Abstract

Evenly distributed production and sales of pot plants throughout the year would be the ideal situation for greenhouse companies with respect to labour input and planning. The market in Sweden, however, has two big selling seasons, the spring sales period with bedding plants, and the Christmas season. During summer, when the climate conditions outdoors are most favourable for cheap production of pot plants, most greenhouse companies chose to stop production since they meet a weak market in late summer and autumn. Blue holly (Ilex × meserveae ‘Blue Princess’ S.Y. Hu) was tested as a potential new greenhouse crop for summer production. This holly hybrid is an evergreen, hardy and woody plant that is normally used as a garden shrub or for Christmas decorations. The advantage of this hardy, woody plant is that the final product can be stored outdoors until the Christmas sales period. The present study investigated the impact of greenhouse climate on growth of blue holly from rooted cuttings to small pot plants. In contrast to outdoor-grown shrubs, holly formed several periodic growth flushes when grown in climate conditions between 17 ºC and 26 ºC and irradiation levels of 0.6 to 5 mol m–2 d–1 (Skye sensors). The duration of one flush and the number of leaves produced were dependent on both irradiation and temperature levels. The individuals within the plant population, however, were poorly synchronized. In order to statistically prove the impact of climate on the growth oscillations, an analytical tool was developed. A sine function was modified to describe the growth oscillations and a value for the amplitude and the frequency of each individual plant could be estimated. The amplitude and frequency values were then analysed further for differences between the treatments. Two growth environments were chosen for the growth analysis in blue holly: a heated greenhouse and climate chambers with artificial lighting (white cool fluorescent tubes). Oscillations in vegetative growth, shoot length, number of side shoots and dry weight were compared in respect of light quality and quantity as well as temperature. Due to the different oscillation patterns within the range of climate conditions tested, the oscillations were categorized to enable comparison. Shoot growth was almost continuous at high irradiation and high temperature conditions, while in cooler temperatures and low light conditions only one flush could be observed. In intermediate climate conditions pronounced oscillations of vegetative growth could be observed with up to three flushes within an experimental period of 93 days. The branching pattern in blue holly was linked to the vegetative growth oscillations. Side shoots only developed after the growth cessation period between two flushes. In treatments with pronounced oscillations, a higher number of side shoots could be observed.