CUMULATIVE HEAT-UNIT METHOD IN PLANNING PLANTING-PROGRAMS FOR SOME PEA VARIETIES IN NORTH-WEST YUGOSLAVIA
The heat unit method for scheduling sowing-programs has not been applied yet, because no long term observations in the production area are available for the existing assortment of varieties. Such a knowledge is essential for practising a heat unit system. The sum of average daily temperatures above 4.5°C from the time of sowing up to technological ripening, is characteristic for each variety. Climatic and edafic factors in each production area, however, and also the management of farming influence the heat unit to a large extent. Therefore, heat units have to be established for each area and each variety on a long term basis.
Climatic conditions, especially temperature extremes during the vegetative period of processing-peas, frequently cause disturbances in the normal rhythm of development. Daily temperatures above 25°C at the time of flowering and the development of pods accelerate the ripening-process. Therefore, the length of the vegetative period under our conditions is unsteady, and varies in number of days and in the heat unit as well. The date for the technological ripening as scheduled by the heat unit method, on the basis of long term averages, deviates from 2 to 4 days in normal years and 7 days in extreme hot or cold years (Lesic, 1966).
According to Katz (1952) the heat unit is lower in cold and higher in warm years, whereas early sowing requires less heat units until the technological ripening than a later one.
In order to make a more successful contribution to application of heat units for scheduling sowing-programs, a study was made during the 8 past years with different varieties and sowing-dates.