Four concentrations of paclobutrazol (PBZ) and two application methods (foliar spray–soil drench) were tested on 3-month-old container-grown cork oak (Quercus suber L.) seedlings. Shoot height was affected by PBZ concentration and application technique. Time course of height was modelled by nonlinear mixed model analysis. Reduced shoot height was the result of shortened and fewer internodes. A 25 to 625 ppm soil drench or 625 to 15,000 ppm foliar spray controlled over vegetative growth, resulting in a reduction between 13% and 90% in plant size relative to untreated controls. Seedlings responded to soil drench at a lower PBZ concentration than for a foliar spray, but the danger of irreversible overdosing seems greater for a soil drench. Soil drench—and foliar spray to a lesser extent—also affected other shoot and root growth traits. PBZ dose decreased root volume and dry weights in a quadratic fashion with increasing dose, but decreased diameter and number of leaves linearly.