Comparing heat unit requirements for flowering and fruit harvest of cucumber in open field, shade net and greenhouse conditions

Cucumber ( Cucumis sativus L.) is cultivated in a wide range of growing environments, and the time from planting to flowering and harvest varies depending on the environment. Studies were conducted to assess flowering and harvest time of three cucumber cultivars grown in parallel under open field, net house and two passive solar greenhouses representing varying thermal regimes. Depending on environment and cultivar, the number of days to flowering varies as much as 41-days, and 61-days difference in first harvest was observed. Total accumulated heat units (HUs) from transplanting to the onset of flowering and first harvest varies significantly under the four production environments. Comparison of seven methods of calculating HUs results in different trends when compared with days to flowering and harvest. The cucumber yield of all the three cultivars followed the same order from high to low: polycarbonate greenhouse (3.7 ± 0.2), polyethylene greenhouse (3.1 ± 0.2), red shade net (1.4 ± 0.4) and open field (1.0 ± 0.2 kg per plant). However, the HUs computed using the seven methods did not follow the same order. Any attempt to explain the flowering and fruit harvest of a particular cultivar using a temperature model alone is an over-simplification. However, when sufficient information is not available and crops are grown under varied production environments, we suggest that calculating growing degree days (GDDs) is a reliable method to determine date of flowering and crop yield in high mountain trans-Himalayan region.