Effects of temperature, drought, and gibberellin A4/7, and timing of treatment, on flowering in potted Piceaengelmannii and Piceaglauca grafts

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Abstract
Four experiments conducted over 2 years on potted 4- to 7-year-old Picea engelmanni and 3-year-old Picea glauca grafts compared the effectiveness of different temperature, drought, and gibberellin A4/7 treatment regimes for promotion of flowering. With the exception of one study in which trees were not properly preconditioned, heat treatment within a polyethylene-covered house (polyhouse) promoted profuse female and male flowering in previously root-pruned P. engelmanni grafts. The optimal daytime temperature of 22-25.degree. C in the polyhouse was well below the 30.degree. C temperature for Picea abies suggested in the literature. Timing of heat treatment was critical. Few trees produced seed or pollen cones if moved indoors before the new shoots were at least 80% elongated, with 85-95% elongation being optimal. Drought also promoted flowering but only if applied outdoors while shoots were actively elongating. Response to these cultural practices was further enhanced by spray applications of gibberellin A4/7 during the rapid growth phase. Younger P. glauca grafts that were not root-pruned or sprayed with gibberellin A4/7 failed to respond to early drought and late heat treatments, but did so the following year when these adjunct treatments were given. Response of P. engelmanni grafts to retreatment indicates that alternate-year reproductive, is practical and will result in sustained abundant cone production in potted trees.