HYDRANGEA
“W
hy doesn’t my hydrangea bloom?”
“Why did my blue hydrangea flowers turn pink?”
These questions are common Hydrangea problems. Hydrangeas may fail to thrive or bloom for the following reasons:
too much or too little sun
incorrect pH
excessive fertilization
improper pruning
improper moisture
improper soil
lack of winter protection on certain varieties
Sunlight vs. shade
It is a common misconception that hydrangeas will thrive in full
s
hade with no direct sun. Although they can be successfully
grown in full shade, they will not bloom well. Depending on the
hydrangea species, some require full sun with adequate soil
moisture, while others will benefit if given shade in the
afternoon when the sun is the strongest.
pH
While most hydrangeas grow better in soil that is lower in pH
(acidic), they will tolerate higher pH soils (alkaline) quite well.
The most common reason for reducing a hydrangea’s soil pH
level is if blue rather than pink flowers are desired. Hy
drangea
flowers will be pink in alkaline soil and blue in acidic soil. Pink
hydrangea flowers can be changed to blue by acidifying soil and
maintaining the pH in the 5.0 to 5.5 range. Adjusting a soil’s pH
will need to be done annually in order to achieve and maintain
proper results. The results from a soil pH test kit will determine
existing pH levels and the need to add lime or sulfur in order to
adjust the soil accordingly. Tap water may alter test results;
therefore, it is important to use deionized water with a soil
testing kit. Sulfur should be added in order to lower a soil’s pH.
Lime should be added in order to raise a soil’s pH. Elemental
sulfur, also called flowers of sulfur, or iron sulfate are better
alternatives to using aluminum sulfate because aluminum can
reach toxic levels in the soil. While aluminum is a key element to
lower a soil’s acidity and change the color of hydrangea flowers,
it can also become toxic at high levels.
Adjusting your soil’s pH to change bloom color is only
applicable for colored hydrangeas. White hydrangeas will never
turn pink or blue; therefore, it is usually not necessary to add
supplemental sulfur to soil where white hydrangeas grow, unless
unusually high alkalinity causes growing problems. Soils in the
Chicago area are normally very high in pH, so adding lime,
which raises a soil’s alkalinity, is rarely needed.
Fertilization
Fertilizing hydrangeas is generally not recommended unless a
nutrient deficiency is observed. Too much nitrogen can produce
excessive green growth at the expense of flowers.
Pruning
Hydrangeas fall into one of two pruning categories. The pruning
c
ategory is determined by whether or not the hydrangea blooms
on old wood or new wood. Old wood is growth produced in a
previous growing season. Hydrangeas that bloom on old wood
should only be pruned immediately after they bloom and
pruning should be kept to a minimum. If this type of hydrangea
is pruned in the fall or winter, it will not bloom in the spring or
summer, and will flower in the following growing season. New
wood is growth produced in the current growing season.
Hydrangeas that belong in this category can be left unpruned
and only blackened winter-injured tips removed in the spring.
Certain hydrangea species can be pruned almost to the ground in
late winter/early spring and will produce flowers on new
branches that grow in the spring. Dead or diseased branches can
be pruned out at any time of the year.
Moisture
Hydrangeas thrive in well-drained soil that is consistently moist
b
ut not soggy. It is not unusual to see some hydrangea varieties
droop and wilt in the hottest months of the summer. A two- to
three-inch layer of mulch will help to conserve moisture.
Soil
Hydrangeas are best planted in soil that has been amended with
a
dequate amounts of organic matter, such as compost, leaf mold,
or well-composted manure.
2
Cold damage
Most hydrangea varieties grow well in the Chicago area and are
r
eliably winter hardy. However, some species such as H.
macrophylla suffer from considerable branch dieback during the
cold months and will benefit from winter protection. Winter
protection can be provided by constructing a wire framework
around the plant and filling the space with shredded leaves to a
depth of approximately 10-12” for insulation. Cold winters and
late spring frosts can often kill flower buds, preventing plants
from blooming in the spring. When unexpected cold weather is
predicted in late spring or early summer after winter protection
has been removed, it may be necessary to cover tender
hydrangea species with cloth, burlap, or heavy paper to protect
flower buds. Plastic should never be used as winter protection
because it may damage tender plant tissue.
