Average Daily Temperature Effects on Paclobutrazol Efficacy of Four Floriculture Crops

The effects of paclobutrazol (PBZ), a plant growth retardant, and average daily temperature (ADT) on geranium (Pelargonium x hortorum), petunia (Petunia x hybrida), marigold (Tagetes erecta), and pineapple mint (Mentha suaveolens) were quantified. Treatments consisted of four PBZ spray concentrations (0, 15, 30, or 45 ppm) and three ADTs (constant day and night greenhouse temperatures set to 16, 22, or 28 degrees C). The effectiveness of PBZ was dependent on species. Greenhouse ADT was significant for all species for both growth index (GI) and dry weight (DW). Whether the GI or DW was impacted by the interaction between ADT and PBZ levels were also species dependent. As ADT increased, the trialed levels of PBZ were less effective. Three species (all but petunia) had a significant ADT and PBZ interaction for DW. The 0 ppm PBZ treatment for geranium exhibited a larger DW at 28 degrees C compared with 16 degrees C, whereas the 30 and 45 ppm PBZ treatments each had smaller DWs at 28 degrees C than at 16 degrees C. However, marigold and pineapple mint generally had larger DWs at higher ADTs than lower ADTs within a PBZ treatment.