We studied flower stimulation in two young miniaturized seed orchards of Douglas-fir (Pseudotsuga menziesii (Mirb.) Franco var. menziesii) in Oregon. In experiment 1, female and male flowering were substantially enhanced when the trees were treated 2–4 years after grafting with stem girdling plus stem-injected gibberellin A4/7 (GA 0.25× rate = ProCone(tm) at 0.084 µL·mm–2 scion cross-sectional area). Comparable results were obtained the following year when the same trees were retreated with 1× GA. In experiment 2, female and male flowering were significantly enhanced when 3-year-old trees were treated with girdling plus either 1× GA, 1.5× GA, or 2× GA. Some treatments had higher mortality and less height growth than the control in the year of cone development. We recommend using a combination of girdling and 1× GA biennially once trees are large enough to produce large per-hectare seed yields and withstand the stress of flower stimulation. At the study orchards, this seems to be about 5 years postgrafting, just before the sixth growing season. Yields were estimated to be 272 963 seeds·ha–1 at age 4 years, or 143 095 seeds·ha–1 annually with stimulation occurring every 2 years. Yields should increase as orchards age, with full stocking, and with higher planting densities.

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We studied miniaturized seed orchards (MSOs) and conventional orchards of coastal Douglas-fir (Pseudotsuga menziesii var. menziesii) in a series of experiments conducted at three locations in Oregon and Washington between 2002 and 2012. In Experiment 1, cone yields were greater using stem girdles plus stem injections of GA 4/7 compared to girdling alone, root pruning, and girdling plus fertilization with Ca(NO 3 ) 2 . In Experiment 2, topping and pruning in the summer following flower stimulation minimized crown volume and maximized female and male flower densities. In Experiment 3, the widest spaced orchard (4 9 6 m) generally produced more flowers and cones per tree, and greater flower and cone densities compared to the narrow spaced orchards (1 9 3 and 2 9 4 m). We conclude that narrow spaced MSOs may produce more cones per hectare in the early years after establishment, but wider spaced orchards will be more productive a few years later. In general, per-hectare cone yields were similar or greater in the MSOs compared to nearby conventionally spaced orchards. Trees grafted using scions collected from juvenile trees (age 6) generally had larger crowns, more female flowers and cones per tree, and greater female flower and cone densities compared to scions collected from middle-aged (age 29) or mature (56-to 99-year-old) trees. We found no evidence that clonal rows resulted in reduced seed quality relative to nearby conventional orchards. Implications for Douglas-fir seed orchard management are discussed.

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