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Because
it's the soil chemistry that determines the bloom color, the
variety names given to these types of hydrangeas means very little
when it comes to bloom color. For instance, ‘Nikko Blue', ‘Pretty
in Pink', ‘Forever Pink' and ‘Blue Deckle', all have an almost
an equal chance of blooming pink or blue, depending on the soil
they are planted in.
So
remember that even if you purchase a hydrangea in bloom, you
cannot be sure the plant will produce the same color flowers
once it's growing in your garden.
To
manipulate the color of a hydrangea’s blooms, you need
to manipulate your soil’s pH level and mineral content.
This is not something you do just once. In order to maintain
growing conditions that result in a specific bloom color, you
may need to apply special soil amendments several times during
the growing season (see instructions at right).
To
really experiment with the color of a hydrangea's blooms, consider
growing the plant in a large pot. Because you will be working
with a much smaller amount of soil, it will be much easier to
maintain the desired soil chemistry.
Start
by testing the pH of your soil. This will give you an idea how
much of an uphill battle you’ll be waging. Be mindful that
the health of the plant should be your first priority. It’s
quite difficult to make a dramatic change in soil pH, and doing
so can adversely affect the overall health of the plant.
To
encourage blue hydrangea flowers, grow the plant in soil that
has a pH of 5.2-5.5. If your soil is more alkaline than this,
you can lower the pH by watering around the plant with aluminum
sulfate (2 Tbsp per gallon of water), taking care to ensure the
soil is moist before application. Soil pH can also be lowered
(more gradually) by applying an acidic organic mulch, such as
pine needles or pine bark.
If
the pH of your soil is naturally quite high (alkaline) it will
be very difficult to get blue flowers--even if there’s
plenty of aluminum in the soil. Alkaline soil tends to "lock
up" the aluminum, making it unavailable to the plant. (However,
you can grow fabulous pink hydrangeas!)
If
you prefer pink blooms, your hydrangea should be deprived of
aluminum by growing it in an alkaline soil with a pH of 6.0-6.2.
You can apply a high phosphorus fertilizer to further discourage
the uptake of aluminum. To raise the pH of a naturally acidic
soil, apply ground, dolomitic limestone several times a year.
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Adjusting the pH of Your Soil
Start by testing the pH level of the soil where you intend
to plant your hydrangea. You can use a home pH kit, pH meter, or submit a soil
sample to your local extension service. If you are trying to maintain a certain
pH level, you should test your soil each year. The effect of adding
materials to raise or lower the pH may not be immediately apparent. You should
also expect that over time, the pH will revert to its original level, which
is dictated by the native soil conditions.
To Raise Soil pH
The most common way to raise the pH of your soil (make
it more alkaline and less acidic) is to add powdered
limestone. Dolomitic limestone will also add manganese
to the soil. Apply it in the fall as it takes several
months for the effects to be noticeable.
Wood ash will also raise soil
pH. It works more quickly than
limestone and adds potassium
and trace elements to the soil.
But use caution when applying
wood ash. It is very concentrated,
and applying too much of it can
drastically alter the pH and
cause nutrient imbalances. Wood
ash can also “burn” foliage,
so for best results, apply it
directly to the soil in the winter.
Apply no more than 2 pounds per
100 square feet, every two to
three years, and test your soil
each year to monitor the effects.
To raise the pH of your soil
by about one point:
In sandy soil: add 3 to 4 pounds of ground limestone per
100 square feet.
In loam (good garden soil): add 7 to 8 pounds per 100 square
feet.
In heavy clay: add 8 to 10 pounds per 100 square feet.
To Lower Soil pH
Soil can be made more acidic by adding ammonium sulfate
or aluminum sulfate. These materials are usually easier
to find than elemental sulfur. Follow application rates
on the packaging. You can also lower pH levels by incorporating
naturally acidic organic materials such as conifer needles,
sawdust, peat moss and oak leaves. Coffee grounds are
also slightly acidic. Remember to retest your soil to
monitor effects over time.
To lower soil pH by about
one point using elemental sulfur:
In sandy soil: add 1 pound ground sulfur per 100 square
feet.
In loam (good garden soil): add 1.5 to 2 pounds per 100
square feet.
In heavy clay: add 2 pounds per 100 square feet.
More Info...
Can't get enough of hydrangeas?
Visit Hydrangeas,Hydrangeas!,
the web site of enthusiast Judith King.
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