Let’s look at lilacs. We call them
lilac trees, but in most parts of
Australia they are really just big,
round-headed deciduous shrubs, most
of which seldom grow more than 4m
tall with a spread of about the same.
We grow them in our gardens for
their beautiful, sweetly fragrant flower
trusses, carried in mid to late spring
in a range of colours including white,
violet, blue, lilac, pink, magenta and
purple. The flowers are clustered in
showy panicles that should be upright,
but tend to droop with the weight of
the densely packed, waxy blooms.
The technique for planting lilacs is different from planting other bare-rooted
plants, and the roots develop above the graft union to form a strong tree.
Lilacs need a cold, frosty winter to
bloom to perfection. They thrive in
slightly alkaline soil and detest acidic
conditions. A dressing of lime every
autumn where soil has a pH of 6.5 or
lower stimulates heavy, leafy growth.
I remember seeing some stunning
lilacs in North America, with rich green
foliage and masses of huge blooms.
The panicles were so large and heavy
that they dragged the branches down
almost to the ground. The secret was
in the soil. It had a pH of 8, which is
surprisingly alkaline, but they clearly
loved growing in such conditions.
Lilacs develop lovely deep roots
over time, and mature specimens are
remarkably drought tolerant. However,
they will certainly benefit from a slow
monthly soak during dry summers.
PHOTOS IAN HOFSTETTER/PETER CUNDALL
There are more than 1500 named
lilac cultivars, some quite vigorous.
The most commonly grown in Australian
gardens are cultivated varieties of
Syringa vulgaris, many of which have
been developed in France. Most of
these have rich green, heart-shaped
leaves and, even when not in flower
in late spring and early summer, make
outstanding garden specimens. They
are excellent hedging plants which,
when planted 3m apart in a row, grow
to form a superb privacy screen.
Dig an extra-deep Place a stake
hole. Form a mound across the hole to
at the base of the hole check that the graft
and sit the roots over it. union is below ground.
Backfill the planting
hole with soil, making
sure the graft union is
buried. Water well.
Perhaps the most
amazing thing about
lilacs is their longevity.
I’ve seen some plants
that were well over
100 years old, and they
were still flourishing.
Lilac plants are best purchased in
winter, while dormant, and are usually
sold as bare-rooted plants. Although
the trend overseas is to grow them
on their own roots, most lilacs sold in
Australia are grafted onto common lilac
or a fellow member of the Oleaceae
family, such as privet. This means
that if they are not planted correctly,
suckering can be a constant problem.
When plants are grafted onto common
lilac stock, the leaves of suckers look
similar to the main plant, making it
difficult to identify them as suckers.
If not controlled, suckers invariably
overwhelm and swamp less vigorous
did you know?
+ Purple lilac flowers are thought to symbolise the emotion of
being in love, while white lilacs represent youthful innocence.
cultivars, and gardeners often don’t
realise this is happening until inferior
flowers of the wrong colour appear.
However, when privet is used as
rootstock, the suckers are easily
identified and they can be cut back
hard. This pruning, which needs to be
carried out when suckers are actively
growing in summer, sharply reduces
their vigour, giving good control.
Plants are grafted onto privet stock
because of a well-understood condition
called delayed incompatibility. Most
privet grafts have a relatively short life
of five to 10 years – just long enough
for the named lilac to form its own
roots before the rootstock and graft
fail. Known as ‘nurse-grafting’, this
technique is used with certain plants
that are hard to strike from cuttings.
It is possible to prevent suckering
by planting lilacs in a manner quite
different to that used for most other
bare-rooted trees or shrubs. For this
method to succeed, the lilac plants
need to be planted in well-drained
ground that doesn’t contain any subsoil
clay. Dig an extra-deep hole and bury
the plants deeper than you normally
would, so the graft union is buried
below the surface. Although this is
contrary to the normal rules of good
planting, it actually works.
Being surrounded by moist soil
encourages the scion, which is the
part of the stem just above the graft
May 2008 Gardening Australia 21