Clematis is an ornamental perennial vine that is among the most well-recognized ornamental plants in the retail market; however, production has not traditionally fit into the greenhouse production cycle for vegetatively propagated, herbaceous perennial crops. The major reason is that the production cycle from propagation to market takes 2 years. The conventional production schedule for a temperate climate zone is described as follows. Propagation from one-node or two-node stem cuttings takes an average of 10 weeks during the summer. Then, rooted liners are transplanted to larger containers and grown for the remainder of the summer. The vining shoots may be trimmed once or twice to minimize the entanglement of the plant canopy during this first bulking period. The plants enter dormancy in the fall and are overwintered in a minimally heated or unheated low or high tunnel during the first dormancy period. The following spring, vining stems emerge from above and/or below ground tissue and are trimmed as needed during the second bulking period. The plants may be transplanted to a larger container at this time. The plants go dormant in the fall and winter (the second dormancy period), and shoots emerge the following spring. Flowering occurs on the new flush of growth in May and June, at which time the plants are sold on the retail market. A commercially acceptable clematis plant consists of three or more flowering stems possessing five or more flowers or buds.
Under conventional production schedules, the major limitation to shortening the production cycle of clematis is the lack of branching observed on 1-year-old plants. As reported for the herbaceous perennial gentian (Gentiana triflora × G. scabra), environmental requirements for promoting bud formation and branching can be replaced by plant growth regulators such as ethephon (Samarakoon et al., 2015) and cytokinins (Grossman et al., 2012). Unpublished reports of clematis indicated no increased shoot formation when cytokinin (thiadiazuron) and ethephon were applied during the shoot development of the current season (Puglisi, 2002). We hypothesized that the shoot number could be improved if an adequate number of buds existed on the perennial structures of clematis before application of the cold treatment. Therefore, our investigation focused on the influence of plant growth regulator applications used during the bulking period before the cold treatment to promote shoot development after the cold treatment, (i.e., during forcing).
Shoot emergence, development, and flowering in clematis may be promoted by the cold treatment, photoperiod, and temperatures in the forcing environment; however, as noted for other herbaceous perennials, plants must be physiologically capable of perceiving the cold treatment (Dole, 2003). Most of the herbaceous perennials in temperate regions use buds produced during the previous growing season for the shoot production of the current season (Vesk and Westoby, 2004). Environmental factors before cold treatment, such as transferring plants from long days to short days, promote bud formation from underground storage structures in some herbaceous perennials as compared to continuous growth under long days (Samarakoon et al., 2015). This response is potentially due to short days being favorable for the partitioning of photo-assimilates from aboveground shoots to underground storage organs (Shillo and Halevy, 1981). Therefore, we hypothesized that the photoperiod during the bulking period could have an impact on branching and flowering after the cold treatment.
Shoot emergence, development, and flowering occur in some temperate perennials only when the requirements for cold temperatures have been satisfied (i.e., vernalization and/or breaking of endodormancy) (Iversen and Weiler, 1994). Inadequate accumulation of chilling units can result in delayed or uneven shoot emergence, reduced budbreak, and slow or weak shoot growth (Lang et al., 1987). Therefore, the duration of the cold treatment required for vernalization and/or breaking dormancy can improve shoot formation and flowering in clematis. Because of the lack of information related to the cold requirement for clematis, 5 °C was used to provide the cold treatments during our experiments because this temperature is recommended for initial studies with herbaceous perennials of temperate origins (Dole, 2003).
Following the cold treatment, the growing environment during forcing can impact shoot development and flowering in perennials. Research reports of the photoperiod and temperature in the growing environment for clematis are limited; however, data have been presented for several herbaceous perennials (Armitage and Garner, 1999). For Oenothera fruticosa (Clough et al., 2001) and Anchusa capensis (Armitage and Garner, 1999), time to flower decreased with the increased photoperiod. Reduced flowering time in response to increased temperature during forcing was noted in Lavendula angustifolia ‘Munstead’ (Whitman et al., 1996) and Companula carpatica ‘Blue Clips’ (Whitman et al., 1997). Therefore, identifying the response to the photoperiod and temperature in the forcing environment is essential to reduce the production schedule for clematis while achieving adequate flowering.
The objectives of this project were as follows: to reduce the conventional production schedule for clematis by providing the following: 1) the appropriate photoperiod and plant growth regulator treatments during the bulking period such that just one bulking period was needed to achieve adequate branching during forcing, 2) one dormancy (cold) period, 3) the optimal duration of cold to achieve adequate flowering, and 4) the response to the photoperiod and temperature during the forcing period to achieve adequate flower numbers.
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