Low Irradiance During Rooting Improves Propagation of Oak and Maple Taxa

Rooting trials were conducted to determine if the duration of exposure to low levels of solar irradiance during root initiation influenced rooting and survival of semihardwood cuttings of six tree species. Cuttings of oak (Quercus imbricaria, Q. nigra, Q. palustris, Q. rubra) and maple (Acer rubrum ‘Bowhall’, and A. truncatum) were treated with IBA and placed in a fog-humidified polyethylene tent rooting chamber for 119 or 117 days (A. rubrum and Q. nigra) under either 82% (control) or 93% (low irradiance) shading of ambient outdoor solar radiation. Cuttings were exposed to either 0, 10, 20, 40, or 119 days of 7% irradiance followed by placement under control conditions (18% irradiance) for the remainder of the treatment. Percentage rooting, the number of roots per cutting, and the number of days to root were significantly influenced by exposure to 7% irradiance during rooting, but responses were dependent on taxa. For most taxa, the highest rooting percentages required at least 10 days of exposure to 7% irradiance. Overwinter survival of rooted cuttings averaged 58.8% and was not affected by treatment. Results of this study demonstrate that low irradiance in the rooting environment can increase the rooting of cuttings of oak and maple taxa.