Number of days from seeding until anthesis, total plant height, inflorescence diameter, and days until anthesis were significant for the PGR treatment and seasonal interaction. Plant growth was greater during summer than spring. Marketable sized plants grown in 15- to 16.5-cm-diameter pots were produced with paclobutrazol drench concentrations of 2 and 4 mg in both seasons, and 8 mg was also effective during the summer. Paclobutrazol drench applications of 0, 2, and 4 mg a.i./pot were applied to 'Pacino' potted sunflowers and 'Red Pigmy' tuberous rooted dahlias grown in substrates containing 50, 60, 70, or 80% (by vol.) Sphagnum peat or coir, with the remainder being perlite, to study the efficacy of paclobutrazol. Potted sunflower plant height significantly differed for peat and coirbased substrates, plant diameter was significantly influenced by the percentage of peat or coir in the substrates and paclobutrazol concentration, and inflorescence diameter was also significantly influenced by paclobutrazol concentration.Plant growth retardant (PGR) foliar sprays (mgà ·L⁻à ¹) of daminozide at concentrations from 1,000 to 16,000; paclobutrazol from 5 to 80; and uniconazole from 2 to 32 were applied to 'Pacino' pot sunflowers (Helianthus annuus L.) to compare their effectiveness at chemical height control. Total plant height, plant diameter, inflorescence diameter, and days until flowering were significant for the PGR treatment interaction. Marketable-sized plants grown in the 1.2-L pots were produced with uniconazole concentrations between 16 and 32 mgà ·L⁻à ¹ or with daminozide concentrations between 4000 and 8000 mgà ·L⁻à ¹. Paclobutrazol foliar sprays up to 80 mgà ·L⁻à ¹ had little effect. Drenches of PGR paclobutrozol were applied at 2, 4, 8, 16, or 32 mg active ingredient (a.i.)/pot, plus an untreated control to potted sunflowers to determine its effect on growth.Though there was a difference in sunflower plant height between the peat and coir-based substrates, with greater plant growth being observed in coir-based substrates, the percent height control from the untreated plants caused by paclobutrazol was similar for both peat and coir-based substrates. This suggests a similar degree of paclobutrazol activity in both coir and peat-based substrates. Quality plants with longer post-harvest life were produced with N at 100 mgà ·L⁻à ¹ and by terminating fertilization 55 days after potting.