into the blue - c.ymcdn.com · oregon state university ... rate was 0, the addition of clay...
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▲Growing Knowledge
An ongoing series provided by Oregon State University in partnership with OAN
Into the blueA comparison of substrate amendments for the adjustment of hydrangea flower color
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Growing hydrangeas that bloom the desirable “true blue” color requires the right soil conditions and amendments. It is not always easy to achieve.
By Heather Stoven and Jim OwenNew hydrangea cultivars are being
introduced on a regular basis, many of which have sepals that vary in color based upon the characteristics of the growing media. It is important to under-stand the color changing mechanism in order to facilitate the desired flower color for production and sales.
The availability of aluminum (Al) is what causes a change of color in the sepals from varying shades of pink to blue. However, the availability of this element is dependent upon the substrate pH. More Al is available to the plant at lower pH, resulting in blue flowers, and less Al is available at higher pH, resulting in pink flowers. The Ball Redbook, a popular grower reference manual, suggests pink flower-ing hydrangeas be grown in a pH of 6.5 and blue at 5.5.
Drenches or substrate amendments of aluminum sulfate (AlSO4) are com-monly used in the industry to provide aluminum and to lower pH, producing
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▲ INTO THE BLUE
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blue flowers. However, it is not always easy to achieve the clear blue color that is desirable even with aluminum sul-fate applications and a sufficiently low pH. Also, if the pH gets too low, plant growth and quality can be insufficient.
Testing with pozzolan clayIn order to address the issue of
blue hydrangea flowers, a study using pozzolan clay (diatomaceous cal-cined clay containing 10 percent Al by weight) was established at the North Willamette Research and Extension Center in Aurora, Ore.
Due to the amorphous structure and high Al content of the clay, it was hypothesized that hydrangea flowers could be produced with a truer blue color and at a higher pH than can be done with typical grower practices. Previous studies have been done with zeolite (Opena and Williams, 2003) and kaolinite (Handreck, 1997) clays to pro-duce blue flowers; however, it was only possible at low pH levels.
Hydrangea macrophylla Endless Summer® was potted into three Douglas-fir-bark-based substrates with different amendments in #2 containers on June 1, 2007. Nine substrate treat-ments were used in the experiment, and each was replicated 10 times.
The substrates were as follows: 9:1 (by volume) DFB to pozzolan clay (Western Pozzolan Corp., Doyle, Calif.), 7:3 (by volume) DFB to sphagnum peat moss , and 3:2 (by volume) coarse DFB to fine DFB. Three rates of AlSO4 were added to the three substrates: a high rate of 12.5 pounds per cubic yard; a medi-um rate of 6.3 pounds per cubic yard; and a control treatment (no AlSO4).
A requisite amount of dolomite lime was added to each of the treat-ments to equalize pH across treatments. Dolomitic lime was incorporated at a rate of 1.2, 0.90, and 0.60 pounds per cubic yard for the high, medium, and control AlSO4 treatments, respectively. Micromax micronutrient package was
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Figure 1. Effect of substrate and aluminum sulfate (AlSO4) on color rating of hydrangea Endless Summer® flowers
Scientists at Oregon State University found that these conditions of amended soil and aluminum sulfate resulted in different hydrangea bloom colors as shown.
Clay Peat None
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▲ INTO THE BLUE
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incorporated at 1.5 pounds per cubic yard into the substrate for all treat-ments. Hydrangeas were top-dressed with a medium-high rate of a controlled release fertilizer and were overhead irri-gated following industry standards.
Growth index [(height + mean width)/2] and flower color were record-ed when the plant was determined to be at a saleable stage on May 21, 2008. At this time substrate solution pH was measured via pour-throughs. Flower color and customer appeal were evalu-ated independently with a rating system of 1-5; with 1 being the least blue and 5 being most blue or most appealing.
Substrate and AlSO4 had a signifi-cant effect on ratings for sepal color (Fig. 1). Increasing AlSO4 and pozzolan clay amounts increased the blue color rating of hydrangea sepals. At each AlSO4 concentration, the highest color ratings for all substrates occurred in combination with clay. The addition of the peat amendment did not significant-ly affect flower color compared to the unamended treatment.
