Comparison of HPS lighting and hybrid lighting with top HPS and intra-canopy LED lighting for high-wire mini-cucumber production

High-pressure sodium lamps (HPS) are the conventional lighting technology used to increase photosynthetically active radiation in greenhouse crop production. Because of its high operating temperature, HPS can only be used as top lighting placed well above crop canopy, which has resulted in uneven vertical light distribution in tall crops such as high-wire mini-cucumbers. LEDs have low surface temperature and can be used as inter-lighting within canopy to improve vertical light distribution and increase whole light use efficiency. Therefore, a trial was conducted at a commercial greenhouse from October 2014 to January 2015 to evaluate the hybrid lighting systems with top HPS and intra-canopy LED, in comparison to the top HPS only, for year-round high-wire mini-cucumber production. Three lighting treatments consisting of i) Top HPS lighting (177 μmol m-2 s-1) only, ii) Top HPS (177 μmol m-2 s-1) plus one row of LED inter-lighting (49 μmol m-2 s-1), and iii) Top HPS lighting (177 μmol m-2 s-1) plus two rows of LEDs inter-lighting (99 μmol m-2 s-1) were evaluated; each was applied to 550 m2 of growing area. Leaf chlorophyll index of the 10th and 15th leaf was improved by the addition of one row and two rows of interlighting LEDs. The visual fruit quality of mini-cucumbers was also improved with the LED inter-lighting. Fruit yield was increased by 22.3 and 30.8% by the addition of one or two rows of inter-lighting LEDs over no inter-lighting, respectively. Combining the electricity and capital costs, the production cost for each kg of mini-cucumbers would be $ 1.30 for top HPS lighting only, $ 1.45 for top HPS plus one row of inter-lighting LEDs and $ 1.72 for top HPS plus two rows of inter-lighting LEDs, based on the current prices of electricity and LED and HPS light fixtures in Ontario, Canada, the largest greenhouse vegetable production area in North America. The high production cost of hybrid lighting systems is due to the high cost of inter-lighting LED fixtures. It is anticipated that with potential price reduction in LED light fixtures, the production cost with the hybrid lighting system will decrease significantly in the near future.