Liner Dips for Growth Regulation of Vigorous Herbaceous Perennials

Many vigorous herbaceous perennials need early growth regulation. Treatment at the plug stage could improve early growth control or maintain product size in the plug tray. Concise (uniconazole, Fine Americas, Inc.), is a growth regulator labeled for use on ornamental greenhouse crops in the US. We treated plugs of five herbaceous perennials with a 2-minute dip into 0, 1, 2, 3, 4 or 5 mg/L Concise the day before planting into 1.1-liter pots filled with a peat-lite medium (ornamental grasses were maintained in the plug pots). Plant height was determined at 2 week intervals after treatment (WAT). Concise reduced shoot growth in a linear or quadratic correlation for all crops except Gaillardia × grandiflora ‘Goblin’ which was not affected by the treatments. The ornamental grasses differed in responsiveness with Miscanthus sinensis ‘Gracillimus’ showing little response until 8 WAT while Calamagrostis × acutifolia ‘Karl Foerster’ exhibited a 22% reduction in plant height at 2 WAT with a 1 mg/L Concise dip followed by a 31% height reduction and 26% reduction in shoot dry weight at 12 WAT. A 1 mg/L liner dip of Concise gave significant height reduction of Phlox paniculata ‘David’ and Rudbeckia fulgida var. sullivantii ‘Goldsturm’ through 8 and 10 WAT, respectively. Liner dips are an effective means of growth regulation that can reduce worker contact with the chemical solutions and provide baseline control of vigorous or hard-to-control herbaceous perennial crops. INTRODUCTION Many vigorous herbaceous plants need growth regulation immediately after transplanting. Dipping the root systems of these crops into a growth retardant solution like uniconazole or paclobutrazol has been shown to reduce the subsequent stem elongation of several bedding plant species (Blanchard and Runkle, 2007) and herbaceous perennials (Latimer and Groover, 2005). Details for optimization of the treatment application have been defined: plugs should be “dry”, i.e., ready for irrigation; dipped for 30 s to 2 min; and treated plugs should be dipped and maintained under normal greenhouse conditions (Schnelle et al., 2005; Schnelle and Barrett, 2006). Depth of the solution had little impact on growth regulation and dipped plugs held up to 10 days before transplanting exhibited the same amount of growth control (Schnelle et al., 2005). The following study was conducted to evaluate the liner dip technique on plants that typically show little response to uniconazole applied by foliar spray techniques. MATERIALS AND METHODS Plugs of five herbaceous perennials, Miscanthus sinensis ‘Gracillimus,’ Calamagrostis × acutifolia ‘Karl Foerster,’ Phlox paniculata ‘David,’ Rudbeckia fulgida var. sullivantii ‘Goldsturm’ and Gaillardia × grandiflora ‘Goblin,’ were tested. Concise (uniconazole-P, 0.5 mg a.i./ml, Fine Americas, Inc., Walnut Creek, CA, USA) was applied as a liner dip at 0, 1, 2, 3, 4 or 5 mg/L to “dry” (ready to irrigate) plugs by setting sections of the plug flat into a solution of the PGR at a depth of one-half the height of the plug root ball for 2 min. Plugs were allowed to dry overnight and potted the next day. Proc. X IS on Flower Bulbs and Herbaceous Perennials Eds.: J.E. van den Ende et al. Acta Hort. 886, ISHS 2011 154 Plants were grown in 1.1-L plastic pots filled with a peat-lite medium (Fafard 3B, Conrad Fafard, Inc, Agawam, MA, USA) and irrigated as needed with 200 mg/L N (Peters 20-1020, The Scotts Co., Marysville, OH, USA). The grasses were evaluated as plugs only, in that the plugs were treated and held in their plug pots (6×6×7.5 cm; 0.27 L) for assessment. Each plant species was set up as an individual experiment with plants arranged in a completely randomized design with five single plant replications. Data collected at 2 week intervals after treatment (WAT) included plant height (from rim of pot to top of plant, vegetative or flowers, unless otherwise noted, in cm) and plant width (average of the largest width and the width perpendicular to the largest width, in cm), and presence of flowers (per cent plants flowering). Plant height and width data were subjected to linear and quadratic regression analysis. RESULTS AND DISCUSSION Concise significantly reduced plant height of Miscanthus but the effects were not apparent until 6 WAT by which time the plants had more than doubled in height (data not presented). The linear regression of height over rate was significant at 6 WAT and each measurement thereafter (Fig. 1). Growth of plants treated with the higher rates, 4 and 5 mg/L, was essentially shut down at about 4 WAT and plants did not grow out of the treatment over the remaining measurement period. Shoot dry weight was linearly reduced up to 26% with increasing rates but the r was very low (0.18) (data not presented). The Miscanthus did not flower during the experimental period. Calamagrostis was very responsive to the liner dips with significant reductions in plant height evident at 2 WAT with all rates. The rate response appeared to be saturated at the 1 mg/L rate with a quadratic growth response to increasing rate (Fig. 1). Plant height did not increase once plants flowered at 6 WAT. Shoot dry weight was reduced linearly with increasing rates of Concise (Fig. 2). Flowering was not affected except for a reduction with the 5 mg/L rate (33% plants flowering vs. 67% for control). Phlox paniculata is one of the herbaceous perennials that has shown little response to growth retardants (Latimer et al., 2003). However, Phlox ‘David’ was responsive to a 2 mg/L or higher liner dip with both height and width reductions at and after 4 WAT (Fig. 3). The response appeared to be saturated at 2 mg/L Concise. Although the statistical response of height to rate was linear, little additional reduction in height occurred with rates above 2 mg/L Concise. Therefore, I would recommend starting with a lower liner dip rate such as 1.25 or 1.5 mg/L for production use. Flowering time and flower height were not affected by Concise even at the high treatment rates. In previous tests, Rudbeckia was responsive to uniconazole sprays (Latimer et al., 2003) and Rudbeckia triloba was responsive to liner dips of paclobutrazol (Latimer and Groover, 2005). In this test, Rudbeckia ‘Goldsturm’ was very responsive to the Concise liner dip treatments. Plant height was reduced with increasing rates of the Concise liner dip by 4 WAT (Fig. 3). Plant width responded in a similar manner (data not presented). Growth responses were quadratic in most cases suggesting that the response was saturated at the higher rates. Concise at the 1 mg/L dip rate gave significant control of plant height without excessive reductions in plant width, and flower height was not adversely affected. Higher rates, especially 4 and 5 mg/L, caused excessive reductions in flower stalk elongation which in some plants resulted in flowers opening below the foliage height. With respect to flowering, there was no consistent rate response to Concise. The 1 mg/L liner dip rate should work well in production greenhouses. In previous tests, Gaillardia ‘Goblin’ was unresponsive to uniconazole spray applications. It is also unresponsive to the liner dip even at the 5 mg/L Concise rate. Flowering also was not affected by liner dip rate. In summary, we found that the grasses, Miscanthus and Calamagrostis were more responsive to the Concise liner dips than to spray applications in previous tests. This gives growers a viable option for maintaining height control, especially during the plug stage where plant height is a greater issue with shipping. Sensitivity to Concise did vary with