Horticultural lighting in the Netherlands: New developments

In the Netherlands about 2000 ha of glasshouses is equipped with supplementary assimilation light (SL), which is about 19% of the total glasshouse area. Besides increased production, SL results in improved product quality, a better control of yield and quality, possibilities for earlier or year-round production and a more regular labor requirement. In this paper several recent experiments with different strategies of SL (33 up to 210 mu mol m(-2) s(-1)) for tomato, sweet pepper, cucumber and eggplant are presented and discussed. In general, it was concluded that SL was not economically feasible. For cucumber SL can only be attractive if the crop is grown at high plant density and according to the high-wire system. Based on 3 plantings per year, a production of 147 kg m(-2) (360 cucumbers) is possible with 210 mu mol m(-2) s(-1) SL during 3000 h/year. Application of 50% of the light within the crop (interlighting) by fluorescent tubes instead of only HPS-lamps above the crop, did not improve production but improved fruit quality in cucumber. Mobile lamps are sometimes used instead of fixed lamps. For sweet pepper and tomato, a fixed-lamp installation was economically more feasible than mobile lamps when compared at the same light intensity. A dynamic simulation model was used to predict effects of different lighting strategies at 500 and 1000 ppm CO2 on potential production. Maximum levels of 110, 64 and 168 kg m(-2) year(-1) were calculated for tomato, sweet pepper and cucumber, respectively.