Foliar Plant Growth Retardant Applications Suppress Height of Seed-propagated New Guinea Impatiens

Flats of bedding impatiens (Impatiens walleriana) are one of the most popular annual bedding plants. However, recent outbreaks of impatiens downy mildew (Plasmopara obducens) are causing producers and consumers to reconsider growing and planting bedding impatiens, respectively. Seed-propagated New Guinea impatiens (Impatiens hawkeri) are a newer crop that may be able to grown and sold in large packs (i.e. 1801) within a flat. However, producing flowering New Guinea impatiens in flats with an acceptable height is a challenge. Our objectives were to quantify the efficacy of foliar plant growth retardant applications on plant height and time to flower of seed-propagated New Guinea impatiens. ‘Divine Cherry Red’, ‘Divine Scarlet Bronze Leaf’, and ‘Divine White Blush’ New Guinea impatiens seedlings were planted in 1801 cell packs. Seven days after planting, deionized water or solutions containing ancymidol (15, 30, or 60 ppm), chlormequat chloride (750, 1500, or 3000 ppm), daminozide (1250, 2500, or 5000 ppm), ethephon (250, 500, or 1000 ppm), flurprimidol (10, 20, or 40 ppm), paclobutrazol (10, 20, or 40 ppm), or uniconazole (5, 10, or 20 ppm) were applied to seedlings. Plant height was measured when plants were treated and again when the first flower opened and time to flower was calculated as the time from transplanting seedlings to the first open flower. Ancymidol, chlormequat chloride, daminozide had little to no impact on stem elongation. Higher concentrations of ethephon resulted in some suppression of stem elongation; however flowering was also delayed.  500 ppm ethephon solutions increased time to flower of ‘Divine Cherry Red’ and ‘Divine Scarlet Bronze Leaf’ by 19 and 18 d, respectively, while 1000 ppm ethephon delayed flowering of ‘Divine Scarlet Bronze Leaf’ by 23 d. Flurprimidol, paclobutrazol, and uniconazole suppressed height at flowering of all three cultivars. For example, final height of ‘Divine Cherry Red’ ‘Divine Scarlet Bronze Leaf’, and ‘Divine White Blush’ treated with 20 ppm flurprimidol was 3.2, 3.5, and 5.0 cm shorter, respectively, than untreated plants. Foliar sprays containing 5 ppm uniconazole resulted in plants that were 4.1 cm (‘Divine Scarlet Bronze Leaf’) to 5.4 cm (‘Divine White Blush’) shorter than untreated plants. The highest concentrations of flurprimidol, paclobutrazol, and uniconazole resulted in excessive suppression of stem elongation and a delay in flowering. Foliar sprays containing flurprimidol, paclobutrazol, or uniconazole are useful to control stem elongation of seed-propagated New Guinea impatiens for production in flats.