Heating system position and vertical microclimate distribution in chrysanthemum greenhouse

In the cultivation of greenhouse chrysanthemums, the main heating system is often positioned overhead. Under unfavourable conditions, this can lead to unacceptable vertical temperature differences or high relative humidity (RH) in the crop and ultimately to increased risk of diseases. Moreover, the overhead heating pipes will result in higher energy loss compared to a heating system at a lower position. Heating circuits at different heights, controlled independently of each other, can help to prevent these problems. To see the effects of the vertical position of independent heating circuits in chrysanthemums, the distribution of air humidity and that of air and leaf temperatures have been measured under a range of ambient conditions, using three combinations of two heating circuits at varying heights. All three configurations provide higher air temperatures above the crop than within, but the air temperature above the crop is decreased under cold outside conditions by about 1.5 degrees C when the primary heating system is mounted low in the crop. The large vertical temperature differences in the crop when using the traditional heating system with overhead pipes can be avoided by applying a crop heating system instead. Using the lower circuit as a primary heating system increases, on cold nights, the air temperature in the lower layers of the crop by about 3 degrees C, reduces the RH there by about 7% and decreases more quickly the dew point below the crop temperature after irrigation. The low positioned heating system had 7-9% lower heating requirement than the traditional system with heating pipes overhead. Between the three compartments, no significant differences occurred in leaf area index (LAI), and length and weight of stems. Nor was any difference found in flower quality. (C) 2000 Elsevier Science B.V. All rights reserved.