hydrangeas - hardiness and pruning

Well, gang, several interesting questions about hydrangeas have been
posted in the not too distant past.  Thought I knew the answers, but
wanted to do a bit of research to make sure and, of course, am behind
with mail...so FWIW.

As to hardiness:  The H. macrophylla hybrids - big leaf hydrangeas,
the mopheads or hortensia types and many of the lace caps - are
considered bud hardy to USDA zone 6 (-10F).  That doesn't mean that a
benevolent microclimate in zone 5 would not produce flowers in mild
winter years, but they are generally going to be very iffy.  They
bloom on old wood.

H. quercifolia - oak leaf hydrangeas - are hardy into zone 5 (-20F)
although they are also subject to getting their flower buds nipped
and may not bloom consistently, depending on siting and weather.
They bloom on old wood.

H. arborescens is hardy to USDA zone 3.  This species includes
'Annabelle' and 'Grandiflora' and is also known as the hills-of-snow
hydrangea because the huge flower heads weigh down the stems so that
a large shrub in bloom can look like a...guess what?  a hill of
snow:-)  They bloom on new wood.

H. paniculata, the panicle hydrangea - commonly known as the P.G. or
pee gee hydrangea - is also hardy to USDA zone 3.   It can be grown
as either a shrub or a tree and can reach 35 feet tall, depending on
pruning.

H. aspera has two sections, large leaf (subspecies sargentiana) and
small-leafed.  These are not terribly hardy but will survive and
bloom in zone 6.

H. anomala subsp. petiolaris, the climbing hydrangea, is a woody
vine, attaching itself via root-like holdfasts to just about
anything.  Lateral branches form that stick out from the main stem so
that the vine gets a very shrubby look to it after time.  It is hardy
to USDA zone 4 (-30F).  Flowers are white lace-cap type.

There are several other hydrangea species, mostly from the far East -
China, Nepal, India, etc.   Hydrangea serratifolia hails from South
America, Argentina and Chile.  New species or forms are being
introduced and finding information on them is not all that easy.  The
above list are going to be the most commonly found in nurseries.

As for pruning:  H. arborescens and H. paniculata can be pruned hard
in late winter as they bloom on new wood.  I tend to cut my
'Annabelle' back to the first set of buds closest to the ground as it
is less floppy that way.  H. arborescens tends to be a floppy
species.  H. paniculata can be trained as a tree or stooled back to a
shrub form.  This is one I don't have...yet...so I have no practical
experience with it.

H. anomala shouldn't be pruned if you can help it.  I've never had
any winter die back on this one, it just keeps getting better and
better and the branching form is part of its beauty.  It is not a
vine for a tight spot...

The macrophylla forms don't require cutting back other than removing
dead wood.  Every year, one should do rejuvenation pruning by cutting
out the three oldest stems at ground level.  These hydrangeas bear a
flower at the tip of a new shoot the first year after the shoot
forms.  Since the shoot is not brand new when flowering takes place,
it is termed 'old wood', even though it's "young" old wood:-)   The
next year two side shoots form and flower and then produce side
shoots, etc.  After a shoot has made 15 to 20 flowers in this way, it
should be pruned off at the ground to make room for younger shoots.
If you don't do this, eventually the older stems die and turn white,
when they break off readily and will fall off eventually.

It's also best to leave the old flower heads on the shrubs for
winter, since they help protect the flower buds from spring frosts.
Nip them off after danger of hard frost just above the first two buds
below them.  If you want heads for dry flowers, cut them when they
are mature - just about to start changing color - and put them in a
container with about a half inch of water in it...just let the water
dry up and by the time it does, the heads will be dry, too.  You can
cut sufficient stem with the head to fit in a vase, but don't take
any more than you absolutely have to or you will reduce your next
year's flower show from those plants that bloom on old wood.  You can
take any amount of stem from H. arborescens cvs and I imagine from H.
paniculata cvs, also.

Flower color in hydrangeas is determined by soil pH and the amount of
aluminum available in the soil.  Acid soil (pH less than 7) has more
aluminum available to the plants and they will then produce blue
flowers - except for clones who are actually mutants who lack the
ability to use aluminum at all and who always produce pink flowers -
such as 'Ami Pasquier', 'Bottstein' and 'Pia'  As soil becomes more
alkaline, the amount of aluminum available to the plant decreases and
flowers turn pink.  You can get varying amounts of blue and pink
pigment in flowers on the same plant just because the soil isn't
totally uniform in pH.

Flowers will also change color as they age (something that happens on
a lot of plants), some becoming deeper pink or purple and some fading
to green.  H. 'Preziosa' has flowers that start white and mature to
pinks and wines.  In shade, there may also be some light blue ones.
Since the flowers open at different rates, this shrub will always
have a mixture of flower colors on it.

All this changing does not affect white flower forms who will be
white, not matter what kind of soil they grow in.  However, they will
signal the pH of the soil by the color of the eye of the male flower,
if you go looking for it.

If you want blue flowers and your soil is being too neutral to
produce them, you can acidify it using aluminum sulfate or sulfur.
Results will not be instantaneous and may not occur until the next
years flowering.  If you want pink and are getting blue, apply garden
lime or wood ashes to the soil around the plant...again, results
won't be instant.  If your water source is alkaline, you need to
adjust for that because watering with mains water will affect the
soil pH.  I guess if you're in an acid rain area, that could also
affect flower color, but that's just a guess...don't really know.

Marge Talt, zone 7 Maryland
mtalt@clark.net
Editor:  Gardening in Shade
-----------------------------------------------
Current Article: Deciduous Flowering Shrubs - Part 3
http://www.suite101.com/welcome.cfm/222
------------------------------------------------
Complete Index of Articles by Category and Date
http://mtalt.hort.net/article-index.html
------------------------------------------------
All Suite101.com garden topics :
http://www.suite101.com/category.cfm/gardening

----------
> From: Gerry/Bob O'Neill <eoneill@IBM.NET>
> Date: Friday, July 07, 2000 7:12 AM
>
> Would any of our Hydrangea experts out there care to discuss the
various
> "types" of hydrangea and how they should be pruned?
>
> Gerry
>
> P.S. Neighbor across the street also has an old mophead with
different
> colors on the same bush. I agree, it is really a wow and I have no
idea how
> that happens..
>
>
> At 07:36 AM 7/6/00 -0500, you wrote:
> >Kay and others, I think I missed some messages for a while. What
kind of
> >hydrangeas are we talking about here? the colored mopheads
(hortensias)?
> >
> >I didn't think mopheads were reliably hardy in zone 5, but there
is a
> >beautiful big bush growing two blocks north of me that has ALL
colors on
> >it--blue, violet, lavender, pink--WOW! What is it and what are
they doing
> >to it to get that effect? How does it work! Do individual fls
change color
> >as they age? I want one! (I'd ask the homeowner but they only
moved in a
> >few months ago and I think the plant has been there a while.)
> >
> >Bobbi Diehl
> >Bloomington, IN
> >zone 5/6
> >
> >
> Oak Ridge, TN
> Z7b
Other Mailing lists | Author Index | Date Index | Subject Index | Thread Index