depth, seedbed conditions, soil water content, surface residue, and soil type. Because of this variation the number of GDD required from planting until emergence is not constant which causes variation in estimated growth stages. Despite this uncertainty, growth stages on NDAWN are calculated based on planting date rather than emergence because planting date is more readily available in producers’ records.

This year has been rather cool and the accumulation of GDD is lagging behind normal. The Figure ‘Departure from Normal GDD’ provides the accumulated GGD with a planting date of May 15th 2009 through July 25th 2009 at Fargo ND. May 15th represents the earliest reported sunflower planting in ND.


During this period 1418 sunflower GGD were accumulated in Fargo compared with the normal accumulation of 1600 GGD. The sunflower GGD can be calculated based on planting date and weather station location using the NDAWN data, which can be found at

The relationship between sunflower development and GDD is displayed in the Table 1. Growing degree days are accumulated beginning with the day after planting. The standardized growth stages used here were described by A. A. Schneiter and J. F. Miller in 1981 (Schneiter and Miller. 1981. Description of Sunflower Growth Stages. Crop Science 11: 635-638).

Table 1. Approximate accumulated GDD needed to
reach the indicated sunflower growth stage

Growth Stage

Accumulated GDD

VE (Emergence)
































Adnan Akyüz
North Dakota State Climatologist

Hans Kandel
NDSU Extension Agronomist, Broadleaf crops



Winter wheat is approaching maturity and early planted barley is not far behind. Not surprisingly, I have had several questions about the use of glyphosate as a pre-harvest desiccant in small grain crops. Glyphosate applied pre-harvest can also aid in the control of perennial and other green weeds though the results are usually disappointing as weeds at this time are tall, nearing maturity and slow growing. Glyphosate is a systemic herbicide and takes from seven to ten days to effectively kill the growing parts of the crop, therefore the dry down process is not immediately visible. Applying glyphosate too early can reduce yield, test weight and seed germination. Because germination can be affected when applied too early, glyphosate should not be used in fields that will be used for seed or on barley intended for malt. The optimum time to apply glyphosate pre-harvest is when the crop has reached physiological maturity which for most varieties of wheat occurs at a grain moisture content of about 30% (20% moisture is the recommended timing for barley). At this moisture content the grain will be in the hard dough stage and if you run your thumb nail across the kernel, the indentation will remain. Since not all kernels arrive at physiological maturity at the same time, be sure to sample multiple kernels to avoid spraying too early. A visual indicator that can also be used to determine physiological maturity is when the peduncle, the portion of the stem just below the spike, turns from green to yellow. There may be occasions when drought or other stress may mask color changes in the peduncle, so it is advisable to also check the hardness of the grain before spraying. Pre-harvest applications of glyphosate must be made at least seven days before harvest. Always read and follow label directions.

Joel Ransom
Extension Agronomist for Cereal Crops

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