Three 2-year field studies were conducted to evaluate 1-naphthaleneacetic acid (NAA) as a suppressant of suckers in European hazelnut (Corylus avellana L.). Treatments were basal-directed applications of NAA at 5, 10, and 20 g·L−1 a.i. applied once per season, and two sequential applications of NAA 10 g·L−1 a.i., 28 days apart, compared with 2,4-D (3.8 g·L−1 acid equivalent), and a nontreated control. Treatments were applied early in spring and repeated the following year. Both NAA and 2,4-D delayed sucker growth by 1.2- to 3.0-fold compared with the nontreated control, and response varied with experimental site and year. Sequential treatments of NAA significantly reduced sucker height and fresh weight 120 days after treatment. NAA applied in sequential treatments increased tree trunk cross-sectional area and canopy volume in two of the three experimental sites. Yield of hazelnuts increased when suckers were removed with NAA or 2,4-D compared with nontreated. Results indicate that NAA is an effective option to control suckers in hazelnuts and can help reduce herbicide and labor in sucker control.