The Effect of Air Temperature on Growth of Eight Herb Species

The effect of different constant air temperatures (18°C, 21°C, 24°C and 27°C) and variable temperatures (24°C/18°C and 27°C/15°C in 12 h/12 h periods) on basil, sage, thyme, lemon balm, cilantro, rosemary, oregano and rocket was studied. Supplementary lighting was given 16 h·day-1 at a photon flux density (PFD) of 150 μmol·m-2·s-1 (corresponding to 8.6 mol·m-2·day-1 photosynthetic active radiation [PAR]). Including daylight the PAR was 29.6 ± 6.9 mol·m-2·day-1 as a mean during the experimental period. Increasing the temperature from 18°C to 27°C increased the fresh weight in basil (106%), sage (95%), rosemary (126%) and rocket (62%), while an increase from 18°C to 24°C increased the weight in lemon balm (78%), cilantro (41%), oregano (40%) and thyme (58%). For the last four species the fresh weight was unaffected by a further increase to 27°C. No significant difference was found between the 24°C/18°C and 27°C/15°C treatments. These treatments gave a mean temperature of about 21°C, and no significant differences were found between these treatments and the constant 21°C treatment. The plant height generally increased in the same proportion as the fresh weight increased in the different species. No differences were visually observed between the treatments after two weeks under indoor conditions. All species remained green except cilantro and rocket, for which some leaf yellowing took place. A simple test indicated that the flavour increased with increasing temperature (from 18°C to 27°C) in all species except cilantro.