Effects of Plant Growth Retardants and Application Timing after Shearing on Growth and Flowering of Coreopsis verticillata `Moonbeam'

Coreopsis verticillata `Moonbeam' is a herbaceous perennial that may grow too tall in small containers under greenhouse conditions for market acceptance, and therefore may benefit from plant growth retardants. Rooted terminal cuttings produced under short days were given a terminal pinch and transplanted to 10 cm pots. Cuttings were sheared to 6 cm above the pot rim 4 weeks later. Growth retardant treatments consisting of ancymidol drench at 0, 0.125, 0.25, or 0.375 mg a.i./pot; paclobutrazol drench at 0, 0.125, 0.25, or 0.375 mg a.i./pot; daminozide spray at 0, 2550, 5100, or 7650 mg·L–1; paclobutrazol spray at 0, 12, 24, 36, 48, or 60 mg·L–1; or flurprimidol spray at 0, 25, 50, 75, 100, 150, or 200 mg·L–1 were applied 10 days after shearing. Night-breaking lighting using incandescent bulbs was started the same day. The highest rate of ancymidol, paclobutrazol drench, daminozide, and flurprimidol decreased plant height compared to controls by 36, 30, 21, and 36%, respectively. Paclobutrazol sprays were not effective. A market quality rating of four or higher (good, salable) was given to plants treated with daminozide at 5100 or 7650 mg·L–1 or flurprimidol at 150 or 200 mg·L–1. A second experiment was conduced to determine application timing. A daminozide spray at 0, 2550, 5100, or 7650 mg·L–1 was applied 0, 3, 6, 9, 12, or 15 days after shearing. Plant height, growth index and lateral shoot length were least and market quality rating highest when 5100 or 7650 mg·L–1 of daminozide was applied between 6 and 9 days after shearing.