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
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26 APRIL 2010 ▲ DIGGER
▲ INTO THE BLUE
Clay amendment No amendment
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.
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
to fine DFB. Three rates of AlSO4 were 3
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
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
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-
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).
Customer Appeal Rating
As AlSO4 rates increased, the flower
color was rated as more blue and con-
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
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
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
28 APRIL 2010 ▲ DIGGER
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▲ INTO THE BLUE
Harmony Hill Hydrangeas
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.
• 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.”
30 APRIL 2010 ▲ DIGGER