Hydrangea species and cultivars
C
limbing hydrangea
Hydrangea anomala petiolaris
Vigorous climbing vine with nice exfoliating bark that attaches itself by rootlet holdfasts
White, fragrant blooms on new wood in mid-summer
Requires little or no pruning
Slow growing until roots are established.
Needs to climb on sturdy structure; can climb to 80’
Smooth hydrangea
Hydrangea arborescens
White blooms on new growth in early to mid-June
Prune hard 12” from ground in late winter or early spring
3-5’ h x 3-5’ w
Flops in too much shade
Annabelle or snowball hydrangea
Hydrangea arborescens
‘Annabelle’
White 6-8” blooms on new growth starting in mid-June
Prune hard 12” from ground in late winter or early spring
3-5’ h x 3-5’ w
Full sun required for good blooms
Hills-of-Snow hydrangea
Hydrangea arborescens
‘Grandiflora’
White blooms on new growth June to August
Prune hard 12” from ground in late winter or early spring
3-5’ h x 3-5’ w
Bigleaf hydrangea
Hydrangea macrophylla
Pink or blue blooms (dependent on soil pH) on old wood
Prune minimally only when necessary immediately after flowering.
3-6’ h x 3-6’ w
Not reliably bloom hardy in zone 5
Endless Summer hydrangea
Hydrangea macrophylla
‘Balimer’
Pink or blue blooms (dependent on soil pH) on both old and new growth in summer
Prune branches with winter dieback in spring
3-5’ h x 3-5’ w
Nikko Blue hydrangea
Hydrangea macrophylla
‘Nikko Blue’
Pink or blue blooms (dependent on soil pH) on old growth in summer
Prune minimally only when necessary immediately after flowering
4-6’ h x 4-6’ w
Not reliably bloom hardy in zone 5
Panicle hydrangea
Hydrangea paniculata
White blooms in midsummer that fade to various shades of pink on new growth
Pruning not essential, however, may prune down to 12-24” in late winter or early spring
Various cultivars up to 20’ h X 20’ w
Extremely cold-hardy hydrangea; full sun required for good blooms
PeeGee hydrangea
Hydrangea paniculata
‘Grandiflora’
White blooms that fade to purplish-pink on new growth July to September
Thin the plant in early spring to 5 to 10 primary shoots for larger blooms
10’ h x 10’ w
Excellent shrub form
Limelight panicle hydrangea
Hydrangea paniculata
‘Limelight’
Lime green to white blooms on new growth
Pruning not essential, however, may prune down to 12-24” in late winter or early spring
5-8’ h x 5-8’ w
Full sun required for good blooms
Late panicle hydrangea
Hydrangea paniculata
‘Tardiva’
White blooms tinted pink on new growth in September
Pruning not essential, however, may prune down to 12-24” in late winter or early spring
6-8’ h x 4-6’ w
Full sun required for good blooms
Unique panicle hydrangea
Hydrangea paniculata
‘Unique’
White blooms tinted pink on new growth in summer
Pruning not essential, however, may prune down to 12-24” in late winter or early spring
Up to 10’ h X 10’ w
Full sun required for good blooms
Oakleaf hydrangea
Hydrangea quercifolia
White blooms on old growth June through July
Minimal pruning; pruning should be done immediately after bloom
4-8’ h x 4-8’ w
Excellent shrub form with good fall color
Little Honey oakleaf hydrangea
Hydrangea quercifolia
‘Little Honey’
Golden yellow foliage with white blooms tinted pink on old growth in summer
Minimal pruning; pruning should be done immediately after bloom
4’ h x 3’ w
Exfoliating bark. Good red fall color
Serrated hydrangea
Hydrangea serrata
Pink or blue blooms (dependent on soil pH)
Certain cultivars bloom on both old and new growth
3-5’ h x 3-5’ w
Hardier than macrophylla
