Promotion of cone production on field-grown eastern white pine grafts by gibberellin application

Field-grown 4- and 9-year-old grafts of eastern white pine (Pinus strobus L.) were sprayed or injected with gibberellin A 4 7 . Treatments in May–June significantly increased pollen-cone production. Spraying produced more pollen-cone clusters per tree. Injection increased the number of clusters per branch and the number of whorls per tree producing pollen cones. Injection, but not spraying, in May–June produced more seed cones. However, spraying in August–September did not promote either pollen-cone or seed-cone production. Injection in August–September decreased the number of pollen cones per cluster and inhibited seed-cone production. Present resutls indicate that treatments for pollen- and seed-cone production are best carried out in May and June during the period of rapid shoot elongation.