Flurprimidol and paclobutrazol drench activity not affected in commercial wood fiber substrates

Commercially available substrates formulated with fibrous wood materials are becoming more widely utilized among greenhouse growers in the United States. Substrate drench applications of plant growth retardants (PGR) such as flurprimidol or paclobutrazol are commonly used to control growth of bedding plants, though substrate composition can affect PGR drench efficacy. Our objective was to determine if flurprimidol and paclobutrazol drench efficacy is affected by two commercially available wood fiber substrates (WFS). Flurprimidol drench applications of 0.5, 1, 2, and 4 mg active ingredient (a.i.) pot-1, paclobutrazol drench applications of 1, 2, 4, and 8 mg a.i. pot-1, or clear tap water (untreated control) drench application of 0 mg a.i. pot-1 were applied to geranium (Pelargonium ×hortorum L.H. Bailey 'Bullseye Red') and petunia (Petunia ×hybrida Vilm. 'Easy Wave Pink Passion' and 'Easy Wave Violet') grown in containers (12.7-cm; 770-mL) filled with either a standard (STD) or high porosity (HP) commercially available WFS. Substrates were composed of wood-fiber and peat moss. Flurprimidol and paclobutrazol drench efficacy was unaffected by substrate composition. For instance, height of geraniums grown in the STD WFS was 69 and 62% less as flurprimidol and paclobutrazol drench applications increased from 0 to 4 and 0 to 8 mg a.i. pot-1, respectively. Similarly, height of geraniums grown in the HP WFS was reduced by 48 and 62% as flurprimidol and paclobutrazol drench applications increased from 0 to 4 and 0 to 8 mg a.i. pot-1, respectively. For both petunia cultivars, a similar trend was observed where plant width and dry mass were smaller as flurprimidol and paclobutrazol drench concentration increased regardless of substrate composition. This research concludes that flurprimidol and paclobutrazol drench activity was not affected by the two commercially available WFS trialed. Furthermore, growers can drench STD or HP WFS with 0.5 to 1 or 1 to 2 mg a.i. pot-1 of flurprimidol or paclobutrazol, respectively, to control growth of geranium and petunia.