Bedding impatiens (Impatiens walleriana) are one of the most popular annual bedding plants. They are used for their ability to thrive under a wide variety of site conditions, including shade (Armitage, 2001; Still, 1993). Beginning in 2011, impatiens downy mildew (Plasmopara obducens) has been identified in landscapes across North America (Caitlin, 2013). As symptoms of impatiens downy mildew develop in bedding impatiens, leaves and flowers senesce prematurely and, therefore, reduce landscape appeal (Warfield, 2012). The increased range and incidence of impatiens downy mildew has caused landscapers and consumers to select alternative species to use where bedding impatiens were previously planted (Caitlin, 2012). New guinea impatiens are tropical annuals that grow well and flower in shade with a wide range of flower and foliage colors (Armitage, 2001; Still, 1993; Whealy, 1995). Furthermore, they are resistant to impatiens downy mildew (Caitlin, 2013). Together, these attributes give new guinea impatiens the potential to replace bedding impatiens in the landscape.
New guinea impatiens are commonly propagated vegetatively using stem tip cuttings and finished in containers with a higher retail value, such as 4- or 5-inch-diameter containers or hanging baskets (Dole and Wilkins, 2005; Nau, 2011). Producing flats of vegetatively propagated new guinea impatiens would not be feasible due to the size of the starting material relative to planting density in a flat and the cost of production. The modest price per plant that contributes to the consumer appeal of bedding impatiens in flats makes production of flats with vegetatively propagated new guinea impatiens uneconomical. The development of F1 hybrid seed-propagated new guinea impatiens provides an opportunity for lower cost propagules that could be produced economically in flats. However, one challenge with producing plants in flats is controlling growth to produce plants that are attractive and appropriately sized for the container.
Vegetatively propagated new guinea impatiens do not routinely require plant growth retardants (PGRs) in production (Dole and Wilkins, 2005). One reason is that they are usually grown in containers that are 4 inches or greater in diameter, and plants in these containers generally require less growth regulation to produce an appropriately sized plant compared with plants produced in smaller containers such as packs and flats. Plant growth can be regulated and influenced in vegetatively propagated new guinea impatiens by irrigation and mineral nutrition levels (Haver and Schuch, 1996). Although modifying crop culture may result in an acceptable level of growth control for vegetatively propagated new guinea impatiens grown in larger containers, it may not provide enough control to suppress new guinea impatiens height produced in flats.
While PGRs are useful in suppressing height of containerized greenhouse flowering crops, we have found no published recommendations for applying PGRs to control growth of seed-propagated new guinea impatiens. The objective of this research was to quantify the efficacy of foliar applications of PGRs for production of seed-propagated new guinea impatiens in flats. This was accomplished in a series of two experiments. In the first experiment several cultivars were treated with a wide range of active ingredients and concentrations. In the second experiment, a broader range of concentrations of the most effective active ingredients from the first experiment were used to model the responses and generate recommendations.
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