Into the blue

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					Growing Knowledge




                         Into the blue
                         A comparison of substrate amendments for the adjustment
                         of hydrangea flower color




                                                                                                  oregon state university
                                                                                                                            By Heather Stoven and Jim Owen
                                                                                                                                 New 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
   Growing hydrangeas that bloom the desirable “true blue” color requires the right soil conditions
   and amendments. It is not always easy to achieve.
                                                                                                                            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
                                                                                                                                                                      27
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26   APRIL 2010    ▲   DIGGER
▲   INTO THE BLUE

                                                                            Clay amendment                      No amendment



                                                             0 AISO4
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 clay
     In order to address the issue of
blue hydrangea flowers, a study using
pozzolan clay (diatomaceous cal-                             6 lbs/yd
cined clay containing 10 percent Al by                       AISO4
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                         12 lbs/yd
done with typical grower practices.
                                                             AISO4
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                                             Scientists at Oregon State University found that these conditions
                                                                          of amended soil and aluminum sulfate resulted in different
Douglas-fir-bark-based substrates with                                    hydrangea bloom colors as shown.
different amendments in #2 containers
on June 1, 2007. Nine substrate treat-
ments were used in the experiment,
and each was replicated 10 times.                            5
     The substrates were as follows:
9:1 (by volume) DFB to pozzolan clay
(Western Pozzolan Corp., Doyle, Calif.),                     4
7:3 (by volume) DFB to sphagnum peat
moss , and 3:2 (by volume) coarse DFB
                                              Color Rating




to fine DFB. Three rates of AlSO4 were                       3
                                                                                                                                                           0 AIS04
added to the three substrates: a high rate
of 12.5 pounds per cubic yard; a medi-                                                                                                                     6 AIS04
um rate of 6.3 pounds per cubic yard;                        2
and a control treatment (no AlSO4).                                                                                                                        12 AIS04

     A requisite amount of dolomite
lime was added to each of the treat-                         1
ments to equalize pH across treatments.
Dolomitic lime was incorporated at a
rate of 1.2, 0.90, and 0.60 pounds per                       0
cubic yard for the high, medium, and                                      Clay                        Peat                       None
control AlSO4 treatments, respectively.
                                                                 Figure 1. Effect of substrate and aluminum sulfate (AlSO4) on color rating of
Micromax micronutrient package was
                                                                 hydrangea Endless Summer® flowers

                                     28
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                                                                                                                                        APRIL 2010   ▲   DIGGER   27
     ▲                   INTO THE BLUE




                                                                                                                   Harmony Hill Hydrangeas
                                                                                                                                                        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.
                                Scientists found they could improve hydrangea flower color by adding calcined                                                As AlSO4 rates increased, the flower
                                clay to the growing medium.                                                                                             color was rated as more blue. The same
                                                                                                                                                        effect was also seen when the AlSO4
                                                                                                                                                        rate was 0, the addition of clay pro-
                            5
                                                                                                                                                        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
                            4
                                                                                                                                                        flowers were preferred (Fig. 2).
Customer Appeal Rating




                                                                                                                                                             As AlSO4 rates increased, the flower
                                                                                                                                                        color was rated as more blue and con-
                            3
                                                                                                                                                        sequently appeal ratings also increased.
                                                                                                                                             0 AIS04    The clay amendment treatments had the
                                                                                                                                                        highest color and appeal ratings, with
                            2                                                                                                                6 AIS04
                                                                                                                                                        the clay with high AlSO4 combination
                                                                                                                                             12 AIS04
                                                                                                                                                        rated highest overall.
                                                                                                                                                             The pH varied slightly between
                            1
                                                                                                                                                        6.63 and 7.02 for the different sub-
                                                                                                                                                        strate treatments, and was significant.
                                                                                                                                                        However, we hypothesize the differenc-
                            0
                                                                                                                                                        es in soilless substrate solution pH were
                                              Clay                        Peat                      None
                                                                                                                                                        due to discrepancies in the dolomite
                                          Figure 2. Effect of substrate and aluminum sulfate (AlSO4) on customer
                                                                                                                                                        and aluminum sulfate rates.
                                          appeal rating of hydrangea Endless Summer® flowers

                                                                                                                                                                                             30
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▲    INTO THE BLUE




                                                                                                                           Harmony Hill Hydrangeas
 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          Since the variability of substrate pH    of this would be the possibility of a
     Flowers Without Aluminum          was not great, it likely did not affect       chemical interaction between injected
     Drenches.” Communications in      flower color or growth. The mean              AlSO4 and liquid feed. An easy solution
     Soil Science and Plant Analysis   height (22.6 inches), mean width (35.7        to this problem would be to incorporate
     28:1191-1198.                     inches) and mean growth index (90.8           the granular form of AlSO4, controlled
     • Opena, G.B. and K.A.            cm.) was not significantly different          release fertilizer or both.
     Williams. 2003. “Use of           between treatments, meaning that the               Aluminum availability is directly
     Precharged Zeolite to Provide     substrate amendments and aluminum             responsible for the production of blue
     Aluminum During Blue              sulfate rate did not affect plant growth.     hydrangea flowers. Results demonstrat-
     Hydrangea Production.”                                                          ed that pozzolan clay is one method
     Journal of Plant Nutrition        Conclusions                                   that can be used to produce enhanced
     26:1825-1840.                          The incorporation of pozzolan clay,      blue sepal color in container-produced
     • Stoven, H.M. and J.S.           in addition to AlSO4 in the substrate,        hydrangeas. Nursery growers who
     Owen. 2008. “Comparison           assists in changing of hydrangea sepals       observe poor growth from low substrate
     of Substrate Amendments           to a blue color even at high pH levels        pH or have difficulty achieving blue
     for the Adjustment of             without negatively affecting growth.          hydrangea sepals could successfully use
     Hydrangea (Hydrangea              This will give more leeway in the pro-        an amorphous aluminosilicate
     macrophylla (THUNB.) SER          duction of blue hydrangeas, allowing          clay (approximately 10 percent alumi-
     ‘Bailmer’, Endless Summer®        for the desired flower color without the      num) to amend their substrate, thus
     Flower Color.” SNA Research       complications of maintaining a low pH.        alleviating these issues.
     Conference 53:30-33.                   If reoccurring issues with aluminum
                                       availability and flower color occur, the      Jim Owen is an assistant professor of
                                       evaluation of growing practices other         nursery crops at Oregon State University
                                       than pH management may be war-                and the North Willamette Research and
                                       ranted. Although we found that conven-        Extension Center in Aurora, Ore. He
                                       tional substrate components (i.e. bark,       can be reached at jim.owen@oregon-
                                       peat) do not appear to affect aluminum        state.edu. Heather Stoven is a research
                                       availability, there are other areas of pro-   assistant at the North Willamette
                                       duction that can.                             Research and Extension Center in
                                            Fertilizer programs can interfere        Aurora, Ore., and can be reached at
                                       with aluminum availability; an example        Heather.Stoven@oregonstate.edu.”


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