Worldwide, freesia is mainly grown as a cut flower, and it is well-adapted to cooler climates. The plant is grown from corms. Using temperature treatments after lifting, flower induction and initiation can be controlled for year-round flowering.
Freesia is a niche crop for potted plant growers. In North America, most plants are grown in a narrow coastal band in California. The cool maritime climate allows outdoor production of compact, high-quality plants. There has long been interest in expanding production to other areas in North America, namely the upper midwest and northeast, where cool winters are ideal for the production of a low-energy, intensive crop. When grown in relatively low-light greenhouses (as in the northeast), however, freesia plants often elongate excessively and the plant quality suffers.
Gianfagna and Wulster (1986) first showed the efficacy of paclobutrazol or ancymidol for freesia growth control. Using five cultivars, drenches of these PGRs (5 mg a.i./pot) reduced plant height by 40% to 60% and inflorescences up to 93%. There was no effect on the number of flowering scapes, although the treated plants flowered ≈10 d after the controls. Preplant soaks in 250 mg·L−1 paclobutrazol were even more effective than substrate drenches. Therefore, it was concluded that “reduced immersion times or rates should result in simple and inexpensive” height control for freesia Gianfagna and Wulster (1986).
Wulster et al. (1989) showed that an average daily temperature (ADT) of 20 °C in the greenhouse caused taller plants than did lower temperatures (15 °C ADT), and plants grown at 10 °C ADT were even shorter. Clearly, forcing temperature is an important component of plant height, and temperature control is critical for potted freesia production. Ancymidol drenches (applied 1 week after planting) up to 3 mg/pot were effective, but less so at warmer temperatures, and the effects varied by cultivar. Regarding freesia production, “from a quality perspective, chemical height control is not an adequate substitute for appropriate greenhouse environment” Wulster et al. (1989).
Berghoef and Zevenbergen (1990) found that ancymidol drenches at 2 to 4 mg/pot applied 2 weeks after planting were effective for freesia height control in October plantings. Under similar growing conditions, the scape and leaf lengths decreased as the ancymidol concentration increased from 50 to 200 mg·L−1 and as the soak length was increased from 1 to 10 h. Therefore, the concept of very long soaks may have been derived from their work.
De Hertogh and Milks (1990) found that freesia ‘Aida’ was responsive to ancymidol at a concentration of 200 mg·L−1, but this is too expensive for commercial use because the full-strength commercial product is only available as 264 mg·L−1. Paclobutrazol soaks (4-h soaks, cultivar Aida) were effective at 50 mg·L−1, with increasing effects to 200 mg·L−1. Drenches of 5 mg/pot a.i. were similar to 100-mg·L−1 soaks. When soaked in 100 to 400 mg·L−1 paclobutrazol, ‘Rossini’ plant height decreased as the soak length increased from 1 to 4 h. Finally, drying times after soaks of only 2 h were suggested because longer drying times resulted in unspecified “erratic” results.
Wulster and Gianfagna (1991) showed that preplant cold storage at 5 °C for 2 to 4 weeks reduced scape height, leaf length, and the number of days to flower after planting. The source material was Rutgers-grown corms that had been stored at 30 °C for 15 weeks to break dormancy before cold storage. They concluded that the time to flower initiation needed to be determined, especially in relation to the cold treatment. Ancymidol was used as drenches of 1 to 3 mg/pot, but it is too expensive for commercial use. Ancymidol at 3 mg/pot reduced the scape height and leaf length by >50% and delayed flowering by ≈9 d.
De Hertogh (1996) listed preplant corm soak guidelines for a variety of freesia cultivars. Soaking for 1 h in 50 to 100 mg·L−1 paclobutrazol was recommended for ≈40% of the cultivars; for ≈60% of the cultivars, the soak recommendations involved 200 to 300 mg·L−1. These rates appeared to have been developed in a warm climate (i.e., North Carolina) (De Hertogh and Milks, 1990).
Cavins and Erickson (2004) reported that the temperature of the soaking solution affected the efficacy of flurprimidol corm soaks. When performing 30- to 60-min soaks, room temperature or warmer solutions (17 to 32 °C) were more effective than cold (7 °C) solutions. Flurprimidol at 54 mg·L−1 and 77 mg·L−1 caused a 30% reduction in height with tepid and cold solutions, respectively. The soak length had no effect on the concentration of flurprimidol that is needed to reduce height by a targeted 30%. As expected, the cultivar response varied significantly.
Much of the literature regarding freesia growth regulation involved now-extinct cultivars or warmer climates, where warmer greenhouse temperature interact with leaf and scape growth, thereby requiring greater PGR concentrations. Additional information about PGR use is needed to expand freesia production in greenhouses in the northern United States. The present work was performed to obtain PGR information regarding modern cultivars grown in northern greenhouse conditions and to further evaluate flurprimidol as a potential PGR for potted freesia.
Berghoef, J. & Zevenbergen, A.P. 1990 The effect of precooling, environmental factors and growth-regulating substances on plant height of Freesia as a pot plant Acta Hort. 266 251 257
De Hertogh, A.A. 1996 Holland bulb forcer’s guide. Alkemade Printing BV, Lisse, The Netherlands
Wulster, G.J., Cartwright, S. & Gianfagna, T.J. 1989 The effects of greenhouse temperature and ancymidol concentration on height and flowering time of Freesia hybrid grown as a container plant Acta Hort. 252 97 103
Wulster, G.J. & Gianfagna, T.J. 1991 Freesia hybrida respond to ancymidol, cold storage of corms, and greenhouse temperatures HortScience 26 1276 1278