© Janet Davis
For most gardeners, wielding a pair of pruning shears
is a terrifying experience. If I cut
too much, will I kill my plant? If I
cut too little, will it stop blooming?
Should I remove branches at the bottom, or just give the top a little
haircut? HELP!
Pruning
is done to remove winter-killed growth, to encourage flowering, to maintain or
reduce a plant’s size, or to admit sun to improve flowering and fruiting. Understanding how and when to prune flowering shrubs
will help them produce strong, shapely vegetative growth and a bonanza of
flowers.
Unless you’re taking drastic
action – rejuvenating an overgrown privet or bridalwreath spiraea hedge, for
example – never “just chop” woody plants back. Instead, make a careful slanted
cut just above a bud that faces the direction in which you want to direct new
growth. To the shrub, your cut is like a detour sign and subsequent growth will
proceed into the next available exit.
Even when you cut a rose for the dining room table, where and how you
cut affects which direction the rose cane will grow.
It is vital to keep pruning shears, clippers, loppers, secateurs and saws very sharp.
Replace your saw blade perioidically, and use a sharpening stone on shears and pruners (or have a professional do
it). And clean the blade as you move
from plant to plant. If you’re pruning
roses, for example, experts recommend using a diluted 1:10 bleach solution to
keep diseases from spreading.
Clematis:
Clematis
vines that bloom in summer or fall on new growth must be pruned in early
spring. These include Jackmanii,
Comtesse de Bouchaud, Ernest Markham, Hagley Hybrid; Viticella hybrids like
Etoile de Violette and Polish Spirit;
late-flowering Texensis hybrids such as Duchess of Albany and Gravetye
Beauty; and sweet autumn clematis (Clematis terniflora). Cut these
back to just above a plump pair of buds, a foot or so above ground.
Clematis
that bloom with large flowers in late spring or early summer (Ramona, Nelly
Moser, The President, Henryi, etc.) and the spring-blooming Clematis alpina or Clematis macropetala cultivars all
flower on last year’s growth, so if pruning is desired to keep them at a
desirable size or to reshape them, it should be done immediately after the
flowers fade.
Shrubs:
In cold
regions, early spring is when we prune back to ground level shrubs that tend to
winter-kill above ground anyway, such as 
butterfly bush (Buddleia).
Also, a few plants that bloom adequately if left unpruned, such as
spirea Anthony Waterer or Goldflame and blue mist bush (Caryopteris x clandonensis), will produce better flowers on
new growth, as well as remaining compact.
Spring-flowering
shrubs such as lilac, forsythia, mock
orange, weigela, deutzia and other spring-bloomers all require periodic
thinning (or renewal pruning) of old canes, trimming of crossed branches and
removal of weak or dead growth.
All renewal pruning of
spring-flowering shrubs should be done immediately after the flowers fade. This
will permit a good flowering show (buds are set in summer for the following
spring) and still allow the shrub to recover in time to make next year’s buds.
When shrubs have become overly dense and flowering declines, remove up to
one-third of the growth annually at ground level, selecting the oldest woody
canes, and pruning to open up the center of the plant to sun.
It’s better to let a shrub
reach its mature dimension, both in size and form, than trim it regularly to
keep its growth artificially in check. As an alternative, many dwarf forms of
large shrubs will give the same effect without the large space requirement. But
if you do find it necessary to contain the size of the shrub, avoid the
“buzz-cut” method that turns beautiful willowy forsythias, for example, into
sheared pompoms. Try to maintain a natural shape and thin old growth at the
base periodically as well.
Hydrangeas:
Both peegee hydrangea (H. paniculata ‘Grandiflora’) and smooth
hydrangea (H. arborescens), including its cultivars ‘Annabelle’
and ‘Grandiflora’, produce summer flowers on new growth made in the current
year. To encourage strong new growth and big blooms, prune back the previous
year’s growth in early spring to two or three buds. Cut on a slant just above
an outfacing bud to direct growth away from the center of the plant (or
in-facing if you wish to direct growth that way). Cut out weak shoots, crossed
branches and suckers.
Both oakleaf hydrangea (H. quercifolia) and the blue or pink
florist’s hydrangea (H. macrophylla) – also known as hortensia, French or bigleaf hydrangea –
produce blossoms on last year’s wood. They should be pruned immediately after
flowering, and given some winter protection.
The exception is the gorgeous, new bigleaf hydrangea ‘Endless Summer’,
which also produces on new growth. 
For that one, experts recommend that in spring, once you can see new
growth, to “prune back the old branches to a finger width above the new green growth”. For more info on the care of this hydrangea,
check out the information contained on the Endless Summer
website.
Roses:
“When the forsythia blooms” is the accepted time in our climate to do regular
spring pruning of established hybrid teas, floribundas and grandifloras. Cut
back to about 4-6 inches (10-15 centimeters) above ground level – or higher for
a tall rose like the grandiflora ‘Queen Elizabeth’ – to healthy, green-barked wood
with white pith in the stems. (It’s prudent to wait through some warm weather
in spring to determine how far up the canes your rose wood is viable.)
Retain vigorous thick canes
and remove old woody ones, as well as thin and crossed branches. Cut on an
upward angle just above an outward facing bud, in the direction of the bud.
Stooling
is the practice of radically cutting back (to as much as six inches or 15
centimeters from ground level) certain types of shrubs. It is usually done to
improve the appearance of plants that tend to get lanky and fruit or flower
poorly as they attain their mature height; for example, summer-flowering
spiraeas. And it’s what many gardeners do with butterfly bush (Buddleia), Russian sage (Perovskia) or blue mist bush (Caryopteris), but usually for reasons
of cold-hardiness, since they often die back to their roots in winter anyway.
But radical pruning done on
perfectly hardy shrubs can result in a compact, dense plant and a lush foliage
effect that might be desirable on its own merits – in a perennial border, for
example. Good candidates for stooling
are purple smoke bush (Cotinus coggygria
‘Royal purple’) and Japanese sand cherry (Prunus
x cistena), which both provide a purple background. For a bright lime-gold
effect, try stooling Sambucus canadensis
‘Aureus’. Stooling for foliage effect
should be done in early spring (flower buds on spring bloomers will be lost)
and only on well-established shrubs growing on their own roots, that is, not
grafted.
Adapted from a column that appeared originally in the Toronto Sun