The effects of crop density on plant growth and variability in cut-flower chrysanthemum (Chrysanthemum morifolium Ramat.)

The effects of crop density on chrysanthemum plant yield and variability were studied using two cultivars, Snowdon and Delta, in four experiments (spring, summer, autumn and winter) using fan design layouts of gradually increasing space per plant. Average total fresh weight per plant varied greatly between experiments, but empirical linear regression relationships showed that changes in space per plant gave near proportional changes in weight per plant over the whole range of densities sampled. This suggests that the density responses were essentially mediated by the available light per plant. A simple three-parameter model was developed incorporating the accumulated light integral and this gave an excellent fit to data in all four experiments. The fresh weight of flowers per plant was allometrically related to "productive" plant weight which was defined as total plant weight minus a fixed weight not contributing to flower production. The relationships showed that the proportional partitioning of fresh weight to the flowers differed between the cultivars and varied between the experiments, being greatest under conditions of an increasing daily light integral (spring) and least under conditions of a declining daily light integral (autumn). The log coefficient of variation of total fresh weight per plant was found to increase with increase in plant density and was greatest under winter conditions. The log coefficient of variation of 'Snowdon' was generally greater than that of;Delta' at harvest, although at planting the two were similar. Polynomial regression relationships between average fresh weight and the log coefficient of variation obtained when data from all four experiments were pooled, suggested that competition for light was a primary determinant of variation. Increasing density was associated with progressively slower flowering and this trend was most pronounced in 'Delta' and in the autumn and winter experiments. Stems were generally reduced in length at high density but this was not always the case. The development of a simple model to predict average stem weights and grade-outs for given spacings at particular times of year is discussed.