The effects of N-source, shading and root zone cooling on two Disa hybrids
Although the Disa genus contains more than 130 species, the most commonly grown is Disa uniflora and its hybrids. The evergreen Disas are indigenous to South Africa and have great potential for marketing as cut flowers or pot plants but production techniques need to be further investigated and improved. Two Disa hybrids ('Unidiorosa' and 'Kewensis') were subjected to different ammonium levels in nutrient solutions under varying environmental conditions. Plants were cultivated in a controlled environment and growth was evaluated after 266 days. Plants were fertigated according to guidelines provided by Naaldwijk for Cymbidium orchids, modified for different ammonium/nitrate ratios with the same amount of total nitrogen. Two levels of shading were provided (56% and 69%) and two root medium temperatures tested. Where most greenhouse crops need less than 10% of their total N supply as ammonium, Cymbidiums need about 20%. 'Unidiorosa' and 'Kewensis' performed best with 40% and 60% of the applied N in the ammonium form respectively. No significant difference was found between the two shading levels, probably due to the cool greenhouse conditions. Lowering of the average mid-day root medium temperature from 21.4°C to 16.8°C had a negative effect on root growth and plant mass, but produced longer leaves and increased leaf area.