Baby primrose (Primula forbesii) is a newly cultivated and valuable ornamental plant with great market potential for both indoor and landscape use. As a container plant, baby primrose has long, weak flower stalks that can easily lodge, resulting in poor-quality plants, especially during transportation. To control plant height and subsequently prevent flower peduncle lodging, we investigated the effects of two plant growth regulators (PGRs), chlormequat chloride (CCC) at 0, 250, 500, or 750 ppm and uniconazole (UNI) at 25, 50, or 75 ppm on growth, development, and flowering of two cultivars of baby primrose, Fragrant Luolan and Red Star. Plant growth regulators at the proposed concentrations were applied twice throughout the experiment. Both PGRs significantly suppressed plant height in both cultivars, with a 16% to 27% reduction by CCC and 50% to 59% by UNI compared with untreated plants. Among CCC-treated groups, plants were shortest when CCC was applied at 500 ppm; plant height was suppressed more when treated with UNI. In both cultivars, UNI significantly suppressed the first, second, and third peduncle lengths. Furthermore, CCC affected peduncle length, but to a lesser extent than UNI. Plant growth regulator applications generally had little effect on flower characteristics of baby primrose. Neither PGRs influenced the inflorescence number and flower size; however, PGRs did increase the number of floral whorls and suppressed pedicel length of ‘Red Star’. New leaf growth was suppressed by both PGRs. In addition, peduncle cell length and width were both significantly suppressed by PGR applications. We concluded that two foliar applications of UNI at 25 ppm comprised the most effective method of controlling baby primrose plant height while maintaining desirable flower traits at a relatively low production cost. Results of this study provide guidance for techniques that can be used to effectively control the plant height of potted baby primrose for commercial greenhouse production.
