Growing chrysanthemums with less electricity and less heat

June 15, 2023

Greenhouse horticulture is an important sector that provides consumers year-round with fresh and healthy food and flowers to improve their quality of life. The sector faces the challenge of reducing energy consumption in order to reduce CO2 emissions. To be able to produce year round in a region where light intensities are low in winter, assimilation lighting is necessary. In recent years, the transition from HPS lighting to LED lighting has been initiated. An important reason for this transition is the high efficiency of LEDs in converting electricity into light (up to 3.6-3.7 µmol/J) compared to HPS (approx. 1.8-1.9 µmol/J).

In addition, LEDs can emit the light spectrum that is most suitable for the crop, variety and crop stage and they can be switched on and off and dimmed quickly. The transition to LED lighting in chrysanthemum is progressing quickly.

Since the autumn of 2021, the prices of electricity and gas have risen sharply, which has further reinforced the urgency of reducing energy consumption for the sector. To enable profitable chrysanthemum cultivation, energy consumption for lighting and heating will have to be reduced. In the project “LED light supplementing sunlight”, we investigated how chrysanthemums can be grown at lower temperatures and lower light levels, and how this can be supported by other measures (e.g. plant density and length of the long day period) to maintain flower quality.

Chrysanthemum cultivars Baltica and Pina colada were grown in two airconditioned greenhouse compartments at two temperatures (“normal” and “low” temperatures of 18.5/18 °C and 17.5/16.5 °C (day/night) respectively) and three light levels (“low” , “middle” and “high” light with light sums of 6.0, 7.5 and 9.1 mol/m2/day). The plant densities and the long day period were adjusted to the light levels. Plant densities for the low, medium and high light treatment were 40, 48 and 56 plants/m2 and the long day period was extended by 1 and 3 days in medium and low light compared to high light. During cultivation, plant length was monitored. Th plant's resistance to Botrytis and thrips was determined via bioassays. At the end of the cultivation, the reaction time was assessed and plants were harvested destructively, determining stem quality, number of flowers and plant weight per stem and per m2.

Lowering the cultivation temperature delayed plant development. Baltica had a response time (cultivation time in number of days short day until the branches could be harvested) that was 2 to 3 days longer at a lower temperature, while the delay of Pina colada was four days. The reaction time was not affected by the light level. Branch weight was reduced at lower light intensities, but still met the minimum weight. The effect of a lower light intensity was therefore not fully compensated by the lower plant density. The production expressed in kg fresh weight per m2 was reduced considerably at lower light intensities, proportional to the decrease in plant density. Branch weight was not affected by temperature. The resistance of the crop to Botrytis and thrips was not affected by the light and temperature treatments.

Branch quality, expressed as length, weight and number of flowers, was good for all light and temperature treatments applied. To realise this, plant density was adjusted to the light levels, which means that the production per m2 was low at low light levels. The low and mid light treatments resulted in electricity conservation of 33 and 18% respectively compared to the high light treatment. The reduction of the temperature by 1.3 °C implies an estimated energy conservation of more than 10%. These results show that it is very well possible to grow chrysanthemums with a lower energy consumption while maintaining a good branch quality.