Kay Easton wrote:
In article , Jason Pope
writes

Victoria Clare wrote:


I piled it all in a heap under the hydrangea, meaning to do something about
it later. This produced a strange effect: the side of the hydrangea next
to the little pile of rusty things is now a vibrant blue, but scales
through purple to pink on the other side!

I've now distributed the items more evenly around the plant, and hope it
will all be blue soon. They are hidden under a layer of mulch.

Victoria

The rust from the nails alters the soil pH which causes the effect you
mention, it is grounded in good science!



Can you elaborate on that? I thought rust was iron oxide, which I
thought was alkaline, but hydrangeas go pink, not blue, in alkaline
soils.

Are you sure it's a change of pH that's having the effect, rather than
greater availability of iron?


From a website:
http://www.nobleplants.com/articles/...20culture.html

Blue or pink colors are predicated on the amount of aluminum in the soil
solution that can be absorbed by the roots. Although pH is often listed
as the agent of color change, it is actually an instigator of (a
precursor to) the process. If soils are acid, aluminum is available; if
more alkaline then aluminum is tied up in insoluble forms and not
readily available for uptake. So the true story is that high acidity,
i.e. low pH, solubilizes (or makes available) aluminum; the reverse at
low acidity (high alkalinity), high pH. Excess phosphorous in the soil
will also tie up the aluminum in insoluble precipitates, even in acid
soils. Hydrangea macrophylla grown in pine bark medium, pH 5 to 6, are
typically pink. Why? The acidity is high, but almost no aluminum is
present in the substrate (bark). Soil is composed of minerals,
typically aluminum, silicon, iron, etc., and therein resides the
difference. So how do growers produce blue hydrangeas in pine bark?
Aluminum sulfate is added to the surface of the container at a
prescribed rate, usually 0.75 to 1.5 ounces evenly distributed on the
surface of the 3-gallon container medium. Greenhouse growers also apply
it as a drench at the rate of 2.4 ounces per gallon solution with 8
ounces applied as a drench per 6 inch container. Greenhouse treatments
start at budbreak and continue every 2 weeks for 3 additional
applications. Growers have variable timetables for application but in
our work as soon as flower buds are visible, a single application is
made. Water thoroughly after application to insure solubilization of
the aluminum and movement into the root zone. Too much is worse than
too little and I have dwarfed and killed plants with excessive applications.

The crux of the color change is that aluminum ions complex with
the pigment, delphinidin-3-monoglucoside, in the sepals to produce the
blue coloration. A positive correlation between intensity of blue and
aluminum foliar concentration has been demonstrated. Growers try to
develop antique shades, somewhere between the rich blues and pinks by
combining lime in the medium and aluminum applications (see photos
below). From my experiences, any pink-sepaled form, perhaps not
‘Preziosa’, can be blued and vice-versa. Although various cultivars are
often listed as pink or blue, I don’t totally believe it. Lacecaps and
mopheads respond in a similar fashion. The white cultivars typically
are not affected and maintain their color.