The Use of Topflor G in Knockout Rose Container Production

Knock Out rose is a popular landscape shrub rose and a major nursery crop in the southeastern United States.  It needs several pruning during production to have dense foliage and abundant flower buds.  Growers are interested in using plant growth regulators to reduce pruning, improve plant quality, or manage crop scheduling.  Topflor G (flurprimidol) is the first granular "Type II" growth retardant that has shown size reduction effects in some crops.  Experiments were conducted in 2012 on Knock Out roses at a local nursery in Louisiana.  Four groups of plants: 1) Knock Out Red transplanted from 4-inch liners; 2) Double Knock Out Red transplanted from 4-inch liners; or 3) from quart liners; and 4) Double Knock Out Pink transplanted from 4-inch liners were potted into 3-gallon pots and treated with Topflor G at 0, 7, 14, and 28 g/pot at four weeks after potting.  Growth and number of flowers were recorded at 2, 4, and 6 WAT.  Significant interactions were found between group and sample date, but not between group and PGR rate.  Analyses for each group indicated that, PGR treatment effects were not significant for Knock Out Red.  For Double Knock Out Red, all rates reduced plant height compared with the untreated, but had no effects on plant width.  Percentage of growth reduction was smaller for the group of Double Red transplanted from quart liners than those from 4-inch liners.  Results suggest that vigorously grown variety such as Double Red responses better than weaker varieties, and effects are more significant when plants were young at the time of treatment.  However, large variation in treatment effects were observed within individual rates; and the participating nursery manager expressed concerns about this inconsistency.  Further research is needed to identify key factors affecting plant response and application methods to improve uniformity.