Bluing of Hydrangea ‘Endless Summer' Sepals Is Influenced by Timing of Aluminum Sulfate Drenches or Aluminum Chelate Foliar Sprays in Three Different Locations and Production Systems

Blue sepal coloration of  hydrangea requires supplemental aluminum (Al) application in nursery and greenhouse production systems.  Sepal bluing of Hydrangea macrophylla ‘Endless Summer’ was evaluated twice during its production cycle with application of two Al sources (Al₂SO₄ drenches or Al-chelate foliar sprays) at combinations of three times (summer, fall, and/or early spring) in three production systems (peat-based substrate in glass greenhouse, Manhattan, KS; Douglas fir-based substrate in an open retractable-roof greenhouse, Aurora, OR; or pine bark-based substrate in open container nursery, Virginia Beach, VA).  Controls included pre-plant incorporation of Al₂SO₄ and no Al treatment.  Sepal coloration was evaluated with both qualitative ratings and quantified colorimetric readings at one site. Substrate characteristics of pH, EC, and Al were measured before and one week after each application. In the greenhouse study, the use of Al-chelate as a foliar spray did not match blueness resulting from either Al₂SO₄ drenches or pre-plant Al₂SO₄ incorporation, though plants treated with Al-chelate had bluer sepals than the untreated control. As in the greenhouse study, Al-chelate applied to bark-based soilless substrates in Oregon and Virginia produced sepals with less blue color than with the application of Al₂SO₄; however, both incorporation and drench methods did not result in a blue color considered acceptable for sale of “blue” hydrangeas.  Results of substrate chemical analyses varied across production systems based on the amount of leaching that occurred. Further research might evaluate the effectiveness of Al-chelate when applied using foliar-sprenches under high humidity conditions or as a substrate drench.