2018 ASHS Annual Conference

Hydrangeas (Hydrangea macrophylla) contain an anthocyanin pigment within the sepal that naturally produces a pink or red color. In the presence of aluminum (Al) the pigment will bind with the Al producing a blue or purple color. The current method for producing blue hydrangeas is to apply 3-4 weekly drench applications of aluminum sulfate (Al₂(SO₄)₃). Growers lack a way to verify if sufficient Al applications were made. The purpose of this study was to determine if early season Al leaf tissue values could be used to predict blue sepal color. We treated ‘Early Blue’ hydrangeas with 0, 6, 8, 9, 10, 12, and 15g of Al₂(SO₄)₃ per pot applied as a drench beginning two weeks after transplanting. The 6, 8, and 10g rates were split over 2 applications and the 9, 12, and 15g were split over 3 applications. We measured Al levels in the leaf tissue and substrate leachate by ICP analysis beginning one week after final drench application and continued weekly until bloom. Sepal colors were evaluated by visual analysis and measured using a handheld colorimeter. The plants receiving 0, 6, 8, and 9g Al₂(SO₄)₃ produced the least blue sepals ranging in color from blueish pink to purple. Plants receiving 12 and 15g Al₂(SO₄)₃ produced the most blue sepals. Plants treated with 15g of Al₂(SO₄)₃ were stunted in growth and displayed leaf scorch symptoms. Leaf symptoms were likely due to stunted root systems caused by higher Al application rates. The Al leaf tissue values were consistently higher in the 12g and 15g treated which produced the bluest sepals. At week 6 the plants treated with 12g (4g Al₂(SO₄)₃ at 3 applications) had Al tissue values of 1724 ppm which was significantly higher than the plants treated with 8g (4g Al₂(SO₄)₃ at 2 applications) which had 867 ppm. At week 7 the plants treated with 15g (5g Al₂(SO₄)₃ at 3 applications) had Al tissue values of 2152 ppm which was significantly higher than the plants treated with 10g (5g Al₂(SO₄)₃ at 2 applications) which had 1454 ppm Al. Based on these findings growers producing ‘Early Blue’ hydrangeas should target leaf tissue values between 1300-2000 ppm Al by weeks 6 to 8 of forcing in order to produce blue hydrangeas.