As AlSO4 rates increased, the flower color was rated as more blue. The same effect was also seen when the AlSO4 rate was 0, the addition of clay pro-duced blue flowers which were rated higher than the flowers from the peat and none substrates. When the flowers were ranked for customer appeal, blue flowers were preferred (Fig. 2).
As AlSO4 rates increased, the flower color was rated as more blue and con-sequently appeal ratings also increased. The clay amendment treatments had the highest color and appeal ratings, with the clay with high AlSO4 combination rated highest overall.
The pH varied slightly between 6.63 and 7.02 for the different sub-strate treatments, and was significant. However, we hypothesize the differenc-es in soilless substrate solution pH were due to discrepancies in the dolomite and aluminum sulfate rates.
Figure 2. Effect of substrate and aluminum sulfate (AlSO4) on customer appeal rating of hydrangea Endless Summer® flowers
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Scientists found they could improve hydrangea flower color by adding calcined clay to the growing medium.
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▲ INTO THE BLUE
Since the variability of substrate pH was not great, it likely did not affect flower color or growth. The mean height (22.6 inches), mean width (35.7 inches) and mean growth index (90.8 cm.) was not significantly different between treatments, meaning that the substrate amendments and aluminum sulfate rate did not affect plant growth.
ConclusionsThe incorporation of pozzolan clay,
in addition to AlSO4 in the substrate, assists in changing of hydrangea sepals to a blue color even at high pH levels without negatively affecting growth. This will give more leeway in the pro-duction of blue hydrangeas, allowing for the desired flower color without the complications of maintaining a low pH.
If reoccurring issues with aluminum availability and flower color occur, the evaluation of growing practices other than pH management may be war-ranted. Although we found that conven-tional substrate components (i.e. bark, peat) do not appear to affect aluminum availability, there are other areas of pro-duction that can.
Fertilizer programs can interfere with aluminum availability; an example
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Acknowledgements: We would like to thank Marta
Mielcarek, Jackson Kowalski, and Kim Phillips for assistance, Bailey Nurseries, Inc for providing plant material and North Pacific for pro-viding the clay aggregate from Western Pozzolan Corporation.
Additional Reading: • Ball, V. 1991. Ball Red Book:
Greenhouse Growing. Geo.J. Ball Publishing, West Chicago, IL.
• Handrick, K.A. 1997. “Production of Blue Hydrangea Flowers Without Aluminum Drenches.” Communications in Soil Science and Plant Analysis 28:1191-1198.
• Opena, G.B. and K.A. Williams. 2003. “Use of Precharged Zeolite to Provide Aluminum During Blue Hydrangea Production.” Journal of Plant Nutrition 26:1825-1840.
• Stoven, H.M. and J.S. Owen. 2008. “Comparison of Substrate Amendments for the Adjustment of Hydrangea (Hydrangea macrophylla (THUNB.) SER ‘Bailmer’, Endless Summer® Flower Color.” SNA Research Conference 53:30-33.
of this would be the possibility of a chemical interaction between injected AlSO4 and liquid feed. An easy solution to this problem would be to incorporate the granular form of AlSO4, controlled release fertilizer or both.
Aluminum availability is directly responsible for the production of blue hydrangea flowers. Results demonstrat-ed that pozzolan clay is one method that can be used to produce enhanced blue sepal color in container-produced hydrangeas. Nursery growers who observe poor growth from low substrate pH or have difficulty achieving blue hydrangea sepals could successfully use an amorphous aluminosilicate clay (approximately 10 percent alumi-num) to amend their substrate, thus alleviating these issues.
Jim Owen is an assistant professor of nursery crops at Oregon State University and the North Willamette Research and Extension Center in Aurora, Ore. He can be reached at [email protected]. Heather Stoven is a research assistant at the North Willamette Research and Extension Center in Aurora, Ore., and can be reached at [email protected].”
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