Potato Growth and Accumulated Heat Units:
How Does This Year Compare?
Mark J. Pavek, Edward P. Driskill Jr., and Zachary J. Holden
Washington State University, Pullman, WA
Soil moisture and temperature are most commonly cited as the major factors that contribute to potato sprout growth and emergence rate. Additional factors include seed size and health, sprout health, sprout/eye location on the mother seed tuber, soil fertility, cultivar, mother-tuber physiological age, volume and mechanical resistance of soil, and seed tuber dormancy. Rapid sprout emergence can promote early-season disease resistance in potato shoots and stems and allow plants to capture solar radiation early in the season. It is important to note, however, that early emergence does not always equate into an increase in yield.
Accumulated heat units, also known as day-degrees and degree-days, are often used to demonstrate or predict sprout emergence. They are calculated by taking the average daily temperature from each day and subtracting the growing base temperature (45° F). The heat units for each day are then added over time to provide accumulated heat units (see figures below). Although potatoes can form sprouts near 40° F, growth is extremely slow. To calculate accumulated heat units, we used a base temperature of 45° F because it is generally more conducive for vegetative growth.
The amount of heat units required in the soil for sprouts to break the soil surface depends on all the factors above and changes for each situation. In general, the faster heat units are accumulated, the quicker plants will emerge. The figures below were calculated with above-ground (ambient temp) heat units because soil temps for all time periods were not available. Above-ground heat units are still relevant because soil temps gradually warm as the average daily air temperatures increase.
How does 2005 compare to years previous?
At Paterson, Basin City, and Moses Lake accumulated heat units on May 30 were above the 5 and 10 year averages and quite similar to 2004 (see figures below). Until late April the heat units at all locations were near the 5 and 10 year averages, but well below the values from 2004. Despite the slow accumulation during early April, heat units accumulated rapidly between the 4 th week of April and early June, due to a rapid increase in the ambient temperature at all locations. Some of the early-planted (March) crops this year were slow to come out of the ground, but those planted in late April emerged quite rapidly when soil moisture was adequate. By comparing the figures to each other, it is easy to see the differences in heat unit accumulation as measurements move between the north and south basin. Between March and June 2005, Paterson accumulated approximately 1000 heat units, Basin City 900, and Moses Lake 800.